Computer Science and Artificial Intelligence Lab (CSAIL)

On the Complexity of Neural Computation in Superposition

Mon, 30 Sep 2024 00:00:00 GMT

On the Complexity of Neural Computation in Superposition Adler, Micah; Shavit, Nir Recent advances in the understanding of neural networks suggest that superposition, the ability of a single neuron to represent multiple features simultaneously, is a key mechanism underlying the computational efficiency of large-scale networks. This paper explores the theoretical foundations of computing in superposition, focusing on explicit, provably correct algorithms and their efficiency. We present the first lower bounds showing that for a broad class of problems, including permutations and pairwise logical operations, a neural net- work computing in superposition requires at least Ω(m′ log m′) parameters and Ω(√(m′ log m′)) neurons, where m′ is the number of output features being computed. This implies that any “lottery ticket” sparse sub-network must have at least Ω(m′ log m′ ) parameters no matter what the initial dense network size. Conversely, we show a nearly tight upper bound: logical operations like pair- wise AND can be computed using O(√(m′) log m′) neurons and O(m′ log^2 m′) parameters. There is thus an exponential gap between computing in superposition, the subject of this work, and representing features in superposition, which can require as little as O(log m′) neurons based on the Johnson-Lindenstrauss Lemma. Our hope is that our results open a path for using complexity theoretic techniques in neural network interpretability research.

Belief Programming Implementation

Mon, 27 Nov 2023 00:00:00 GMT

Belief Programming Implementation Atkinson, Eric

Speranza: Usable, privacy-friendly software signing

Tue, 19 Sep 2023 00:00:00 GMT

Speranza: Usable, privacy-friendly software signing Merrill, Kelsey; Newman, Zachary; Torres-Arias, Santiago; Sollins, Karen Software repositories, used for wide-scale open software distribu- tion, are a significant vector for security attacks. Software signing provides authenticity, mitigating many such attacks. Developer- managed signing keys pose usability challenges, but certificate- based systems introduce privacy problems. This work, Speranza, uses certificates to verify software authenticity but still provides anonymity to signers using zero-knowledge identity co-commitments. In Speranza, a signer uses an automated certificate authority (CA) to create a private identity-bound signature and proof of authoriza- tion. Verifiers check that a signer was authorized to publish a pack- age without learning the signer’s identity. The package repository privately records each package’s authorized signers, but publishes only commitments to identities in a public map. Then, when issuing certificates, the CA issues the certificate to a distinct commitment to the same identity. The signer then creates a zero-knowledge proof that these are identity co-commitments. We implemented a proof-of-concept for Speranza. We find that costs to maintainers (signing) and end users (verifying) are small (sub-millisecond), even for a repository with millions of packages. Techniques inspired by recent key transparency systems reduce the bandwidth for serving authorization policies to 2 KiB. Server costs in this system are negligible. Our evaluation finds that Speranza is practical on the scale of the largest software repositories. We also emphasize practicality and deployability in this project. By building on existing technology and employing relatively sim- ple and well-established cryptographic techniques, Speranza can be deployed for wide-scale use with only a few hundred lines of code and minimal changes to existing infrastructure. Speranza is a practical way to bring privacy and authenticity together for more trustworthy open-source software. This is an extended version of the shorter paper by the same name, published in ACM CCS 2023.

How Can Large Language Models Help Humans in Design And Manufacturing?

Thu, 27 Jul 2023 00:00:00 GMT

How Can Large Language Models Help Humans in Design And Manufacturing? Makatura, Liane; Foshey, Michael; Wang, Bohan; Hähnlein, Felix; Ma, Pingchuan; Deng, Bolei; Tjandrasuwita, Megan; Spielberg, Andrew; Owens, Crystal Elaine; Chen, Peter Yichen; Zhao, Allan; Zhu, Amy; Norton, Wil J; Gu, Edward; Jacob, Joshua; Li, Yifei; Schulz, Adriana; Matusik, Wojciech

Counterfactual Explanations and Predictive Models to Enhance Clinical Decision-Making in Schizophrenia using Digital Phenotyping

Thu, 15 Jun 2023 00:00:00 GMT

Counterfactual Explanations and Predictive Models to Enhance Clinical Decision-Making in Schizophrenia using Digital Phenotyping Canas, Juan Sebastian; Gomez, Francisco; Costilla Reyes, Omar Clinical practice in psychiatry is burdened with the increased demand for healthcare services and the scarce resources available. New paradigms of health data powered with machine learning techniques could open the possibility to improve clinical workflow in critical stages of clinical assessment and treatment in psychiatry. In this work, we propose a machine learning system capable of predicting, detecting, and explaining individual changes in symptoms of patients with Schizophrenia by using behavioral digital phenotyping data. We forecast symptoms of patients with an error rate below 10%. The system detects decreases in symptoms using changepoint algorithms and uses counterfactual explanations as a recourse in a simulated continuous monitoring scenario in healthcare. Overall, this study offers valuable insights into the performance and potential of counterfactual explanations, predictive models, and change-point detection within a simulated clinical workflow. These findings lay the foundation for further research to explore additional facets of the workflow, aiming to enhance its effectiveness and applicability in real-world healthcare settings. By leveraging these components, the goal is to develop an actionable, interpretable, and trustworthy integrative decision support system that combines real-time clinical assessments with sensor-based inputs.

The AEGIS Processor Architecture for Tamper-Evident and Tamper-Resistant Processing

Wed, 01 Jan 2003 00:00:00 GMT

The AEGIS Processor Architecture for Tamper-Evident and Tamper-Resistant Processing Suh, G. Edward; Clarke, Dwaine; Gassend, Blaise; van Dijk, Marten; Devadas, Srinivas We describe the architecture for a single-chip AEGIS processor which can be used to build computing systems secure against both physical and software attacks. Our architecture assumes that all components external to the processor, such as memory, are untrusted. We show two different implementations. In the first case, the core functionality of the operating system is trusted and implemented in a security kernel. We also describe a variant implementation assuming an untrusted operating system. AEGIS provides users with tamper-evident, authenticated environments in which any physical or software tampering by an adversary is guaranteed to be detected, and private and authenticated tamper-resistant environments where additionally the adversary is unable to obtain any information about software or data by tampering with, or otherwise observing, system operation. AEGIS enables many applications, such as commercial grid computing, secure mobile agents, software licensing, and digital rights management. We also present a new encryption/decryption method that successfully hides a significant portion of encryption/decryption latency, in comparison to a conventional direct encryption scheme. Efficient memory encryption and integrity verification enable the implementation of a secure computing system with the only trusted component being a single-chip AEGIS CPU. Preliminary simulation results indicate that the overhead of security mechanisms in AEGIS is reasonable.

The AEGIS Processor Architecture for Tamper-Evident and Tamper-Resistant Processing

Wed, 01 Jan 2003 00:00:00 GMT

The AEGIS Processor Architecture for Tamper-Evident and Tamper-Resistant Processing Suh, G. Edward; Clarke, Dwaine; Gassend, Blaise; van Dijk, Marten; Devadas, Srinivas We describe the architecture of the AEGIS processor which can be used to build computing systems secure against both physical and software attacks. AEGIS assumes that the operating system and all components external to it, such as memory, are untrusted. AEGIS provides tamper-evident, authenticated environments in which any physical or software tampering by the adversary is guaranteed to be detected, and private and authenticated, tamper-resistant environments where additionally the adversary is unable to obtain any information about software or data by tampering with, or otherwise observing, system operation. AEGIS enables many applications, such as commercial grid computing, software licensing, and digital rights management. We present a new encryption/decryption method that successfully hides a significant portion of encryption/decryption latency, in comparison to a conventional direct encryption scheme. Efficient memory encryption and integrity verification enable the implementation of a secure computing system with the only trusted component being a single-chip AEGIS CPU. Detailed simulation results indicate that the performance overhead of security mechanisms in AEGIS is reasonable.

AEGIS: Architecture for Tamper-Evident and Tamper-Resistant Processing

Wed, 01 Jan 2003 00:00:00 GMT

AEGIS: Architecture for Tamper-Evident and Tamper-Resistant Processing Suh, G. Edward; Clarke, Dwaine; Gassend, Blaise; van Dijk, Marten; Devadas, Srinivas We describe the architecture for a single-chip AEGIS processor which can be used to build computing systems secure against both physical and software attacks. Our architecture assumes that all components external to the processor, such as memory, are untrusted. We show two different implementations. In the first case, the core functionality of the operating system is trusted and implemented in a security kernel. We also describe a variant implementation assuming an untrusted operating system. AEGIS provides users with tamper-evident, authenticated environments in which any physical or software tampering by an adversary is guaranteed to be detected, and private and authenticated tamper-resistant environments where additionally the adversary is unable to obtain any information about software or data by tampering with, or otherwise observing, system operation. AEGIS enables many applications, such as commercial grid computing, secure mobile agents, software licensing, and digital rights management. Preliminary simulation results indicate that the overhead of security mechanisms in AEGIS is reasonable.

Hybrid I/O Automata*

Wed, 01 Jan 2003 00:00:00 GMT

Hybrid I/O Automata* Lynch, Nancy A.; Segala, Roberto; Vaandrager, Frits Hybrid systems are systems that exhibit a combination of discrete and continuous behavior. Typical hybrid systems include computer components, which operate in discrete program steps, and real-world components, whose behavior over time intervals evolves according to physical constraints. Important examples of hybrid systems include automated transportation systems, robotics systems, process control systems, systems of embedded devices, and mobile computing systems. Such systems can be very complex, and very difficult to describe and analyze. This paper presents the Hybrid Input/Output Automaton (HIOA) modeling framework, a basic mathematical framework to support description and analysis of hybrid systems. An important feature of this model is its support for decomposing hybrid system descriptions. In particular, the framework includes a notion of external behavior for a hybrid I/O automaton, which captures its discrete and continuous interactions with its environment. The framework also defines what it means for one HIOA to implement another, based on an inclusion relationship between their external behavior sets, and defines a notion of simulation, which provides a sufficient condition for demonstrating implementation relationships. The framework also includes a composition operation for HIOAs, which respects external behavior, and a notion of receptiveness, which implies that an HIOA does not block the passage of time. The framework is intended to support analysis methods from both computer science and control theory. This work is a simplification of an earlier version of the HIOA model [49, 50]. The main simplification in the new model is a clearer separation between the mechanisms used to model discrete and continuous interaction between components. In particular, the new model removes the dual use of external variables for discrete and continuous interaction.

Hybrid I/O Automata*

Fri, 01 Feb 2002 00:00:00 GMT

Enhanced Certificate Revocation

Fri, 01 Mar 1996 00:00:00 GMT

Enhanced Certificate Revocation Micali, Silvio We apply off-line/on-line signatures to provide an alternative solution to the problem of certificate revocation. The new systems dismiss with traditional CRLs (Certificate Revocation Lists) and yield public-key infrastructures that are substantially cheaper to run than traditional ones.

What are principal typings and what are they good for?

Wed, 01 Nov 1995 00:00:00 GMT

What are principal typings and what are they good for? Jim, Trevor We demonstrate the pragmatic value of the principal typing property, a property more general than ML's principal type property, by studying a type system with principal typings. The type system is based on rank 2 intersection types and is closely related to ML. Its principal typing property provides elegant support for separate compilation, including "smartest recompilation" and incremental type inference, and for accurate type error messages. Moreover, it motivates a novel rule for typing recursive definitions that can type many examples of polymorphic recursion.

Rank 2 Type Systems and Recursive Definitions

Wed, 01 Nov 1995 00:00:00 GMT

Rank 2 Type Systems and Recursive Definitions Jim, Trevor We demonstrate an equivalence between the rank 2 fragments of the polymorphic lambda calculus (System F) and the intersection type discipline: exactly the same terms are typable in each system. An immediate consequence is that typability in the rank 2 intersection system is DEXPTIME-complete. We introduce a rank 2 system combining intersections and polymorphism and prove that it types exactly the same terms as the other rank 2 systems. The combined system suggests a new rule for typing recursive definitions. The result is a rank 2 type system with decidable type inference that can type some interesting examples of polymorphic recursion. Finally, we discuss some applications of the type system in data representation optimizations such as unboxing and overloading.

An Autoimmune Mechanism for AIDS' T4 Lymphopenia

Wed, 01 May 1991 00:00:00 GMT

An Autoimmune Mechanism for AIDS' T4 Lymphopenia Micali, Silvio We put forward a new model for the T4 lymphopenia occuring in AIDS by suggesting a mechanism whose net effect is blocking the generation of T4 cells during HIV infection. Supporting evidence for this mechanism is derived from the experiments in the recent literature.

Randomness-efficient Sampling of Arbitrary Functions

Sun, 01 Jul 1990 00:00:00 GMT

Randomness-efficient Sampling of Arbitrary Functions Bellare, Mihir; Rompel, John

A Modular Drinking Philosophers Algorithm

Thu, 01 Oct 1992 00:00:00 GMT

A Modular Drinking Philosophers Algorithm Welch, Jennifer Lundelius; Lynch, Nancy A. A variant of the drinking philosphers algorithm of Chandy and Misra is described and proved correct in a module way, using the I/O automaton model of Lynch and Tuttle. The algorithm of Chandy and Misra is based on an particular dining philosophers algorithm, and relies on certain properties of its implementation. The drinking philosophers algorithm presented in this paper is able to use an arbitrary dining philosophers algorithm as a true subroutine; nothing about the implementation needs to be known, only that is solves the dining philosophers problem. An important advantage of this modularity is that by substituting a more time-efficient dining philosophers algorithm with O(1) worst-case waiting time is obtained, whereas the drinking philosophers algorithm of Chandy and Misra has O(n) worst-case waiting time (for n philosophers). Formal definitions are given to distinguish the drinking and dining philosophers problems and to specify precisely varying degrees of concurrency.

Bandwidth Management in Wireless Sensor Networks

Tue, 01 Apr 2003 00:00:00 GMT

Bandwidth Management in Wireless Sensor Networks Hull, Bret; Jamieson, Kyle; Balakrishnan, Hari Wireless sensor networks are often used in monitoring and control applications, where software running on generalpurpose computers ÔøΩpullÔøΩ information from remote sensors and ÔøΩpushÔøΩ actuations into the network. The sensors themselves form a multihop wireless network communicatingwith one or more sensor access points (SAPs) that interface between application software and the sensor network. This paper addresses the problem of managing wireless network bandwidth and improving network capacity in a sensor network deployed as a shared infrastructure, concurrently used by different applications. Our bandwidth management architecture incorporates three ideas: first, we develop a simple rule system that allows applications and the network administrator to specify how traffic generated by sensors should be treated by the sensor network. Each rule is a function that maps a sensor data type and generated value to a transmission rate and a traffic class. Second, we show how using multiple SAPs and SAP selection method that considers packet loss probabilities, path load, and path lengths improves the capacity of the network and the performance of individual sensor streams. Third, we show that hopby- hop flow control, rather than end-to-end congestion control, is a better way to cope with the nature of sensor network traffic and avoids unnecessary packet losses that waste valuable wireless network bandwidth. Our experimental results from a 40-node indoor wireless sensor testbed show that these three techniques are simple to implement and allow scarce network bandwidth to be used efficiently.

Computer-Enforced Immutability for the Java Language

Sun, 01 Jun 2003 00:00:00 GMT

Computer-Enforced Immutability for the Java Language Birka, Adrian This thesis presents the design, implementation, and evaluation of an extension to the Java language, ConstJava, that is capable of expressing immutability constraints and verifying them at compile time. The specific constraint expressed in ConstJava is that the transitive state of the object to which a given reference refers cannot be modified using that reference. In addition to the ability to specify and enforce this basic constraint, ConstJava includes several other features, such as mutable fields, immutable classes, templates, and the const cast operator, that make ConstJava a more useful language. The thesis evaluates the utility of ConstJava via experiments involving writing ConstJava code and converting Java code to ConstJava code. The evaluation of ConstJava shows that the language provides tangible benefits in early detection and correction of bugs that would otherwise be difficult to catch. There are also costs associated with the use of ConstJava. These are minimized by ConstJavaÔøΩs backward compatibility with Java, and by the high degree of inter-operability of the two languages, which allows for a less painful transition from Java to ConstJava. This technical report is a revision of the authorÔøΩs MasterÔøΩs thesis, which was advised by Prof. Michael D. Ernst.

Compositionality for Probabilistic Automata

Compositionality for Probabilistic Automata Lynch, Nancy A.; Segala, Roberto; Vaandrager, Frits We establish that on the dfomain of probabilistic automata, the trace distribution preorder coincides with the simulation preorder.

Subexponential Parameterized Algorithms on Graphs of Bounded Genus and H-minor-free Graphs

Sun, 01 Jun 2003 00:00:00 GMT

Subexponential Parameterized Algorithms on Graphs of Bounded Genus and H-minor-free Graphs Demaine, Erik D.; Fomin, Fedor V.; Hajiaghayi, Mohammad Taghi; Thilikos, Dimitrios M. We introduce a new framework for designing fixed-parameter algorithms with subexponential running time---2^O(sqrt k) n^O(1). Our results apply to a broad family of graph problems, called bidimensional problems, which includes many domination and covering problems such as vertex cover, feedback vertex set, minimum maximal matching, dominating set, edge dominating set, clique-transversal set, and many others restricted to bounded genus graphs. Furthermore, it is fairly straightforward to prove that a problem is bidimensional. In particular, our framework includes as special cases all previously known problems to have such subexponential algorithms. Previously, these algorithms applied to planar graphs, single-crossing-minor-free graphs, and/or map graphs; we extend these results to apply to bounded-genus graphs as well. In a parallel development of combinatorial results, we establish an upper bound on the treewidth (or branchwidth) of a bounded-genus graph that excludes some planar graph H as a minor. This bound depends linearly on the size |V(H)| of the excluded graph H and the genus g(G) of the graph G, and applies and extends the graph-minors work of Robertson and Seymour. Building on these results, we develop subexponential fixed-parameter algorithms for dominating set, vertex cover, and set cover in any class of graphs excluding a fixed graph H as a minor. In particular, this general category of graphs includes planar graphs, bounded-genus graphs, single-crossing-minor-free graphs, and any class of graphs that is closed under taking minors. Specifically, the running time is 2^O(sqrt k) n^h, where h is a constant depending only on H, which is polynomial for k = O(log^2 n). We introduce a general approach for developing algorithms on H-minor-free graphs, based on structural results about H-minor-free graphs at the heart of Robertson and Seymour's graph-minors work. We believe this approach opens the way to further development on problems in H-minor-free graphs.

Fixed Parameter Algorithms for Minor-Closed Graphs (of Locally Bounded Treewidth)

Sun, 01 Jun 2003 00:00:00 GMT

Fixed Parameter Algorithms for Minor-Closed Graphs (of Locally Bounded Treewidth) Demaine, Erik D.; Hajiaghayi, Mohammad Taghi Frick and Grohe showed that for each property phi that is definable in first-order logic, and for each class of minor-closed graphs of locally bounded treewidth, there is an O(n^(1+epsilon))-time algorithm deciding whether a given graph has property phi. In this paper, we extend this result for fixed-parameter algorithms and show that any minor-closed [contraction-closed] bidimensional parameter which can be computed in polynomial time on graphs of bounded treewidth is also fixed-parameter tractable on general minor-closed graphs [minor-closed class of graphs of locally bounded treewidth]. These parameters include many domination and covering parameters such as vertex cover, feedback vertex set, dominating set, and clique-transversal set. Our algorithm is very simple and its running time is explicit (in contrast to the work of Frick and Grohe). Along the way, we obtain interesting combinatorial bounds between the aforementioned parameters and the treewidth of the graphs.

Equivalence of Local Treewidth and Linear Local Treewidth and its Algorithmic Applications

Thu, 01 May 2003 00:00:00 GMT

Equivalence of Local Treewidth and Linear Local Treewidth and its Algorithmic Applications Demaine, Erik D.; Hajiaghayi, Mohammad Taghi We solve an open problem posed by Eppstein in 1995 and re-enforced by Grohe concerning locally bounded treewidth in minor-closed families of graphs. A graph has bounded local treewidth if the subgraph induced by vertices within distance r of any vertex has treewidth bounded by a function of r (not n). Eppstein characterized minor-closed families of graphs with bounded local treewidth as precisely minor-closed families that minor-exclude an apex graph, where an apex graph has one vertex whose removal leaves a planar graph. In particular, Eppstein showed that all apex-minor-free graphs have bounded local treewidth, but his bound is doubly exponential in r, leaving open whether a tighter bound could be obtained. We improve this doubly exponential bound to a linear bound, which is optimal. In particular, any minor-closed graph family with bounded local treewidth has linear local treewidth. Our bound generalizes previously known linear bounds for special classes of graphs proved by several authors. As a consequence of our result, we obtain substantially faster polynomial-time approximation schemes for a broad class of problems in apex-minor-free graphs, improving the running time from 2^(2^(2^O(1/epsilon))) n^O(1) to 2^O(1/epsilon) n^O(1).

Snapshots in a Distributed Persistent Object Storage System

Thu, 01 May 2003 00:00:00 GMT

Snapshots in a Distributed Persistent Object Storage System Moh, Chuang-Hue

Incremental Multiset Hash Functions and their Application to Memory Integrity Checking

Thu, 01 May 2003 00:00:00 GMT

Incremental Multiset Hash Functions and their Application to Memory Integrity Checking Clarke, Dwaine; Devadas, Srinivas; van Dijk, Marten; Gassend, Blaise; Suh, G. Edward We introduce a new cryptographic tool: multiset hash functions. Unlike standard hash functions which take strings as input, multiset hash functions operate on multisets (or sets). They map multisets of arbitrary finite size to strings (hashes) of fixed length. They are incremental in that, when new members are added to the multiset, the hash can be updated in time proportional to the change. The functions may be multiset-collision resistant in that it is diÔøΩcult to find two multisets which produce the same hash, or just set-collision resistant in that it is diÔøΩcult to find a set and a multiset which produce the same hash. In particular, we introduce four multiset hash functions, each with its own advantages. MSet-XOR-Hash uses the XOR operation and is very eÔøΩcient; however, it uses a secret key and is only set-collision resistant. MSet-Add-Hash uses addition modulo a large integer and, thus, is slightly less eÔøΩcient than MSet-XOR-Hash; MSet-Add-Hash also uses a secret key but it is multiset-collision resistant. MSet-Mu-Hash uses finite field arithmetic and is not as eÔøΩcient as the other two hash functions; however, MSet-Mu-Hash is multiset-collision resistant, and unlike the other two hash functions, does not require a secret key. MSet-VAdd-Hash is more eÔøΩcient than MSet-Mu-Hash; it is also multiset-collision resistant, and does not use a secret key, but the hashes it produces are significantly longer than the hashes of the other functions. The proven security of MSet-XOR-Hash and MSet-Add-Hash is quantitative. We reduce the hardness of finding collisions to the hardness of breaking the underlying pseudorandom functions. The proven security of MSet-Mu-Hash is in the random oracle model and is based on the hardness of the discrete logarithm problem. The proven security of MSet-VAdd-Hash is also in the random oracle model and is based on the hardness of the worst-case shortest vector problem. We demonstrate how set-collision resistant multiset hash functions make an existing oÔøΩine memory integrity checker secure against active adversaries. We improve on this checker such that it can use smaller time stamps without increasing the frequency of checks. The improved checker uses multiset-collision resistant hash functions

Improving Application-level Network Services with Regions

Thu, 01 May 2003 00:00:00 GMT

Improving Application-level Network Services with Regions Li, Ji The underlying premise of the Region Project is that the concept of a region should be a new architecture capability in networking. A region is an entity that encapsulates and implements scoping, grouping, subdividing, and crossing boundaries of sets of entities. It is a powerful tool for managing the increasingly complex demands on the Internet and its successors, and thus should be made into an explicit, first-class component of the network architecture. Autonomous Systems and peer-to-peer networks can be viewed as two simple forms of existing regions. In this work, we explore the utility of informing members in one region of the membership of those same entities in different regions. Specifically, we improve peer-to-peer networks with information derived from Autonomous Systems. This thesis makes three notable contributions. Firstly, we provide a general peer-to-peer simulation framework for different optimization schemes. Secondly, we achieve performance improvements in the lookup, caching and replication of peer-to-peer system. Finally, we enhance our overall understanding of regions through the simulation, as well as their utilities to improve system performance.

Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks

Thu, 01 May 2003 00:00:00 GMT

Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks Jamieson, Kyle; Balakrishnan, Hari; Tay, Y.C. Nodes in sensor networks often encounter spatially-correlated contention, where multiple nodes in the same neighborhood all sense an event they need to transmit information about. Furthermore, in many sensor network applications, it is sufficient if a subset of the nodes that observe the same event report it. We show that traditional carrier-sense multiple access (CSMA) protocols like 802.11 do not handle the first constraint adequately, and do not take advantage of the second property, leading to degraded latency and throughput as the network scales in size. We present Sift, a medium access protocol for wireless sensor networks designed with the above observations in mind. Sift is a randomized CSMA protocol, but unlike previous protocols, does not use a time-varying contention window from which a node randomly picks a transmission slot. Rather, to reduce the latency for the delivery of event reports, Sift uses a fixed-size contention window and a carefully-chosen, non-uniform probability distribution of transmitting in each slot within the window. We show using simulations that Sift can offer up to a 7-fold latency reduction compared to 802.11 as the size of the sensor network scales up to 500 nodes. We then analytically prove bounds on the best latency achievable by a decentralized CSMA-based MAC protocol for sensor networks where one report of each event is enough, and show that Sift comes close to meeting this bound.

Anchor-free Distributed Localization in Sensor Networks

Tue, 01 Apr 2003 00:00:00 GMT

Anchor-free Distributed Localization in Sensor Networks Priyantha, Nissanka B.; Balakrishnan, Hari; Demaine, Erik; Teller, Seth Many sensor network applications require that each node's sensor stream be annotated with its physical location in some common coordinate system. Manual measurement and configuration methods for obtaining location don't scale and are error-prone, and equipping sensors with GPS is often expensive and does not work in indoor and urban deployments. Sensor networks can therefore benefit from a self-configuring method where nodes cooperate with each other, estimate local distances to their neighbors, and converge to a consistent coordinate assignment. This paper describes a fully decentralized algorithm called AFL (Anchor-Free Localization) where nodes start from a random initial coordinate assignment and converge to a consistent solution using only local node interactions. The key idea in AFL is fold-freedom, where nodes first configure into a topology that resembles a scaled and unfolded version of the true configuration, and then run a force-based relaxation procedure. We show using extensive simulations under a variety of network sizes, node densities, and distance estimation errors that our algorithm is superior to previously proposed methods that incrementally compute the coordinate of nodes in the network, in terms of its ability to computer correct coordinates under a wider variety of conditions and its robuestness to measurement errors.

System Dependence Graph Construction for Aspect-Oriented Programs

Sat, 01 Mar 2003 00:00:00 GMT

System Dependence Graph Construction for Aspect-Oriented Programs Zhao, Jianjun; Rinard, Martin We extend previous dependence-based representations called system dependence graphs (SDGs) to represent aspect-oriented programs and present an SDG construction algorithm. This algorithm first constructs a module dependence graph (MDG) for each piece of advice, introduction, and method in aspects and classes. It then uses existing techniques to connect the MDGs at call sites to form a partial SDG. Finally, it weaves the MDG for each piece of advice into the partial SDG for those methods whose behavior may be affected by the advice. The result is the complete SDG. Our SDGs capture the additional structure present in many aspect-oriented features such as join points, advice, introduction, aspects, and aspect inheritance, and various types of interactions between aspects and classes. They also correctly reflect the semantics of aspect-oriented concepts such as advice precedence, introduction scope, and aspect weaving. SDGs therefore provide a solid foundation for the further analysis of aspect-oriented programs.

RAMBO II: Rapidly Reconfigurable Atomic Memory for Dynamic Networks

Sat, 01 Mar 2003 00:00:00 GMT

RAMBO II: Rapidly Reconfigurable Atomic Memory for Dynamic Networks Gilbert, Seth; Lynch, Nancy A.; Shvartsman, Alexander A. Future civilian rescue and military operations will depend on a complex system of communicating devices that can operate in highly dynamic environments. In order to present a consistent view of a complex world, these devices will need to maintain data objects with atomic (linearizable) read/write semantics.

Inference of Generic Types in Java

Sat, 01 Mar 2003 00:00:00 GMT

Inference of Generic Types in Java Donovan, Alan; Ernst, Michael D. Future versions of Java will include support for parametric polymorphism, or generic classes. This will bring many benefits to Java programmers, not least because current Java practise makes heavy use of pseudo-generic classes. Such classes (for example, those in package java.util) have logically generic specifications and documentation, but the type system cannot prove their patterns of use to be safe. This work aims to solve the problem of automatic translation of Java source code into Generic Java (GJ) source code. We present two algorithms that together can be used to translate automatically a Java source program into a semantically-equivalent GJ program with generic types. The first algorithm infers a candidate generalisation for any class, based on the methods of that class in isolation. The second algorithm analyses the whole program; it determines a precise parametric type for every value in the program. Optionally, it also refines the generalisations produced by the first analysis as required by the patterns of use of those classes in client code.

Building Data Structures on Untrusted Peer-to-Peer Storage with Per-participant Logs

Sat, 01 Mar 2003 00:00:00 GMT

Building Data Structures on Untrusted Peer-to-Peer Storage with Per-participant Logs Chen, Benjie; Gil, Thomer M.; Muthitacharoen, Athicha; Morris, Robert T. L* is a technique for building multi-user distributed data structures out of untrusted peer-to-peer distributed hash tables (DHTs). L* uses multiple logs, one log per participant, to store changes to the data structure. Each participant finds data by consulting all logs, but performs modifications by appending only to its own log. This dencentralized structure allows L* to maintain meta-data consistency without locking and to isolate users' changes from each other, an appropriate arrangement for unreliable users. Applications use L* to maintain consistent data structures. L* interleaves multiple logs deterministically so that decentralized clients can agree on the order of completed operations, even if those operations where issued concurrently. When the data structure is quiescent, L* guarantees that clients agree on the state of the data structure. L* optionally provides mutual exclusion for applications that need to ensure atomicity for multi-step operations. The Ivy file system, built on top of L*, demonstrates that L*'s consistency guarantees are useful and can be used and implemented efficiently.

Efficient Consistency Proofs on a Committed Database

Sat, 01 Feb 2003 00:00:00 GMT

Efficient Consistency Proofs on a Committed Database Ostrovsky, Rafail; Rackoff, Charles; Smith, Adam A consistent query protocol allows a database owner to publish a very short string c which commits her to a particular database D with special consistency property (i.e., given c, every allowable query has unique and well-defined answer with respect to D.) Moreover, when a user makes a query, any server hosting the database can answer the query, and provide a very short proof P that the answer is well-defined, unique, and consistent with c (and hence with D). One potential application of consistent query protocols is for guaranteeing the consistency of many replicated copies of D---the owner can publish c, and users can verify the consistency of a query to some copy of D by making sure P is consistent with c. This strong guarantee holds even for owners who try to cheat, while creating c. The task of consistent query protocols was originally proposed for membership queries by Micali and Rabin, and subsequently and independently, by Kilian. In this setting a server can prove to a client whether or not a given key is present or not in a database, based only on a short public commitment c. We strengthen their results in several ways. For membership queries, we improve the communication complexity; more importantly, we provide protocols for more general types of queries and more general relational databases. For example, we consider databases in which entries have several keys and where we allow range queries (e.g. we allow a client to ask for all entries within a certain age range and a certain salary range). Towards this goal, we introduce query algorithms with certain inherent robustness properties---called data-robust algorithms---and show how this robustness can be achieved. In particular, we illustrate our general technique by constructing an efficient data-robust algorithm for proving consistency of orthogonal range queries (a particular case of a ``join''query). The server's proof convinces the client not only that all the matching entries provided are in D, but also that no others are present. Our guarantees hold even if the answer is the empty set. In the case of one-dimensional range queries we also show a new data-hiding technique---called explicit hashing---which allows us to a execute consistent query protocol P and at the same time protect the privacy of all other information in the database efficiently. In particular, we avoid the NP reductions required in a generic zero-knowledge proof.

REX: Secure, modular remote execution through file descriptor passing

Wed, 01 Jan 2003 00:00:00 GMT

REX: Secure, modular remote execution through file descriptor passing Kaminsky, Michael; Peterson, Eric; Fu, Kevin; Mazières, David; Kaashoek, M. Frans The ubiquitous SSH package has demonstrated the importance of secure remote login and execution. This paper presents a new system, REX, designed to provide remote login and execution in the context of the SFS secure distributed file system. REX departs from traditional remote login design and is built around two main mechanisms---file descriptor passing and a user agent process. File descriptor passing allows REX to be split into several smaller pieces; privileged code can run as its own process to provide enhanced security guarantees. REX also emulates secure file descriptor passing over network connections, allowing users to build extensions to REX outside of the core REX software. REX uses and extends SFS's agent mechanism to provide a transparent distributed computing environment to users. The agent stores private keys, server nicknames, and other per-user configuration state; REX makes the SFS agent available to programs that it executes on remote machines. We have an implementation of REX and demonstrate that its flexibility does not come at the cost of performance. Initial REX connections are comparable to those of SSH in speed, while subsequent connections are much faster because REX exploits the SFS agent to cache connection state to avoid costly public-key operations.

The AEGIS Processor Architecture for Tamper-Evident and Private Tamper-Resistant Processing

Wed, 01 Jan 2003 00:00:00 GMT

The AEGIS Processor Architecture for Tamper-Evident and Private Tamper-Resistant Processing Suh, G. Edward; Clarke, Dwaine; Gassend, Blaise; van Dijk, Marten; Devadas, Srinivas We describe the architecture of the AEGIS processor which can be used to build computing systems secure against both physical and software attacks. AEGIS assumes that the operating system and all components external to it, such as memory, are untrusted. AEGIS provides tamper-evident, authenticated environments in which any physical or software tampering by the adversary is guaranteed to be detected, and private and authenticated, tamper-resistant environments where additionally the adversary is unable to obtain any information about software or data by tampering with, or otherwise observing, system operation. AEGIS enables many applications, such as commercial grid computing, software licensing, and digital rights management. We present a new encryption/decryption method that successfully hides a significant portion of encryption/decryption latency, in comparison to a conventional direct encryption scheme. Efficient memory encryption and integrity verification enable the implementation of a secure computing system with the only trusted component being a single-chip AEGIS CPU. Detailed simulation results indicate that the performance overhead of security mechanisms in AEGIS is reasonable.

Efficient Web Browsing for Mobile Clients using HTTP Compression

Wed, 01 Jan 2003 00:00:00 GMT

Efficient Web Browsing for Mobile Clients using HTTP Compression Krashinsky, Ronny Efficient web browsing on mobile computers presents a unique challenge. These machines are different from other classes of client computers since they have relatively low-bandwidth connections and they are battery-powered and therefore limited by their energy consumption. However, they tend to interact with the same servers for the delivery of web content. This project investigates optimizing the final critical link between a mobile client and a stationary base station by compressing HTTP request and response messages. Using a split proxy design, compression of individual request messages reduces bandwidth by 26% to 34% across a variety of benchmark traces, and applying compression to response messages yields savings of 59% to 82% of the compressible data. Higher compression rates are achieved by using streaming compression algorithms to compress the streams of request and response messages. In this case, the bandwidth for requests sees an order of magnitude improvement, and the response stream obtains additional savings of 7% to 25% on top of the savings achieved with per-response compression.

Physical Random Functions

Sat, 01 Feb 2003 00:00:00 GMT

Physical Random Functions Gassend, Blaise In general, secure protocols assume that participants are able to maintain secret key information. In practice, this assumption is often incorrect as an increasing number of devices are vulnerable to physical attacks. Typical examples of vulnerable devices are smartcards and Automated Teller Machines. To address this issue, Physical Random Functions are introduced. These are Random Functions that are physically tied to a particular device. To show that Physical Random Functions solve the initial problem, it must be shown that they can be made, and that it is possible to use them to provide secret keys for higher level protocols. Experiments with Field Programmable Gate Arrays are used to evaluate the feasibility of Physical Random Functions in silicon.

On the Theory of Structural Subtyping

Wed, 01 Jan 2003 00:00:00 GMT

On the Theory of Structural Subtyping Kuncak, Viktor; Rinard, Martin We show that the first-order theory of structural subtyping of non-recursive types is decidable. Let Sigma be a language consisting of function symbols (representing type constructors) and C a decidable structure in the relational language L containing a binary relation <. C represents primitive types; < represents a subtype ordering. We introduce the notion of Sigma-term-power of C, which generalizes the structure arising in structural subtyping. The domain of the Sigma-term-power of C is the set of Sigma-terms over the set of elements of C. We show that the decidability of the first-order theory of C implies the decidability of the first-order theory of the Sigma-term-power of C. This result implies the decidability of the first-order theory of structural subtyping of non-recursive types. Our decision procedure is based on quantifier elimination and makes use of quantifier elimination for term algebras and Feferman-Vaught construction for products of decidable structures. We also explore connections between the theory of structural subtyping of recursive types and monadic second-order theory of tree-like structures. In particular, we give an embedding of the monadic second-order theory of infinite binary tree into the first-order theory of structural subtyping of recursive types.

Boosting Fault-Tolerance in Asynchronous Message Passing Systems is Impossible

Sun, 01 Dec 2002 00:00:00 GMT

Boosting Fault-Tolerance in Asynchronous Message Passing Systems is Impossible Attie, Paul C.; Lynch, Nancy A.; Rajsbaum, Sergio

Economic Mechanisms for Efficient Wireless Coexistence

Thu, 01 Aug 2002 00:00:00 GMT

Economic Mechanisms for Efficient Wireless Coexistence Aftab, Omar

Automatic Detection and Repair of Errors in Data Structures

Sun, 01 Dec 2002 00:00:00 GMT

Automatic Detection and Repair of Errors in Data Structures Demsky, Brian; Rinard, Martin We present a system that accepts a specification of key data structure constraints, then dynamically detects and repairs violations of these constraints. Our experience using our system indicates that the specifications are relatively easy to develop once one understands the data structures. Furthermore, for our set of benchmark applications, our system can effectively repair errors to deliver consistent data structures that allow the program to continue to operate successfully within its designed operating envelope.

A Type System and Analysis for the Automatic Extraction and Enforcement of Design Information

Sun, 01 Dec 2002 00:00:00 GMT

A Type System and Analysis for the Automatic Extraction and Enforcement of Design Information Lam, Patrick; Rinard, Martin We present a new type system and associated type checker, analysis, and model extraction algorithms for automatically extracting models that capture aspects of the design of the program. Our type system enables the developer to place a _token_ on each object; this token serves as the object's representative during the analysis and model extraction. The polymorphism in our type system enables the use of general-purpose classes whose instances may serve different purposes in the computation; programmers may also hide the details of internal data structures by placing the same token on all of the objects in these data structures. Our combined type system and analysis provide the model extraction algorithms with sound heap aliasing information. Our algorithms can therefore extract both structural models that characterize object referencing relationships and behavioral models that capture indirect interactions mediated by objects in the heap. Previous approaches, in contrast, in the absence of aliasing information, have focused on control-flow interactions that take place at procedure call boundaries. We have implemented our type checker, analysis, and model extraction algorithms and used them to produce design models. Our experience indicates that it is straightforward to produce the token annotations and that the extracted models provide useful insight into the structure and behavior of the program.

A Dynamic Primary View Group Communication Service

Fri, 01 Nov 2002 00:00:00 GMT

A Dynamic Primary View Group Communication Service De Prisco, Roberto; Fekete, Alan; Lynch, Nancy A.; Shvartsman, Alexander A.

Hardware Mechanisms for Memory Integrity Checking

Fri, 01 Nov 2002 00:00:00 GMT

Hardware Mechanisms for Memory Integrity Checking Suh, G. Edward; Clarke, Dwaine; Gassend, Blaise; van Dijk, Marten; Devadas, Srinivas Memory integrity verification is a useful primitive when implementing secure processors that are resistant to attacks on hardware components. This paper proposes new hardware schemes to verify the integrity of untrusted external memory using a very small amount of trusted on-chip storage. Our schemes maintain incremental multiset hashes of all memory reads and writes at run-time, and can verify a {\\em sequence} of memory operations at a later time. We study the advantages and disadvantages of the two new schemes and two existing integrity checking schemes, MACs and hash trees, when implemented in hardware in a microprocessor. Simulations show that the new schemes outperform existing schemes of equivalent functionality when integrity verification is infrequent.

Offline Integrity Checking of Untrusted Storage

Fri, 01 Nov 2002 00:00:00 GMT

Offline Integrity Checking of Untrusted Storage Clarke, Dwaine; Gassend, Blaise; Suh, G. Edward; van Dijk, Marten; Devadas, Srinivas

Access-Controlled Resource Discovery for Pervasive Networks

Sun, 01 Sep 2002 00:00:00 GMT

Access-Controlled Resource Discovery for Pervasive Networks Raman, Sanjay; Clarke, Dwaine; Burnside, Matt; Devadas, Srinivas; Rivest, Ronald L. Networks of the future will be characterized by a variety of computational devices that display a level of dynamism not seen in traditional wired networks. Because of the dynamic nature of these networks, resource discovery is one of the fundamental problems that must be faced. While resource discovery systems are not a novel concept, securing these systems in an efficient and scalable way is challenging. This paper describes the design and implementation of an architecture for access-controlled resource discovery. This system achieves this goal by integrating access control with the Intentional Naming System (INS), a resource discovery and service location system. The integration is scalable, efficient, and fits well within a proxy-based security framework designed for dynamic networks. We provide performance experiments that show how our solution outperforms existing schemes. The result is a system that provides secure, access-controlled resource discovery that can scale to large numbers of resources and users

A Type System for Safe Region-Based Memory Management in Real-Time Java

Fri, 01 Nov 2002 00:00:00 GMT

A Type System for Safe Region-Based Memory Management in Real-Time Java Salcianu, Alexandru; Boyapati, Chandrasekhar; Beebee, William S., Jr.; Rinard, Martin The Real-Time Specification for Java (RTSJ) allows a program to create real-time threads with hard real time constraints. Real-time threads use immortal memory and region-based memory management to avoid unbounded pauses caused by interference from the garbage collector. The RTSJ uses runtime checks to ensure that deleting a region does not create dangling references and that real-time threads do not access references to objects allocated in the garbage-collected heap. This paper presents a static type system that guarantees that these runtime checks will never fail for well-typed programs. Our type system therefore 1) provides an important safety guarantee for real-time programs and 2) makes it possible to eliminate the runtime checks and their associated overhead. Our system also makes several contributions over previous work on region types. For object-oriented programs, it combines region types and ownership types in a unified type system framework. For multithreaded programs, it allows long-lived threads to share objects without using the heap and without having memory leaks. For real-time programs, it ensures that real-time threads do not interfere with the garbage collector. We have implemented several programs in our system. Our experience indicates that our type system is sufficiently expressive and requires little programming overhead. We also ran these programs on our RTSJ platform. Our experiments show that eliminating the RTSJ runtime checks using a static type system can significantly decrease the execution time of a real-time program.

A Formal Venture into Reliable Multicast Territory

Fri, 01 Nov 2002 00:00:00 GMT

A Formal Venture into Reliable Multicast Territory Livadas, Carolos; Lynch, Nancy A.

The Case for Exploiting Packet Loss Locality in Multicast Loss Recovery

Tue, 01 Oct 2002 00:00:00 GMT

The Case for Exploiting Packet Loss Locality in Multicast Loss Recovery Livadas, Carolos; Keidar, Idit

Blueware: Bluetooth Simulator for ns

Tue, 01 Oct 2002 00:00:00 GMT

Blueware: Bluetooth Simulator for ns Tan, Godfrey

Tetris is Hard, Even to Approximate

Tue, 01 Oct 2002 00:00:00 GMT

Tetris is Hard, Even to Approximate Demaine, Erik D.; Hohenberger, Susan; Liben-Nowell, David

The Facility Location Problem with Concave Cost Functions

Sun, 01 Sep 2002 00:00:00 GMT

The Facility Location Problem with Concave Cost Functions Hajiaghayi, Mohammad Taghi; Mahdian, Mohammad; Mirrokni, Vahab S.

Scalar Operand Networks: On-chip interconnect for ILP in Partitioned Architechures

Mon, 01 Jul 2002 00:00:00 GMT

Scalar Operand Networks: On-chip interconnect for ILP in Partitioned Architechures Taylor, Michael Bedford; Lee, Walter; Amarasinghe, Saman; Agarwal, Anant

Ownership Types and Safe Lazy Upgrades in Object-Oriented Databases

Mon, 01 Jul 2002 00:00:00 GMT

Ownership Types and Safe Lazy Upgrades in Object-Oriented Databases Boyapati, Chandrasekhar; Liskov, Barbara H.; Shrira, Liuba

Caches and Merkle Trees for Efficient Memory Authentication

Mon, 01 Jul 2002 00:00:00 GMT

Caches and Merkle Trees for Efficient Memory Authentication Gassend, Blaise; Suh, G. Edward; Clarke, Dwaine; van Dijk, Marten; Devadas, Srinivas

Delay-Based Circuit Authentication With Application to Key Cards

Sat, 01 Jun 2002 00:00:00 GMT

Delay-Based Circuit Authentication With Application to Key Cards Gassend, Blaise; Clarke, Dwaine; van Dijk, Marten; Devadas, Srinivas

Safe Runtime Downcasts With Ownership Types

Sat, 01 Jun 2002 00:00:00 GMT

Safe Runtime Downcasts With Ownership Types Boyapati, Chandrasekhar; Lee, Robert; Rinard, Martin

Automatic Generation and Checking of Program Specifications

Sat, 01 Jun 2002 00:00:00 GMT

Automatic Generation and Checking of Program Specifications Nimmer, Jeremy

Safe Lazy Software Upgrades in Object-Oriented Databases

Sat, 01 Jun 2002 00:00:00 GMT

Safe Lazy Software Upgrades in Object-Oriented Databases Liskov, Barbara H.; Moh, Chuang-Hue; Richman, Steven; Shrira, Liuba; Chueng, Yin; Boyapati, Chandrasekhar

Combining Abstraction with Byzantine Fault-Tolerance

Tue, 01 May 2001 00:00:00 GMT

Combining Abstraction with Byzantine Fault-Tolerance Rodrigues, Rodrigo

A Scalable Byzantine Fault Tolerant Secure Domain Name Service

Mon, 01 Jan 2001 00:00:00 GMT

A Scalable Byzantine Fault Tolerant Secure Domain Name Service Ahmed, Sarah

Improving Test Suites via Generated Specifications

Sat, 01 Jun 2002 00:00:00 GMT

Improving Test Suites via Generated Specifications Harder, Michael

A Trusted Third-Party Computation Service

Tue, 01 May 2001 00:00:00 GMT

A Trusted Third-Party Computation Service Ajmani, Sameer; Morris, Robert T.; Liskov, Barbara H.

A Trusted Execution Platform for Multiparty Computation

Fri, 01 Sep 2000 00:00:00 GMT

A Trusted Execution Platform for Multiparty Computation Ajmani, Sameer

Controlled Physical Unknown Functions: Applications to Secure Smartcards and Certified Execution

Sat, 01 Jun 2002 00:00:00 GMT

Controlled Physical Unknown Functions: Applications to Secure Smartcards and Certified Execution Gassend, Blaise; Clarke, Dwaine; van Dijk, Marten; Devadas, Srinivas

A Double-Pulsed Set-Conditional-Reset Flip-Flop

Wed, 01 May 2002 00:00:00 GMT

A Double-Pulsed Set-Conditional-Reset Flip-Flop Ma, Albert; Asanovi_, Krste

The IOA Simulator

Mon, 01 Jul 2002 00:00:00 GMT

The IOA Simulator Kaynar, Dilsun Kirh; Chefter, Anna; Dean, Laura; Garland, Stephen J.; Lynch, Nancy A.; Ne Win, Toh; Ramírez-Robredo, Antonio

Towards An Extensible Virtual Machine

Mon, 01 Apr 2002 00:00:00 GMT

Towards An Extensible Virtual Machine Boyapati, Chandrasekhar

Verifying Distributed Algorithms via Dynamic Analysis and Theorem Proving

Wed, 01 May 2002 00:00:00 GMT

Verifying Distributed Algorithms via Dynamic Analysis and Theorem Proving Ne Win, Toh; Ernst, Michael D.

Early-Delivery Dynamic Atomic Broadcast

Mon, 01 Apr 2002 00:00:00 GMT

Early-Delivery Dynamic Atomic Broadcast Bar-Joseph, Ziv; Keidar, Idit; Lynch, Nancy A.

A Type System for Preventing Data Races and Deadlocks in Java Programs

Fri, 01 Mar 2002 00:00:00 GMT

A Type System for Preventing Data Races and Deadlocks in Java Programs Boyapati, Chandrasekhar; Lee, Robert; Rinard, Martin

Exponential Speedup of Fixed Parameter Algorithms K_{3,3}-minor-free or K_5-minor-free Graphs

Fri, 01 Mar 2002 00:00:00 GMT

Exponential Speedup of Fixed Parameter Algorithms K_{3,3}-minor-free or K_5-minor-free Graphs Demaine, Erik D.; Hajiaghayi, Mohammad Taghi; Thilikos, Dimitrios M.

L+: Scalable Landmark Routing and Address Lookup for Multi-hop Wireless Networks

Fri, 01 Mar 2002 00:00:00 GMT

L+: Scalable Landmark Routing and Address Lookup for Multi-hop Wireless Networks Chen, Benjie; Morris, Robert T.

Effects of Loss Rate on Ad Hoc Wireless Routing

Fri, 01 Mar 2002 00:00:00 GMT

Effects of Loss Rate on Ad Hoc Wireless Routing DeCouto, Douglas S.J.; Aguayo, Daniel; Chambers, Benjamin A.; Morris, Robert This paper uses measurements from two deployed wireless ad hoc networks to illustrate the effects of link loss rates on routing protocol performance. Measurements of these networks show that the radio links between the majority of nodes have substantial loss rates. These loss rates are high enough to prevent existing ad hoc routing protocols from using the links. Link-level retransmission can mask high loss rates, at the cost of substantial decreases in throughput. Simulations, driven by the observed loss rates, show that the shortest paths chosen by existing routing protocols tend to find routes with much less capacity than is available along the best route. Based on these observations, we present a routing metric intended to allow routing protocols to find good routes in wireless ad hoc networks. The metric is the expected total number of transmissions required to deliver a packet along a route. This metric favors routes with high throughput and low total impact on spectrum. It is expected to perform better than existing techniques that eliminate links based on loss rate thresholds.

Write Barrier Removal by Static Analysis

Fri, 01 Feb 2002 00:00:00 GMT

Write Barrier Removal by Static Analysis Zee, Karen; Rinard, Martin

Silicon Physical Unknown Functions and Secure Smartcards

Wed, 01 May 2002 00:00:00 GMT

Silicon Physical Unknown Functions and Secure Smartcards Gassend, Blaise; Clarke, Dwaine; van Dijk, Marten; Devadas, Srinivas

Fine-Grain Dynamic Leakage Reduction

Tue, 01 Jan 2002 00:00:00 GMT

Fine-Grain Dynamic Leakage Reduction Heo, Seongmoo; Barr, Kenneth; Hampton, Mark; Asanovi_, Krste

Leakage-Biased Domino Circuits for Dynamic Fine-Grain Leakage Reduction

Tue, 01 Jan 2002 00:00:00 GMT

Leakage-Biased Domino Circuits for Dynamic Fine-Grain Leakage Reduction Heo, Seongmoo; Asanovi_, Krste

An Information-Theoretic Approach to Interest Making

Tue, 01 May 2001 00:00:00 GMT

An Information-Theoretic Approach to Interest Making Koh, Waikit The Internet has brought a new meaning to the term communities. Geography is no longer a barrier to international communications. However, the paradigm of meeting new interesting people remains entrenched in traditional means; meeting new interesting people on the Internet still relies on chance and contacts. This thesis explores a new approach towards matching users in online communities in an effective fashion. Instead of using the conventional feature vector scheme to profile users, each user is represented by a personalized concept hierarchy (or an ontology) that is learnt from the user's behavior in the system. Each concept hierarchy is then interpreted within the Information Theory framework as a probabilistic decision tree. The matching algorithm uses the Kullback-Leiber distance as a measure of deviation between two probabilistic decision trees. Thus, in an online community, where a personalized concept hierarchy represents each user, the Kullback-Leiber distance imposes a full- order rank on the level of similarity of all the users with respect to a particular user in question. The validity and utility of the proposed scheme of matching users is then applied in a set of simulations, using the feature-vector-overlap measure as a baseline. The results of the simulations show that the Kullback Leiber distance, when used in conjunction with the concept hierarchy, is more robust to noise and is able to make a stronger and more distinctive classification of users into similar groups in comparison to the conventional keyword-overlap scheme. A graphical agent system that relies upon the ontology-based interest matching algorithm, called the Collaborative Sanctioning Network, is also described in this thesis.

2RegionRED: a Congestion Control Mechanism for the High Speed Internet

Sat, 01 Dec 2001 00:00:00 GMT

2RegionRED: a Congestion Control Mechanism for the High Speed Internet Wang, Karen This thesis proposes a new Active Queue Management (AQM) scheme called 2RegionRED. It is superior to the classic Random Early Detection (RED) algorithm in that there is an intuitive way to set its parameters and it is self-tuning. Its design is motivated by an original principle to sustain the smallest queue possible while still allowing for maximum link utilization. 2RegionRED uses the number of competing TCPs as its measure of load. However it does not keep an explicit count. The result is a novel algorithm that adjusts the drop rate according to two regions of operation: that requiring less than and greater than one drop per round-trip time (RTT). This thesis also analyzes methods for measuring the persistent queue and proposes the ABSMIN method. Simulations of 2RegionRED using ABSMIN reveal some difficulties and insights. Basic comparisons to the Adaptive RED and Flow Proportional Queuing (FPQ) adaptive algorithms are also demonstrated through simulation

Inferring Congestion Sharing and Path Characteristics from Packet Interarrival Times

Sat, 01 Dec 2001 00:00:00 GMT

Inferring Congestion Sharing and Path Characteristics from Packet Interarrival Times Katabi, Dina; Blake, Charles

Hybrid I/O Automata*

Sat, 01 Sep 2001 00:00:00 GMT

Forming Scatternets from Bluetooth Personal Area Networks

Sat, 01 Sep 2001 00:00:00 GMT

Forming Scatternets from Bluetooth Personal Area Networks Tan, Godfrey; Mui, Allen; Guttag, John V.; Balakrishnan, Hari There is increasing interest in wireless ad hoc networks built from portable devices equipped with short-range wireless network interfaces. This paper addresses issues related to internetworking such networks to form larger ÔøΩscatternets.ÔøΩ Within the constraints imposed by the emerging standard Bluetooth link layer and MAC protocol, we describe an efficient online topology formation algorithm, called TSF (Tree Scatternet Formation) to build scatternets. TSF connects nodes in a tree structure that simplifies packet routing and scheduling. The design allows nodes to arrive and leave arbitrarily, incrementally building the topology and healing partitions when they occur. We present simulation results that show that TSF has low tree formation latency and also generates an efficient topology for forwarding packets.

Scalable, Controlled Imagery Capture in Urban Environments

Sat, 01 Sep 2001 00:00:00 GMT

Scalable, Controlled Imagery Capture in Urban Environments Teller, Seth We describe the design considerations underlying a system for scalable, automated capture of precisely controlled imagery in urban scenes. The system operates for architectural scenes in which, from every camera position, some two vanishing points are visible. It has been used to capture thousands of controlled images in outdoor environments spanning hundreds of meters. The proposed system architecture forms the foundation for a future, fully robotic outdoor mapping capability for urban areas, analogous to existing, satellite-based robotic mapping systems which acquire images and models of natural terrain. Four key ideas distinguish our approach from other methods. First, our sensor acquires georeferencing metadata with every image, enabling related images to be efficiently identified and registered. Second, the sensor acquires omni-directional images; we show strong experimental evidence that such images are fundamentally more powerful observations than conventional (narrow-FOV) images. Third, the system uses a probabilistic, projective error formulation to account for uncertainty. By treating measurement error in an appropriate depth-free framework, and by deferring decisions about camera calibration and scene structure until many noisy observations can be fused, the system achieves superior robustness and accuracy. Fourth, the system's computational requirements scale linearly in the input size, the area of the acquisition region, and the size of the output model. This is in contrast to most previous methods, which either assume constant-size inputs or exhibit quadratic running time (or worse) asymptotically. These attributes enable the system to operate in a regime of scale and physical extent which is unachievable by any other method, whether manual or automated. Consequently, it can acquire the most complex calibrated terrestrial image sets in existence, while operating faster thanany existing manual or algorithmic method.

Location Proxies and Intermediate Node Forwarding for Practical Geographic Forwarding

Tue, 01 May 2001 00:00:00 GMT

Location Proxies and Intermediate Node Forwarding for Practical Geographic Forwarding De Couto, Douglas S.J.; Morris, Robert T.

Automatic Generation and Checking of Program Specifications

Automatic Generation and Checking of Program Specifications Nimmer, Jeremy W.; Ernst, Michael D.

Roles Are Really Great!

Wed, 01 Aug 2001 00:00:00 GMT

Roles Are Really Great! Kuncak, Viktor; Lam, Patrick; Rinard, Martin

On the Cost of Fault-Tolerant Consensus When There Are No Faults - A Tutorial

Tue, 01 May 2001 00:00:00 GMT

On the Cost of Fault-Tolerant Consensus When There Are No Faults - A Tutorial Keidar, Idit; Rajsbaum, Sergio

Using precise feedback for controlling congestion in the Internet

Tue, 01 May 2001 00:00:00 GMT

Using precise feedback for controlling congestion in the Internet Katabi, Dina; Handley, Mark

Chord: A scalable peer-to-peer lookup service for Internet applications

Thu, 01 Mar 2001 00:00:00 GMT

Chord: A scalable peer-to-peer lookup service for Internet applications Stoica, Ion; Morris, Robert T.; Karger, David R.; Kaashoek, M. Frans; Balakrishnan, Hari

Client Authentication on the Web

Thu, 01 Mar 2001 00:00:00 GMT

Client Authentication on the Web Fu, Kevin; Sit, Emil; Smith, Kendra; Feamster, Nick

Practical Byzantine Fault Tolerance

Mon, 01 Jan 2001 00:00:00 GMT

Practical Byzantine Fault Tolerance Castro, Miguel Our growing reliance on online services accessible on the Internet demands highly-available systemsthat provide correct service without interruptions. Byzantine faults such as software bugs, operatormistakes, and malicious attacks are the major cause of service interruptions. This thesis describesa new replication algorithm, BFT, that can be used to build highly-available systems that tolerateByzantine faults. It shows, for the first time, how to build Byzantine-fault-tolerant systems that canbe used in practice to implement real services because they do not rely on unrealistic assumptionsand they perform well. BFT works in asynchronous environments like the Internet, it incorporatesmechanisms to defend against Byzantine-faulty clients, and it recovers replicas proactively. Therecovery mechanism allows the algorithm to tolerate any number of faults over the lifetime of thesystem provided fewer than 1=3 of the replicas become faulty within a small windowof vulnerability.The window may increase under a denial-of-service attack but the algorithm can detect and respondto such attacks and it can also detect when the state of a replica is corrupted by an attacker.BFT has been implemented as a generic program library with a simple interface. The BFTlibrary provides a complete solution to the problem of building real services that tolerate Byzantinefaults. We used the library to implement the first Byzantine-fault-tolerant NFS file system, BFS. TheBFT library and BFS perform well because the library incorporates several important optimizations.The most important optimization is the use of symmetric cryptography to authenticate messages.Public-key cryptography, which was the major bottleneck in previous systems, is used only toexchange the symmetric keys. The performance results show that BFS performs 2% faster to 24%slower than production implementations of the NFS protocol that are not replicated. Therefore, webelieve that the BFT library can be used to build practical systems that tolerate Byzantine faults.

Object Models, Heaps and Interpretations

Mon, 01 Jan 2001 00:00:00 GMT

Object Models, Heaps and Interpretations Rinard, Martin; Kuncak, Viktor This paper explores the use of object models for specifying verifiable heap invariants. We define a simple language based on sets and relations and illustrate its use through examples. We give formal semantics of the laguage by translation into predicate calculus and interpretation of predicates in terms of objects and references in the program heap.

Perspectives on the Use of the Internet in Sri Lanka

Wed, 01 Nov 2000 00:00:00 GMT

Perspectives on the Use of the Internet in Sri Lanka Shrestha, Govinda; Amarasinghe, Saman The survey examines the use of computers and the Internet in Sri Lanka from the perspective of the Internet Service Provider (ISP) members. It attempts to describe the general nature of IT use in terms of the availability, access, familiarity and general conditions associated with using computers and the Internet in the country. The survey was conducted in July 1999. Questionnaires were e-mailed to 9448 ISP members in Sri Lanka, using e-mail addresses available to us at that time. Altogether, 560 members completed and returned questionnaires via e-mail to MIT's Laboratory for Computer Science. Descriptive analysis of both quantitative and qualitative data was then conducted. Major quantitative findings include: *Over 60% of the respondents were members of their respective ISPs for two or less years, and over half had first used a computer sometime during the 1990-99 period. *Sixty-two percent of the respondents had sent 10 or more e-mails per week over the past six (or less) months, and 52% had received 15 or more e-mails per week during the same period. *Nearly half of the respondents used a computer at home, and 48% indicated 33.6K as the baud rate to connect their ISPs. *Seventy-eight percent of the respondents spent 1-9 hours per week sending and receiving e-mails, and a large majority (68%) spent 1-9 hours surfing the Web. *A majority of the respondents were positive about conditions in the workplace, such as the number and quality of opportunities for training and skill development, the quality of telecommunications facilities, and the quality and reliability of Internet connections. *An overwhelming majority of the respondents indicated that ISP subscriber fees, computer hardware and software costs, and telecommunications charges were generally high. *Most respondents were generally positive about 1) the quality of access to the Internet, 2) the quality of access to e-mails, Web pages and other Internet-based features, and 3) various benefits of Internet access. *Seventy-one percent of the respondents were male; nearly half were younger than 35, and a large majority were educated (with at least a high school diploma.) Private company employees and people in business comprised over half of the respondents. Major qualitative findings include: * It is crucially important to have faster access to information, increased communication at low costs, online-education and training, and increased efficiency in business, professional and organizational activities. * Matters of considerable concern include the low bandwidth, the high telecommunications charges, the low quality of Internet services, and the lack of organized information and databases. * Greatly needed is a raising of awareness, a change in the current regulatory environment, an open government, and a set of local information resources to support commerce.

Automatic Recovery of Camera Positions in Urban Scenes

Fri, 01 Dec 2000 00:00:00 GMT

Automatic Recovery of Camera Positions in Urban Scenes Antone, Matthew E.; Teller, Seth Accurate camera calibration is crucial to the reconstruction of three-dimensional geometry and the recovery of photometric scene properties. Calibration involves the determination of intrinsic parameters (e.g. focal length, principal point, and radial lens distortion) and extrinsic parameters (orientation and position). In urban scenes and other environments containing sufficient geometric structure, it is possible to decouple extrinsic calibration into rotational and translational components that can be treated separately, simplifying the registration problem. Here we present such a decoupled formulation and describe methods for automatically recovering the positions of a large set of cameras given intrinsic calibration, relative rotations, and approximate positions. Our algorithm first estimates the directions of translation (up to an unknown scale factor) between adjacent camera pairs using point features but without requiring explicit correspondence between them. This technique combines the robustness and simplicity of a Hough transform with the accuracy of Monte Carlo expectation maximization. We then find a set of distances between the pairs that produces globally-consistent camera positions. Novel uncertainty formulations and match plausibility criteria improve reliability and accuracy. We assess our system's performance using both synthetic data and a large set of real panoramic imagery. The system produces camera positions accurate to within 5 centimeters in image networks extending over hundreds of meters.

Fine-Grained Failover Using Connection Migration

Wed, 01 Nov 2000 00:00:00 GMT

Fine-Grained Failover Using Connection Migration Snoeren, Alex C.; Andersen, David G.; Balakrishnan, Hari This paper presents a set of techniques for providing fine-grained failover of long-running connections across a distributed collection of replica servers, and is especially useful for fault-tolerant and load-balanced delivery of streaming media and telephony sessions. Our system achieves connection-level failover across both local- and wide-area server replication, without requiring a front-end transport- or application-layer switch. Our approach is enabled by the recently-developed end-to-end ``connection migration'' mechanism for transport protocols such as TCP, combined with a soft-state session synchronization protocol between replica servers. The end result is a robust, fast, and fine-grained server failover mechanism that is transparent to both the client and server applications. We describe the details of our design and Linux implementation, as well as experiments with our implementation that show that this approach to failover is an attractive way to engineer robust systems for distributing long-running streams; connections suffer relatively low performance degradation even when server redirection occurs every few seconds, and overhead is negligible when compared to standard techniques. In particular, we observe the performance impact of migrating TCP connections depends on the length of time between migration and the most recent loss-recovery event.

Programming Language Techniques for Modular Router Configurations

Tue, 01 Aug 2000 00:00:00 GMT

Programming Language Techniques for Modular Router Configurations Kohler, Eddie; Chen, Benjie; Kaashoek, M. Frans; Morris, Robert T.; Poletto, Massimiliano This paper applies programming language techniques to a high-level system description, both to optimize the system and to prove useful properties about it. The system in question is Click, a modular software router framework. Click routers are built from components called elements. Elements are written in C++, but the user creates a configuration using a simple, declarative data flow language. This language is amenable to data flow analysis and other conventional programming language techniques. Applied to a router configuration, these techniques have high-level results---for example, optimizing the router or verifying its high-level properties. This paper describes several programming language techniques that have been useful in practice, including optimization tools that remove virtual function calls from router definitions and remove redundant parts of adjacent routers. We also present performance results for an extensively optimized standards-compliant IP router. On conventional PC hardware, this router can forward up to 456,000 64-byte packets per second.

An Efficient Boosting Algorithm for Combining Preferences

Sun, 01 Aug 1999 00:00:00 GMT

An Efficient Boosting Algorithm for Combining Preferences Iyer, Raj Dharmarajan, Jr. The problem of combining preferences arises in several applications, such as combining the results of di_x000B_erent search engines. This work describes an effcient algorithm for combining multiple preferences. We _x000C_rst give a formal framework for the problem. We then describe and analyze a new boosting algorithm for combining preferences called RankBoost. We also describe an effcient implementation of the algorithm for certain natural cases. We discuss two experiments we carried out to assess the performance of RankBoost. In the _x000C_rst experiment, we used the algorithm to combine di_x000B_erent WWW search strategies, each of which is a queryexpansion for a given domain. For this task, we compare the performance of RankBoost to the individual search strategies. The second experiment is a collaborative-filtering task for making movie recommendations. Here, we present results comparing RankBoost to nearest-neighbor and regression algorithms.

TrendFinder: Automated Detection of Alarmable Trends

Sat, 01 Jan 2000 00:00:00 GMT

TrendFinder: Automated Detection of Alarmable Trends Tsien, Christine L.

System Support for Bandwidth Management and Content Adaptation in Internet Applications

Mon, 01 May 2000 00:00:00 GMT

System Support for Bandwidth Management and Content Adaptation in Internet Applications Andersen, David; Bansal, Deccuk; Curtis, Dorothy; Seshan, Srinivasan; Balakrishnan, Hari

An Analysis of Short-Term Fairness in Wireless Media Access Protocols

An Analysis of Short-Term Fairness in Wireless Media Access Protocols Koksal, Can Emre; Kassab, Hisham; Balakrishnan, Hari We investigate the problem of unfairness over short time scales in decentralized wireless media access (MAC) protocols. Motivated by experimental results over a CSMA/CA-based WaveLAN wireless LAN that shows starvation and degraded TCP performance, we see

TCP-friendly Congestion Control for Real-time Streaming Applications

Mon, 01 May 2000 00:00:00 GMT

TCP-friendly Congestion Control for Real-time Streaming Applications Bansal, Deepak; Balakrishnan, Hari This paper introduces and analyzes a class of nonlinear congestion control algorithms called binomial algorithms, motivated in part by the needs of streaming audio and video applications for which a drastic reduction in transmission rate upon congestion i

On Building Blocks for Distributed Systems

Wed, 01 Dec 1999 00:00:00 GMT

On Building Blocks for Distributed Systems De Prisco, Robert

The Static Single Information Form

Wed, 01 Sep 1999 00:00:00 GMT

The Static Single Information Form Ananian, C. Scott The Static Single Information (SSI) form is a compiler intermediate representation that allows efficient sparse implementations of predicated analysis and backward dataflow algorithms. It possesses several attractive graph-theoretic properties which aid

Maps: A Compiler-Managed Memory System for Software-Exposed Architectures

Sat, 01 Jan 2000 00:00:00 GMT

Maps: A Compiler-Managed Memory System for Software-Exposed Architectures Barua, Rajeev Microprocessors must exploit both instruction-level parallelism (ILP) and memory parallelism for high performance. Sophisticated techniques for ILP have boosted the ability of modern-day microprocessors to exploit ILP when available. Unfortunately, impro

QoS Preserving Totally Ordered Multicast

Sat, 01 Jan 2000 00:00:00 GMT

QoS Preserving Totally Ordered Multicast Bar-Joseph, Ziv; Keidar, Idit; Anker, Tal; Lynch, Nancy A. This paper studies the Quality of Service (QoS) guarantees of totally ordered multicast algorithms. The paper shows that totally ordered multicast can coexist with guaranteed predictable delays in certain network models. The paper considers two reservatio

Compositional Pointer and Escape Analysis for Multithreaded Java Programs

Mon, 01 Nov 1999 00:00:00 GMT

Compositional Pointer and Escape Analysis for Multithreaded Java Programs Rinard, Martin; Whaley, John

A Client-Server Approach to Virtually Synchronous Group Multicast: Specifications, Algorithms, and Proofs

A Client-Server Approach to Virtually Synchronous Group Multicast: Specifications, Algorithms, and Proofs Keidar, Idit; Khazan, Roger This paper presents a formal design for a novel group multicast service that provides virtually synchronous semantics in asynchronous fault-prone environments. The design employs a client-server architecture in which group membership is maintained not by

Information Technology Use in Developing Countries

Sat, 01 Jul 2000 00:00:00 GMT

Information Technology Use in Developing Countries Shrestha, Govinda

Group Communication Specifications: A Comprehensive Study

Wed, 01 Sep 1999 00:00:00 GMT

Group Communication Specifications: A Comprehensive Study Vitenberg, Roman; Keidar, Idit; Chockler, Gregory V.; Dolev, Danny View-oriented group communication is an important and widely used building block for many distributed applications. Much current research has been dedicated to specifying the semantics and services of view-oriented Group Communication Systems (GCSs). Howe

I/O Automaton Models and Proofs for Shared-Key Communication Systems

Sun, 01 Aug 1999 00:00:00 GMT

I/O Automaton Models and Proofs for Shared-Key Communication Systems Lynch, Nancy A. The combination of two security protocols, a simple shared-key communication protocol and the Diffie-Hellman key distribution protocol, is modeled formally and proved correct. The modeling is based on the I/O automaton model for distributed algorithms, an

Natural Selection and Loop Analysis

Natural Selection and Loop Analysis Mohtashemi, Mojdeh

Weak Consistency: A Generalized Theory and Optimistic Implementations for Distributed Transactions

Mon, 01 Mar 1999 00:00:00 GMT

Weak Consistency: A Generalized Theory and Optimistic Implementations for Distributed Transactions Adya, Atul Current commercial databases allow application programmers to trade off consistency for performance. However, existing definitions of weak consistency levels are either imprecise or they disallow efficient implementation techniques such as optimism. Rul

Portable High-Performance Programs

Tue, 01 Jun 1999 00:00:00 GMT

Portable High-Performance Programs Frigo, Matteo This dissertation discusses how to write computer programs that attain both high performance and portability, despite the fact that current computer systems have different degrees of parallelism, deep memory hierarchies, and diverse processor architecture

Immediate-Mode Ray-Casting

Tue, 01 Jun 1999 00:00:00 GMT

Immediate-Mode Ray-Casting Alex, John; Teller, Seth We propose a simple modification to the classical polygon rasterization pipeline that enables exact, efficient raycasting of bounded implicit surfaces without the use of a global spatial data structure bounding hierarchy. Our algorithm requires two descr

Mostly-Static Decentralized Information Flow Control

Fri, 01 Jan 1999 00:00:00 GMT

Mostly-Static Decentralized Information Flow Control Myers, Andrew C. The growing use of mobile code in downloaded programs such as applets and servlets has increased interest in robust mechanisms for ensuring privacy and secrecy. Common security mechanisms such as sandboxing and access control are either too restrictive or

Performance Nonmonotonicities: A Case Study of the UltraSPARC Processor

Mon, 01 Jun 1998 00:00:00 GMT

Performance Nonmonotonicities: A Case Study of the UltraSPARC Processor Kushman, Nathaniel A. Modern microprocessor architectures are very complex designs. Consequently, they exhibit many idiosyncrasies. In fact, situations exist in which the addition or removal of a single instruction changes the performance of a program by a factor of 3 to 4. I

Regions: A Scalable Infrastructure for Scoped Service Location in Ubiquitous Computing

Sat, 01 May 1999 00:00:00 GMT

Regions: A Scalable Infrastructure for Scoped Service Location in Ubiquitous Computing Benedicto, Kathryn Flores Until recently, most efforts in service location have focused on finding local services. However, service location is also useful in large-scale networked environments containing numerous, possibly non-local services. Regions address this need for scala

Creating and Rendering Image-Based Visual Hulls

Sat, 01 May 1999 00:00:00 GMT

Creating and Rendering Image-Based Visual Hulls Buehler, Chris; Matusik, Wojciech; McMillan, Leonard In this paper, we present efficient algorithms for creating and rendering image-based visual hulls. These algorithms are motivated by our desire to render real-time views of dynamic, real-world scenes. We first describe the visual hull, an abstract geomet

Providing QoS Guarantees in Input Buffered Crossbar Switches with Speedup

Sat, 01 Aug 1998 00:00:00 GMT

Providing QoS Guarantees in Input Buffered Crossbar Switches with Speedup Charney, Anna

Dynamically Reparameterized Light Fields

Dynamically Reparameterized Light Fields Isaksen, Aaron; McMillan, Leonard; Gortler, Steven J.

Teaching Policy to Computer Science Students

Tue, 01 Dec 1998 00:00:00 GMT

Teaching Policy to Computer Science Students Blumenthal, Marjory S. Computing motivates more and more attention by policy-makers at all levels of government, and policy interests of all kind can touch on computer scienceÔøΩboth inspiring new research directions or constraining technology development. Understanding public p

Credible Compilers

Mon, 01 Mar 1999 00:00:00 GMT

Credible Compilers Rinard, Martin C. This paper presents a new concept in compiler correctness: instead of proving that the compiler performs all of its transformations correctly, the compiler generates a proof that the transformed program correctly implements the input program. A simple pro

An Architecture for Intentional Name Resolution and Application-level Routing

Mon, 01 Feb 1999 00:00:00 GMT

An Architecture for Intentional Name Resolution and Application-level Routing Adjie-Winoto, William; Schwartz, Elliot; Balakrishnan, Hari

An Integrated Congestion Management Architecture for Internet Hosts

Mon, 01 Feb 1999 00:00:00 GMT

An Integrated Congestion Management Architecture for Internet Hosts Balakrishnan, Hari; Rahul, Hariharan S.; Seshan, Srinivasan This paper presents a novel framework for managing network congestion from an end-to-end perspective. Our work is motivated by several trends in traffic patterns that threaten the long-term stability of the Internet. These trends include the use of multi

Fast Place and Route Approaches for FPGAs

Mon, 01 Feb 1999 00:00:00 GMT

Fast Place and Route Approaches for FPGAs Tessier, Russell G.

The Case for SRPT Scheduling in Web Servers

Thu, 01 Oct 1998 00:00:00 GMT

The Case for SRPT Scheduling in Web Servers Harchol-Balter, Mor; Crovella, Mark E.; Park, SungSim

Polygonal Approximation of Voronoi Diagrams of Set of Triangles in Three Dimensions

Polygonal Approximation of Voronoi Diagrams of Set of Triangles in Three Dimensions Teichmann, Marek; Teller, Seth

A Model for Window Based Flow Control Packet-Switched Networks

Sun, 01 Mar 1998 00:00:00 GMT

A Model for Window Based Flow Control Packet-Switched Networks Yang, Xiaowei Recently, networks have increased rapidly both in scale and speed. Problems related to the control and management are of increasing interest. However, there is no satisfactory tool to study the behavior of such networks. The traditional event driven simul

Providing QoS Guarantees in Input Buffered Crossbar Switches with Speedup

Tue, 01 Sep 1998 00:00:00 GMT

Providing QoS Guarantees in Input Buffered Crossbar Switches with Speedup Charny, Anna

Fast Thread Communication and Synchronization Mechanisms for a Scalable Single Chip Multiprocessor

Mon, 01 Jun 1998 00:00:00 GMT

Fast Thread Communication and Synchronization Mechanisms for a Scalable Single Chip Multiprocessor Keckler, Stephen William

Scalable Inter-Cluster Communications Systems for Clustered Microprocessors

Mon, 01 Jun 1998 00:00:00 GMT

Scalable Inter-Cluster Communications Systems for Clustered Microprocessors Jiang, Xiaohu; Yeung, Donald As workstation clusters move away from uniprocessors in favor of multiprocessors to support the increasing computational needs of distributed applications, greater demands are placed on the communication interfaces that couple individual workstations. th

Algorithms for Data-Race Detection in Multithreaded Programs

Wed, 01 Jul 1998 00:00:00 GMT

Algorithms for Data-Race Detection in Multithreaded Programs Cheng, Guang-Ien Two parallel accesses to the same location, at least one of which is a write, form a race. Debugging such races is complicated by atomic critical sections. In programs without critical sections, a race is usually a bug causing nondeterminism. In programs

On Choosing a Task Assignment Policy for a Distributed Server System

On Choosing a Task Assignment Policy for a Distributed Server System Harchol-Balter, Mor; Crovella, Mark E.; Murta, Cristina D. We consider a distributed server system model and ask which policy should be used for assigning tasks to hosts. In our model each host processes tasks in First-Come-First-Serve order and the task's service demand is known in advance. We consider four ta

Exploring Optimal Cost-Performance Designs for RAW processors

Mon, 01 Jun 1998 00:00:00 GMT

Exploring Optimal Cost-Performance Designs for RAW processors Moritz, Csaba Andras; Yeung, Donald; Agarwal, Anant The semiconductor industry roadmap projects that advances in VLSI technology will permit more than one billion transistors on a chip by the year 2010. The MIT Raw microprocessor is a proposed architecture that strives to exploit these chip-level resource

A Model for Interactive Computation: Applications to Speech Research

Mon, 01 Jun 1998 00:00:00 GMT

A Model for Interactive Computation: Applications to Speech Research McCandless, Michael Kyle The speech research community has developed numerous toolkits to support ongoing research, e.g. Sapphire, Spire, ISP, ESPS/Waves+, HTK, CSLU Toolkit, LNKNet. While these toolkits contain extensive and useful functionality, they typically offer limited en

Debugging Multithreaded Programs that Incorporate User-Level Locking

Fri, 01 May 1998 00:00:00 GMT

Debugging Multithreaded Programs that Incorporate User-Level Locking Stark, Andrew F. A multithreaded program with a bug may behave nondeterministically, and this nondeterminism typically makes the bug hard to localize. This thesis presents a debugging tool, the Nondeterminator-2, which automatically finds certain nondeterminacy bugs in pr

Cilk: Efficient Multithreaded Computing

Fri, 01 May 1998 00:00:00 GMT

Cilk: Efficient Multithreaded Computing Randall, Keith H.

Bounded-Error Interactive Ray Tracing

Sun, 01 Mar 1998 00:00:00 GMT

Bounded-Error Interactive Ray Tracing Bala, Kavita; Dorsey, Julie; Teller, Seth

An Efficient Virtual Network Interface in the Fugu Scalable Workstation

Thu, 01 Jan 1998 00:00:00 GMT

An Efficient Virtual Network Interface in the Fugu Scalable Workstation Mackenzie, Kenneth Martin A scalable workstation is one vision of a mainstream parallel computer: a machine that combines scalable, fine-grain communication facilities for parallel applications with virtual memory and pre-emptive multiprogramming to support general-purpose workloa

Integrated Shared-Memory and Message-Passing Communication in the Alewife Multiprocessor

Thu, 01 Jan 1998 00:00:00 GMT

Integrated Shared-Memory and Message-Passing Communication in the Alewife Multiprocessor Kubiatowicz, John David To date, MIMD multiprocessors have been divided into two classes based on hardware communication models: those supporting shared memory and those supporting message passing. Breaking with tradition, this thesis argues that multiprocessors should integrate

Multigrain Shared Memory

Thu, 01 Jan 1998 00:00:00 GMT

Multigrain Shared Memory Yeung, Donald Designers of parallel computers have to decide how to apportion a machine's resources between processing, memory, and communication. How these resources are apportioned determine the grain and balance of the resulting machine. Often, these design decisio

An Interactive Approach to the Identification and Extraction of Visual Events

Sun, 01 Feb 1998 00:00:00 GMT

An Interactive Approach to the Identification and Extraction of Visual Events Stasior, William F.

An Interactive Approach to the Indentification and Extraction of Visual Events

Sun, 01 Feb 1998 00:00:00 GMT

An Interactive Approach to the Indentification and Extraction of Visual Events Stasior, William F. This report describes an interactive approach to the computerized processing and interpretation of visual information. The objective is to facilitate the development of interactive applications that analyze and interpret video input. The approach is to

Frustum Casting for Progressive, Interactive Rendering

Thu, 01 Jan 1998 00:00:00 GMT

Frustum Casting for Progressive, Interactive Rendering Teller, Seth; Alex, John Efficient visible surface determination algorithms have long been a fundamental goal of computer graphics. We discuss the well-known ray casting problem: given a geometric scene description, a synthetic camera, and a viewport which discretizes the camer

Planning and control in stochastic domains with imperfect information

Thu, 01 Aug 1996 00:00:00 GMT

Planning and control in stochastic domains with imperfect information Hauskrechts, Milos

Speech Perception Using Real-Time Phoneme Detection: The BeBe System

Wed, 01 Apr 1998 00:00:00 GMT

Speech Perception Using Real-Time Phoneme Detection: The BeBe System Sweeny, Latanya; Thompson, Patrick We define a new approach to speech recognition based on auditory perception and modeled after the human brain's tendency to automatically categorize speech sounds [House 1962; Liberman 1957]. As background, today's speech recognition systems are knowle

Boolean Compilation of Relational Specifications

Thu, 01 Jan 1998 00:00:00 GMT

Boolean Compilation of Relational Specifications Jackson, Daniel A new method for analyzing relational specifications is described. A property to be checked is cast as a relational formula, which, if the property holds, has no finite models. The relational formula is translated into a boolean formula that has a model f

Decoding Reed Solomon Codes beyond the Error-Correction Diameter

Wed, 01 Jan 1997 00:00:00 GMT

Decoding Reed Solomon Codes beyond the Error-Correction Diameter Sudan, Madhu

Algorithmic issues in coding theory

Wed, 01 Oct 1997 00:00:00 GMT

Algorithmic issues in coding theory Sudan, Madhu

Formal Verification of Safety-Critical Hybrid Systems

Mon, 01 Sep 1997 00:00:00 GMT

Formal Verification of Safety-Critical Hybrid Systems Livadas, Carolos This thesis investigates how the formal modeling and verification techniques of computer science can be used for the analysis of hybrid systems [1,2,3,4]---systems involving both discrete and continuous behavior. The motivation behind such research lies i

Automatic Extraction of Textured Vertical Facades from Pose Imagery

Thu, 01 Jan 1998 00:00:00 GMT

Automatic Extraction of Textured Vertical Facades from Pose Imagery Coorg, Satvan; Teller, Seth Extracting 3-dimensional structure from real-world imagery and rendering it from unrestricted viewpoints is an important problem in computer vision, and increasingly, computer graphics. Despite many years of research, a system that automatically recovers

The Fastest Fourier Transform in the West

Mon, 01 Sep 1997 00:00:00 GMT

The Fastest Fourier Transform in the West Frigo, Matteo; Johnson, Steven G.

Garbage Collection in a Large, Distributed Object Store

Mon, 01 Sep 1997 00:00:00 GMT

Garbage Collection in a Large, Distributed Object Store Maheshwari, Umesh

View-based abstraction: Enhancing Maintainability and Modularity in the presence of Implementation Dependencies

Mon, 01 Sep 1997 00:00:00 GMT

View-based abstraction: Enhancing Maintainability and Modularity in the presence of Implementation Dependencies Rodriguez, Luis H., Jr. This dissertation presents a new, backwards compatible, language independent, and incremental programming methodology called view-based abstraction. Unlike the well-known black-box abstraction approach, view-based abstraction enables programmers to maintain program modularity even in the presence of implementation couplings, i.e., dependencies among the code modules that rely on otherwise "hidden" implementation details not specified in the module interfaces. This dissertation also presents a transformation-based implementation of view-based abstraction, called ViewForm. ViewForm acts as a source-to-source preprocessor that automatically performs an implementation coupling expressed by the programmer. When the original code is later updated, ViewForm automatically attempts to reapply the implementation coupling to the updated code. ViewForm will modify the updated source code only if the coupling is still valid. In this way, by performing some extra work up front, the programmer performing an implementation coupling saves future programmers from having to pay for the consequences of broken modularity. To aid in writing this up-front ViewForm code, this dissertation presents a structured approach for using view-based abstraction and writing ViewForm transformations constructs. To demonstrate view-based abstraction, ViewForm is used to produce automated, performance-based implementation couplings in three example programs: an amorphous computing simulator, a conditional-probability pedigree computation, and ViewForm itself. Unlike other approaches that also use interprocedural program analyses, the results indicate that view-based abstraction is practical and scales gracefully - the extra automation increased compilation time from a typical 34%, to 40% in the worst case, despite a less than fully optimized ViewForm implementation. Each optimization required the programmer to write only 65 to 137 lines of ViewForm code for programs of size 167 lines to 7,616 lines. This work is amortized as time saved by programmers modifying the original program in the future. In all three examples, ViewForm maintained modularity by regenerating correct code when the original modules were modified - even when those modifications were to the optimization-dependent sections of the original code.

Managing Scheduled Routing with a High-level Communication Language

Fri, 01 Aug 1997 00:00:00 GMT

Managing Scheduled Routing with a High-level Communication Language Metcalf, Christopher D.

An Optimized Hardware Architecture and Communication Protocol for Scheduled Communication

Fri, 01 Aug 1997 00:00:00 GMT

An Optimized Hardware Architecture and Communication Protocol for Scheduled Communication Shoemaker, David

Building an Active Node on the Internet

Thu, 01 May 1997 00:00:00 GMT

Building an Active Node on the Internet Murphy, David M.

Message-Driven Dynamics

Tue, 01 Jul 1997 00:00:00 GMT

Message-Driven Dynamics Lethin, Richard Anton

HULA: An Efficient Protocol for Reliable Delivery of Messages

Tue, 01 Jul 1997 00:00:00 GMT

HULA: An Efficient Protocol for Reliable Delivery of Messages Maheshwari, Umesh We present a new protocol for reliable delivery of messages over a network that might lose, duplicate, reorder, or arbitrarily delay packets. It is the first protocol that guarantees exactly-once and ordered delivery on a connection while avoidin

Experimental Study of Minimum Cut Algorithms

Thu, 01 May 1997 00:00:00 GMT

Experimental Study of Minimum Cut Algorithms Levine, Matthew S. Recently, several new algorithms have been developed for the minimum cut problem that substantially improve worst-case time bounds for the problem. These algorithms are very different from the earlier ones and from each other. We conduct an experimental

A Model-Based Expert System for interpretation of hemodynamic data from ICU patients

Thu, 01 May 1997 00:00:00 GMT

A Model-Based Expert System for interpretation of hemodynamic data from ICU patients Zhao, Ruilin

Revisiting the Paxos Algorithm

Sun, 01 Jun 1997 00:00:00 GMT

Revisiting the Paxos Algorithm De Prisco, Roberto The Paxos algorithm is an efficient and highly fault-tolerant algorithm, devised by Lamport, for reaching consensus in a distributed system. Although it appears to be practical, it seems to be not widely known or understood. This thesis contains a new p

Relieving Hot Spots on the World Wide Web

Sun, 01 Jun 1997 00:00:00 GMT

Relieving Hot Spots on the World Wide Web Panigrahy, Rina We describe a family of caching protocols for distributed networks that can be used to decrease or eliminate the occurrence of hot spots in the network. Hot spots are web sites that swamped by a large number of requests for their pages. Our protocols are

A Random Server Model for Private Information Retrieval (or Information Theoretic PIR Avoiding Database Replication

Tue, 01 Apr 1997 00:00:00 GMT

A Random Server Model for Private Information Retrieval (or Information Theoretic PIR Avoiding Database Replication Gertner, Yael; Goldwasser, Shafi; Malkin, Tal Private information retrieval (PIR) schemes provide a user with information from a database while keeping his query secret from the database manager. We propose a new model for PIR, utilizing auxiliary random servers providing privacy services for databas

Efficient distributed 1 out of n oblivious transfer

Tue, 01 Apr 1997 00:00:00 GMT

Efficient distributed 1 out of n oblivious transfer Gertner, Yael; Malkin, Tal

Fine-Grained Control of Java Applets Using a Simple Constraint Language

Sun, 01 Jun 1997 00:00:00 GMT

Fine-Grained Control of Java Applets Using a Simple Constraint Language Mehta, Nimisha V. The use of the internet has increased extensively with a growing number of inexperienced users surfing the Web. Lurking in Web pages, Java applets are automatically executed on users' machines. As a result, popular Web browsers are understandably con

Engineering a Global Resolution Service

Sun, 01 Jun 1997 00:00:00 GMT

Engineering a Global Resolution Service Slottow, Edward C. As the World Wide Web continues to balloon in size the issue of a robust information infrastructure has become increasingly important. Currently, Web links are based on fragile names that have limited life due to semantic content. Uniform Resource Na

Modularity in the Presence of Subclassing

Tue, 01 Apr 1997 00:00:00 GMT

Modularity in the Presence of Subclassing Stata, Raymie Classes are harder to subclass than they need be. This report addresses this problem, showing how to design classes that are more modular and easier to subclass without sacrificing the extensibility that makes subclassing useful. In the context of singl

Demand-Based Coscheduling of Parallel Jobs on Multiprogrammed Multiprocessors

Tue, 01 Apr 1997 00:00:00 GMT

Demand-Based Coscheduling of Parallel Jobs on Multiprogrammed Multiprocessors Sobalvarro, Patrick Gregory

Baring it all to Software: The Raw Machine

Sat, 01 Mar 1997 00:00:00 GMT

Baring it all to Software: The Raw Machine Waingold, Elliot; Taylor, Michael; Sarkar, Vivek; Lee, Walter; Lee, Victor; Kim, Jang; Frank, Matthew; Finch, Peter; Devabhaktuni, Srikrishna; Barua, Rajeev; Babb, Jonathan; Amarasinghe, Saman; Agarwal, Anant Rapid advances in technology force a quest for computer architectures that exploit new opportunities and shed existing mechanisms that do not scale. Current architectures, such as hardware scheduled superscalars, are already hitting performance and comple

Optimism vs. Locking: A Study of Concurrency Control for Client-Server Object-Oriented Databases

Wed, 01 Jan 1997 00:00:00 GMT

Optimism vs. Locking: A Study of Concurrency Control for Client-Server Object-Oriented Databases Gruber, Robert Edward Many client-server object-oriented database systems (OODBs) run applications at clients and perform all accesses on cached copies of database objects. Moving both data and computation to the clients can improve response time, throughput, and scalability.

Replication Control in Distributed B-Trees

Sat, 01 Feb 1997 00:00:00 GMT

Replication Control in Distributed B-Trees Cosway, Paul R. B-trees are a commonly used data structure to associate symbols with related information, as in a symbol table or file index. The performance of B-tree algorithms is well understood for sequential processing and even concurrent processing on small-scale

Learning Algorithms with Applications to Robot Navigation and Protein Folding

Sun, 01 Dec 1996 00:00:00 GMT

Learning Algorithms with Applications to Robot Navigation and Protein Folding Singh, Mona

Public-Key Cryptosystems from Lattice Reduction Problems

Fri, 01 Nov 1996 00:00:00 GMT

Public-Key Cryptosystems from Lattice Reduction Problems Goldreich, Oded; Goldwasser, Shafi; Halevi, Shai We present a new proposal for a trapdoor one-way function, from which we derive public-key encryption and digital signatures. The security of the new construction is based on the conjectured computational difficulty of lattice-reduction proble

Code Importing Techniques for Fast, Safe Client/Server Access

Sun, 01 Sep 1996 00:00:00 GMT

Code Importing Techniques for Fast, Safe Client/Server Access Bank, Joseph A.

The Cilk System for Parallel Multithreaded Computing

Mon, 01 Jan 1996 00:00:00 GMT

The Cilk System for Parallel Multithreaded Computing Joerg, Christopher Frank

On Consulting a Set of Experts and Searching

Sun, 01 Sep 1996 00:00:00 GMT

On Consulting a Set of Experts and Searching Galperin, Igal Two chapters of this thesis analyze expert consulting problemas via game theoretic models; the first points out a close connectionn between the problem of consulting a set of experts and the problem of searching. The last chapter presents a solution to th

Partitioned Garbage Collection of a Large Object Store

Sat, 01 Feb 1997 00:00:00 GMT

Partitioned Garbage Collection of a Large Object Store Maheshwari, Umesh; Liskov, Barbara H. This paper describes a new garbage collection scheme for large persisten object stores that makes efficient use of the disk and main memory. The heap is divided into partitions that are collected independently using information about inter-partit

Shared Memory Versus Message Passing for Iterative Solution of Sparse, Irregular Problems

Tue, 01 Oct 1996 00:00:00 GMT

Shared Memory Versus Message Passing for Iterative Solution of Sparse, Irregular Problems Chong, Frederic T.; Agarwal, Anant The benefits of hardware support for shared memory versus those formessage passing are difficult to evaluate without an in-depth study ofreal applications on a common platform. We evaluate the communicationmechanisms of the MIT Alewife machine, a multipr

The Computer Science Technical Report (CS-TR) Project: Considerations from the Library Perspective

Sat, 01 Jun 1996 00:00:00 GMT

The Computer Science Technical Report (CS-TR) Project: Considerations from the Library Perspective Anderson, Greg; Lasher, Rebecca; Reich, Vicky In 1992 the Advanced Research Projects Agency (ARPA) funded a three year grant to investigate the questions related to large-scale, distributed, digital libraries. The award focused research on Computer Science Technical Reports (CS-TR) and was granted to

Efficient Garbage Collection for Large Object-Oriented Databases

Wed, 01 May 1996 00:00:00 GMT

Efficient Garbage Collection for Large Object-Oriented Databases Ng, Tony C. This thesis presents the design of an efficient garbage collection scheme for large, persistent object-oriented databases in a client-server environment. The scheme uses a partitioned approach. A database is divided into disjoint partitions and each parti

A Security Model for the Information Mesh

Sat, 01 Jun 1996 00:00:00 GMT

A Security Model for the Information Mesh Condell, Matthew N. Many distributed systems that are currently being designed are object based. These sytems require a model for authentication and access control which conforms to the object model. They need a model that allows objects to control their own security. In

Encapsulated Key Escrow

Mon, 01 Apr 1996 00:00:00 GMT

Encapsulated Key Escrow Bellare, Mihir; Goldwasser, Shafi

Phonological Parsing for Bi-directional Letter-to-Sound/Sound-to-Letter Generation

Thu, 01 Jun 1995 00:00:00 GMT

Phonological Parsing for Bi-directional Letter-to-Sound/Sound-to-Letter Generation Meng, Helen Mei-Ling This thesis proposes a unified framework for integrating a variety of linguistic knowledge sources for representing speech, in order to facilitiate their concurrent utilization in spoken language systems. The feasibility of the proposed methodology is de

On the Structure of the Scaffolding Core of Bacteriophage T4 and Its Role in Head Length

Mon, 01 Jan 1996 00:00:00 GMT

On the Structure of the Scaffolding Core of Bacteriophage T4 and Its Role in Head Length Berger, Bonnie A.; Hoest, Gunnar W.; Paulson, James R.; Shor, Peter W.

Correctness of Vehicle Control Systems: A Case Study

Thu, 01 Feb 1996 00:00:00 GMT

Correctness of Vehicle Control Systems: A Case Study Weinberg, Henri B.

Time-lock Puzzles and Timed-release Crypto

Thu, 01 Feb 1996 00:00:00 GMT

Time-lock Puzzles and Timed-release Crypto Rivest, Ronald L.; Shamir, Adi; Wagner, David A. Our motivation is the notion of ``timed-release crypto,'' where the goal is to encrypt a message so that it can not be decrypted by anyone, not even the sender, until a pre-determined amount of time has passed. The goal is to ``send information into the

Translucent Cyrptography: An Alternative to Key Escrow, and its Implementation via Fractional Oblivious Transfer

Thu, 01 Feb 1996 00:00:00 GMT

Translucent Cyrptography: An Alternative to Key Escrow, and its Implementation via Fractional Oblivious Transfer Bellare, Mihir; Rivest, Ronald L.

Adaptively Secure Multi-party Computation

Thu, 01 Feb 1996 00:00:00 GMT

Adaptively Secure Multi-party Computation Canetti, Ran; Feige, Uri; Goldreich, Oded; Naor, Moni A fundamental problem in designing secure multi-party protocols is how to deal with adaptive adversaries (i.e., adversaries that may choose the corrupted parties during the course of the computation), in a setting where the channels are insecure and secur

A Theory of Clock Synchronization

Sat, 01 Oct 1994 00:00:00 GMT

A Theory of Clock Synchronization Patt, Boaz We consider the problem of clock synchronization in a system with uncertain message delays and clocks with bounded drift. To analyze this classical problem, we introduce the concept of synchronization graphs, and show that the tightest achievable synchron

Admission Control and Routing: Theory and Practice

Thu, 01 Jun 1995 00:00:00 GMT

Admission Control and Routing: Theory and Practice Gawlick, Rainer Emerging high speed Broadband Integrated Services Digital Networks (B-ISDN) will carry traffic for services such as video-on-demand and video teleconferencing, which require resource reservation along the path on which the traffic is sent. As a result, su

Identifying and Merging Related Bibliographic Records

Thu, 01 Feb 1996 00:00:00 GMT

Identifying and Merging Related Bibliographic Records Hylton, Jeremy A. Bibliographic records freely available on the Internet can be used to construct a high-quality, digital finding aid that provides the ability to discover paper and electronic documents. The key challenge to providing such a service is integrating mixed-

Executing Multithreaded Programs Efficiently

Fri, 01 Sep 1995 00:00:00 GMT

Executing Multithreaded Programs Efficiently Blumofe, Robert D. This thesis presents the theory, design, and implementation of Cilk (pronounced "silk") and Cilk-NOW. Cilk is a C-based language and portable runtime system for programming and executing multithreaded parallel programs. Cilk-NOW is an implementation of

Modeling and Verification of Randomized Distributed Real -Time Systems

Sat, 01 Jun 1996 00:00:00 GMT

Modeling and Verification of Randomized Distributed Real -Time Systems Segala, Roberto Randomization is an excellent tool for the design of distributed algorithms, sometimes yielding efficient solutions to problems that are inherently complex, or even unsolvable, in the setting of deterministic algorithms. However, this tool has a price: e

High-Performance All-Software Distributed Shared Memory

Thu, 01 Feb 1996 00:00:00 GMT

High-Performance All-Software Distributed Shared Memory Johnson, Kirk L. The C Region Library (CRL) is a new all-software distributed shared memory (DSM) system. CRL requires no special compiler, hardware, or operating system support beyond the ability to send and receive messages between processing nodes. It provides a simp

Aurora at MIT

Fri, 01 Dec 1995 00:00:00 GMT

Aurora at MIT Clark, David D; Houh, Henry; Tennenhouse, David L.

Decentralized Channel Management in Scalable Multihop Spread-Spectrum Packet Radio Networks

Sat, 01 Jul 1995 00:00:00 GMT

Decentralized Channel Management in Scalable Multihop Spread-Spectrum Packet Radio Networks Shepard, Timothy Jason This thesis addresses the problems of managing the transmissions of stations in a spread-spectrum packet ratio network so that the system can remain effective when scaled to millions of nodes concentrated in a metropolitan area. The principal difficulty

Theta Reference Manual

Wed, 01 Feb 1995 00:00:00 GMT

Theta Reference Manual Liskov, Barbara; Curtis, Dorothy; Day, Mark; Ghemawat, Sanjay; Gruber, Robert; Johnson, Paul; Myers, Andrew C.

Lottery and Stride Scheduling: Flexible Proportional-share Resource Management

Fri, 01 Sep 1995 00:00:00 GMT

Lottery and Stride Scheduling: Flexible Proportional-share Resource Management Waldspurger, Carl A. This thesis presents flexible abstractions for specifying resource management policies, together with efficient mechanisms for implementing those abstractions. Several novel scheduling techniques are introduced, including both randomized and deterministi

The Modified Object Buffer: A Storage Management Technique for Object-Oriented Databases

Fri, 01 Sep 1995 00:00:00 GMT

The Modified Object Buffer: A Storage Management Technique for Object-Oriented Databases Ghemawat, Sanjay

Dynamic Computation Migration in Distributed Shared Memory Systems

Fri, 01 Sep 1995 00:00:00 GMT

Dynamic Computation Migration in Distributed Shared Memory Systems Hsieh, Wilson Cheng-Yi

Reactive Synchronization Algorithms for Multiprocessors

Thu, 01 Jun 1995 00:00:00 GMT

Reactive Synchronization Algorithms for Multiprocessors Lim, Beng-Hong Efficient synchronization algorithms are hard to design because their performance depends on run-time factors that are hard to predict. In particular, the designer has a choice of protocols to implement the synchronization operation, and a choice of wait

Restricted Branching Programs and Hardware Verification

Tue, 01 Aug 1995 00:00:00 GMT

Restricted Branching Programs and Hardware Verification Ponzio, Stephen J. Recent developments in the field of digital design and hardware verification have found great use for restricted forms of branching programs. In particular, oblivious read-once branching programs (also called "OBDD's") are central to a very common techni

Computationally Efficient Error-Correcting Codes and Holographic Proofs

Thu, 01 Jun 1995 00:00:00 GMT

Computationally Efficient Error-Correcting Codes and Holographic Proofs Spielman, Daniel Alan We present computationally efficient error-correcting codes and holographic proofs.Our error-correcting codes are asymptotically good and can be encoded and decoded in linear time.Our construction of holographic proofs provide, for every proof of any theo

Link Architecture for a Global Information Infrastructure

Thu, 01 Jun 1995 00:00:00 GMT

Link Architecture for a Global Information Infrastructure Van Dyke, Jeffrey R.

Increasing Cross-Domain Call Batching Using Promises and Batched Control Structures

Thu, 01 Jun 1995 00:00:00 GMT

Increasing Cross-Domain Call Batching Using Promises and Batched Control Structures Zondervan, Quinton Y. In a client-server system, it may be possible for the client to corrupt server data through unsafe access methods or programming error. A common method for protecting the server data is to separate the client and server into distinct protection domains,

Noise Tolerant Algorithms for Learning and Searching

Wed, 01 Feb 1995 00:00:00 GMT

Noise Tolerant Algorithms for Learning and Searching Aslam, Javed Alexander We consider the problem of developing robust algorithms which cope with noisy data. In the Probably Approximately Correct model of machine learning, we develop a general technique which allows nearly all PAC learning algorithms to be converted into highly

Quantitative Performance Modeling of Scientific Computations

Mon, 01 May 1995 00:00:00 GMT

Quantitative Performance Modeling of Scientific Computations Toledo, Sivan Abraham The first part of the thesis demonstrates that the performance of programs can be predicted accurately, automatically, and rapidly using a method called benchmapping. The key aspects benchmapping are: automatic creation of detailed performance models, pr

Reducing Synchronization Overhead in Parallel Simulation

Mon, 01 May 1995 00:00:00 GMT

Reducing Synchronization Overhead in Parallel Simulation Legedza, Ulana Synchronization is often the dominant cost in conservative parallel simulation, particularly in simulations of parallel computers, in which low-latency simulated communication requires frequent synchronization. This thesis presents local barriers and pre

Connecting Homes to the Internet: An Engineering Cost Model of Cable vs. ISDN

Thu, 01 Jun 1995 00:00:00 GMT

Connecting Homes to the Internet: An Engineering Cost Model of Cable vs. ISDN Gillett, Sharon Eisner Using the World Wide Web at 28.8 Kbps (or less) can be a frustrating experience: a multimedia page that takes a fraction of a second to download at Ethernet speeds takes many seconds at modem rates. Two enhancements to existing infrastructure have the pot

An Interchange Standard and System for Browsing Digital Documents

Mon, 01 May 1995 00:00:00 GMT

An Interchange Standard and System for Browsing Digital Documents Kass, Andrew Jonathan With the advent of fast global digital communication networks, information will increasingly be delivered in electronic form. In addition, as libraries become increasingly more computerized, not just card catalogs but entire books will be stored on-line.

Client Cache management in a Distributed Object Database

Mon, 01 May 1995 00:00:00 GMT

Client Cache management in a Distributed Object Database Day, Mark Stuart A distributed object database stores persistently at servers. Applications run on client machines, fetching objects into a client-side cache of objects. If fetching and cache management are done in terms of objects, rather than fixed-size units such as

Randomness Versus Non-Determinism in Distributed Computing

Sat, 01 Oct 1994 00:00:00 GMT

Randomness Versus Non-Determinism in Distributed Computing Saias, Alain Isaac This thesis is devoted to the analysis and illustration of the effects of the interplay between randomness and non-determinism in randomized computing. Using ideas from game theory , we provide a general model for randomized computing which formalizes th

Quickstep: A System for Performance Monitoring and Debugging Parallel Applications on the Alewife Multiprocessor

Sun, 01 Jan 1995 00:00:00 GMT

Quickstep: A System for Performance Monitoring and Debugging Parallel Applications on the Alewife Multiprocessor Mitra, Sramana

A Distributed Programming System for Media Applications

Wed, 01 Feb 1995 00:00:00 GMT

A Distributed Programming System for Media Applications Phillips, Brent M.

Functional Encapsulation and Type Reconstruction in a Strongly-typed, Polymorphic Language

Wed, 01 Feb 1995 00:00:00 GMT

Functional Encapsulation and Type Reconstruction in a Strongly-typed, Polymorphic Language Gupta, Shail Aditya

A Statistical Approach to Language Modelling for the ATIS Problem

Wed, 01 Feb 1995 00:00:00 GMT

A Statistical Approach to Language Modelling for the ATIS Problem Koppelman, Joshua D.

Synchronized MIMD Computing

Sun, 01 May 1994 00:00:00 GMT

Synchronized MIMD Computing Kuszmaul, Bradley C. Fast global synchronization provides simple, efficient solutions to many of the system problems of parallel computing. It achieves this by providing composition of both performance and correctness. If you understand the performance and meaning of parall

Mechanisms and Interfaces for Software-Extended Coherent Shared Memory

Sun, 01 Jan 1995 00:00:00 GMT

Mechanisms and Interfaces for Software-Extended Coherent Shared Memory Chaiken, David L. Software-extended systems use a combination of hardware and software to implement shared memory on large-scale multiprocessors. Hardware mechanisms accelerate common-case accesses, while software handles exceptional events. This dissertation proposes, d

Small-Depth Counting Networks and Related Topics

Thu, 01 Sep 1994 00:00:00 GMT

Small-Depth Counting Networks and Related Topics Klugerman, Michael Richard

Statistical Trajectory Models for Phonetic Recognition

Mon, 01 Aug 1994 00:00:00 GMT

Statistical Trajectory Models for Phonetic Recognition Goldenthal, William David The main goal of this work is to develop an alternative methodology for acoustic-phonetic modelling of speech sounds. The approach utilizes a segment-based framework to capture the dynamical behavior and statistical dependencies of the acoustic attribute

On-Line Algorithms for Robot Navigation and Server Problems

Sun, 01 May 1994 00:00:00 GMT

On-Line Algorithms for Robot Navigation and Server Problems Kleinberg, Jon M.

An Interactive Programming System for Media Computation

Thu, 01 Sep 1994 00:00:00 GMT

An Interactive Programming System for Media Computation Wetherall, David J. As digital video is manipulated by increasingly powerful computers, many new applications are becoming viable. This report investigates the programming language aspects of controlling such video applications. It presents the design, implementation, and

The Structure of Near-minimum Edge Cuts

The Structure of Near-minimum Edge Cuts Benczúr, András A.

Serializing Parallel Programs by Removing Redundant Computation

Mon, 01 Aug 1994 00:00:00 GMT

Serializing Parallel Programs by Removing Redundant Computation Ernst, Michael D.

A Programming system for the Dynamic Manipulation of Temporally Sensitive Data

Mon, 01 Aug 1994 00:00:00 GMT

A Programming system for the Dynamic Manipulation of Temporally Sensitive Data Lindblad, Christopher J. In computer-participative multimedia applications, the computer not only manipulates media, but also digests it and performs independent actions based on media content. In this report I discuss an approach to the design of environments to support the dev

Failsafe Key Escrow

Mon, 01 Aug 1994 00:00:00 GMT

Failsafe Key Escrow Kilian, Joseph; Leighton Frank Thomson

A Scheme Shell

Fri, 01 Apr 1994 00:00:00 GMT

A Scheme Shell Shivers, Olin

Time Optimal Self-Stabilizing Spanning Tree Algorithms

Sat, 01 Jan 1994 00:00:00 GMT

Time Optimal Self-Stabilizing Spanning Tree Algorithms Aggarwal, Sudhanshu Madan

An Integrated Approach to Dynamic Decision Making under Uncertainty

Mon, 01 Aug 1994 00:00:00 GMT

An Integrated Approach to Dynamic Decision Making under Uncertainty Leong, Tze-Yun

Global Partitioning of Parallel loops and Data Arrays for Caches and Distributed Memory in Multiprocessors

Sat, 01 Jan 1994 00:00:00 GMT

Global Partitioning of Parallel loops and Data Arrays for Caches and Distributed Memory in Multiprocessors Barua, Rajeev K.

Using Specifications to Check Source Code

Wed, 01 Jun 1994 00:00:00 GMT

Using Specifications to Check Source Code Evans, David

Automatic Acquisition of Language Models for Speech Recognition

Wed, 01 Jun 1994 00:00:00 GMT

Automatic Acquisition of Language Models for Speech Recognition McCandless, Michael Kyle

Transaction Management for Mobile Objects Using Optimistic Concurrency Control

Fri, 01 Jul 1994 00:00:00 GMT

Transaction Management for Mobile Objects Using Optimistic Concurrency Control Adya, Atul We present computationally efficient error-correcting codes and holographic proofs. Our error-correcting codes are asymptotically good and can be encoded and decoded in linear time. Our construction of holographic proofs provide, for every proof of any t

A Safe, Efficient Object Database Interface Using Batched Futures

Fri, 01 Jul 1994 00:00:00 GMT

A Safe, Efficient Object Database Interface Using Batched Futures Bogle, Phillip Lee For many systems such as operating systems and databases it is important to run client code in a separate protection domain so that it cannot interfere with the correct operation of the system. Clients communicate with the server by making cross domain c

Time Surveying: Clock Synchronization over packet Networks

Sun, 01 May 1994 00:00:00 GMT

Time Surveying: Clock Synchronization over packet Networks Troxel, Gregory D.

Investigation of a Preemptive Network Architecture

Sun, 01 May 1994 00:00:00 GMT

Investigation of a Preemptive Network Architecture Lefelhocz, Christopher James Two network architectures, cell and packet, form the basis of most high bandwidth network research. If analyzed from the perspective of building a switch, both architectures have unique advantages. The preemptive architecture described herein proposes

Formal Specification Techniques for Promoting Software Modularity, Enhancing Documentation, and Testing Specifications

Wed, 01 Jun 1994 00:00:00 GMT

Formal Specification Techniques for Promoting Software Modularity, Enhancing Documentation, and Testing Specifications Tan, Yang Meng This thesis presents three ideas. First, it presents a novel use of formal specification to promote a programming style based on specified interfaces and data abstraction in a programming language that lacks such supports. Second, it illustrates the uses of claims about specifications.

Observing "True" Concurrency

Wed, 01 Sep 1993 00:00:00 GMT

Observing "True" Concurrency Jategaonkar, Lalita A. In concurrent process theory, processors are often modeled by state machines and Petri Nets. Algebraic process theories based on state machines, exemplified by Milner's CCS and Hoare's CSP, have been more fully developed than Net-based theories, but are inadequate for modeling "true" concurrency concepts such as non-atomic actions, action refinement, locality of actions, and multithreadedness.

Efficient Implementation of High-Level Languages on User-Level Communications Architectures

Sun, 01 May 1994 00:00:00 GMT

Efficient Implementation of High-Level Languages on User-Level Communications Architectures Hsieh, Wilson C.; Johnson, Kirk L.; Kaashoek, M. Frans; Wallach, Deborah A.; Weihl, William E. User-level communication architectures --- parallel architectures that give user code direct but protected access to the network --- provide communication performance that is an order of magnitude higher than previous-generation message-passing architectures. Unfortunately, in order to take advantage of his level of performance, programmers must concern themselves with low-level issues that are often hardware dependent (e.g., what primitives to use for large and small data transfers, and either to use interrupts or polling).

Cellular Automata Methods in Mathematical Physics

Sun, 01 May 1994 00:00:00 GMT

Cellular Automata Methods in Mathematical Physics Smith, Mark Andrew Cellular automata (CA) are fully discrete, spatially-distributed dynamical systems which can serve as an alternative framework for mathematical descriptions of physical systems. Furthermore, they constitute intrinsically parallel models of computation which can be efficiently realized with special-purpose cellular automata machines.

A Timing Analysis and Optimization System for Level-clocked Circuitry

Wed, 01 Sep 1993 00:00:00 GMT

A Timing Analysis and Optimization System for Level-clocked Circuitry Papaefthymiou, Marios Christos This thesis investigates timing analysis and optimization issues in synchronous circuitry. The major thrust of our work is a collection of provably correct and efficient algorithms that perform a variety of architectural-level operations on level-clocked

Guardian Angel: Patient-Centered Health Information Systems

Sun, 01 May 1994 00:00:00 GMT

Guardian Angel: Patient-Centered Health Information Systems Szolovits, Peter; Doyle, Jon; Long, William J.; Kohane, Isaac; Pauker, Stephen G.

Distributing Information for Collaborative Filtering on Usenet Net News

Sun, 01 May 1994 00:00:00 GMT

Distributing Information for Collaborative Filtering on Usenet Net News Maltz, David A. As part of the Information Revolution," the amount of raw information available to computer users has increased as never before. Unfortunately , there has been a corresponding jump in the amount of unrelated information users must search through in order

The Vidboard:A Video Capture and Processing Peripheral for the ViewStation System

Tue, 01 Sep 1992 00:00:00 GMT

The Vidboard:A Video Capture and Processing Peripheral for the ViewStation System Adam, Joel F.; Tennenhouse, David L. With the growth of multimedia applications, video is increasingly being handled within the computing environment. Since video presents serious technological challenges to the current generation of personal computers and networks, other systems based on t

An Algorithm for Rate Allocation in a Packet-Switching Network With Feedback

Fri, 01 Apr 1994 00:00:00 GMT

An Algorithm for Rate Allocation in a Packet-Switching Network With Feedback Charny, Anna As the speed and complexity of computer networks evolve, sharing network resources becomes increasingly important. thus, the issue of how to allocate the available bandwidth among the multitude of users needs to be addressed. Such allocation needs to be

Function-Based Indexing for Object-Oriented Databases

Tue, 01 Feb 1994 00:00:00 GMT

Function-Based Indexing for Object-Oriented Databases Hwang, Deborah Jing-Hwa Object-oriented databases should support queries over user-defined sets based on properties computed using user-defined functions. This dissertation presents a new function-based indexing scheme to make these queries run faster. These indexes are diffic

Fast Object Operations in a Persistent Programming System

Sat, 01 Jan 1994 00:00:00 GMT

Fast Object Operations in a Persistent Programming System Myers, Andrew C.

Exploiting Specifications to Improve Program Performance

Tue, 01 Feb 1994 00:00:00 GMT

Exploiting Specifications to Improve Program Performance Vandevoorde, Mark T. Although programmers benefit from interface specifications when reasoning about programs, existing compilers do not. In this thesis, I discuss how to incorporate specifications into a programming language to improve performance. I use specifications in

Run-time Thread Management for Large-Scale Distributed-Memory Multiprocessors

Wed, 01 Sep 1993 00:00:00 GMT

Run-time Thread Management for Large-Scale Distributed-Memory Multiprocessors Nussbaum, Daniel Effective thread management is crucial to achieving good performance on large-scale distributed-memory multiprocessors that support dynamic threads. For a given parallel computation with some associated task constraints imposed by the task graph, a thread-management algorithm produces a running schedule as output, subject to the precedence constraints imposed by the task graph and the constraints imposed by the interprocessor communications network.

compiler analysis to implement point-to-point synchronization in parallel programs

Wed, 01 Sep 1993 00:00:00 GMT

compiler analysis to implement point-to-point synchronization in parallel programs Nguyen, John The shared-memory data-parallel model presents an attractive interface for programming multiprocessors by allowing for easy management of parallel tasks while hiding details of the underlying machine architecture.

Closing the Window of Vulnerability in Multiphase memory transaction: The alewife transaction store

Mon, 01 Feb 1993 00:00:00 GMT

Closing the Window of Vulnerability in Multiphase memory transaction: The alewife transaction store Kubiatowicz, John David Multiprocessor architects have begun to explore several mechanisms such as prefetching, context-switching and software-assisted dynamic cache-coherence, which transform single-phase memory transactions in conventional memory systems into multi-phase operations.

Automatic language Identification Using a Segment-Based Approach

Sun, 01 Aug 1993 00:00:00 GMT

Automatic language Identification Using a Segment-Based Approach Hazen, Timothy J. Automatic language Identification (ALI) is the problem of automatically identifying the language of an utterance through the use of a computer. In 1977, House and Neuburg proposed an approach to ALI which focused on the phonotactic constraints of different languages.

Correctness of Communications Protocols, A case Study

Mon, 01 Nov 1993 00:00:00 GMT

Correctness of Communications Protocols, A case Study Søgaard-Andersen, Jørgen; Lynch, Nancy A.; Lampson, Butler W. During the past few years, the technology for formal specification and verification of communication protocols has matured to the point where we believe that it now provides practical assistance for protocol design and validation.

Correctness Proof for a Network Synchronizer

Wed, 01 Dec 1993 00:00:00 GMT

Correctness Proof for a Network Synchronizer Devarajan, Harish; Fekete, Alan; Lynch, Nancy A.; Shrira, Liuba In this paper we offer a formal, rigorous proof of the correctness of Awerbuch's algorithm for network synchronization [1]. We specify both the algorithm and the correctness condition using the I/O automaton model.

Liveness in Timed and Untimed Systems

Wed, 01 Dec 1993 00:00:00 GMT

Liveness in Timed and Untimed Systems Gawlick, Rainer; Segala, Roberto; Søgaard-Andersen, Jørgen; Lynch, Nancy A. When proving the correctness of algorithms in distributed systems, one generally considers safety conditions and liveness conditions. The Input /Output (I/)0 automaton model and its timed version have used successfully, but have focused on safety conditions and on a restricted from of liveness called fairness.

Virtual Wires: Overcoming Pin Limitations in FPGA-based Logic Emulation

Mon, 01 Nov 1993 00:00:00 GMT

Virtual Wires: Overcoming Pin Limitations in FPGA-based Logic Emulation Babb, Jonathan William Existing FPGA-based logic emulators are limited by inter-chip communication bandwidth, resulting in low gate utilization (10 to 20 percent of usable gates). This resource imbalance increases the number of chips needed to emulate a particular logic design and thereby decreases emulation speed, since signals must cross more chip boundaries.

Reordering with Hindsight

Fri, 01 Oct 1993 00:00:00 GMT

Reordering with Hindsight Spiers, Bradford T. This report presents the reordering technique for parallel debugging. This technique is useful for debugging ordering errors, caused when actions a programmer meant to occur in a specific order occur in a different, unintended order.

Self-stabilization By Local Checking and Correction

Thu, 01 Oct 1992 00:00:00 GMT

Self-stabilization By Local Checking and Correction Varghese, George A self-stabilizing protocol begins to behave correctly in bounded time, no matter what state it starts in. Self-stabilization abstracts the ability to tolerate arbitrary faults that stop. This thesis describes a simple paradigm called local checking and correction for the design of stabilizing network protocols.

Cache Performance of Garbage-collected Programming Languages

Wed, 01 Sep 1993 00:00:00 GMT

Cache Performance of Garbage-collected Programming Languages Reinhold, Mark B. As processor speeds continue to improve relative to main-memory access times, cache performance is becoming an increasingly important component of program performance.

Structured Video: A Data Type with Content-based Access

Wed, 01 Sep 1993 00:00:00 GMT

Structured Video: A Data Type with Content-based Access Duda, Andrzej; Weiss, Ron We describe structured video, a general video data model allowing free form annotation, composition, and content-based access to video segments. The structured video abstraction provides an efficient means of organizing and manipulating video data by assigning logical representations to the underlying video streams and their contents.

Fair Cryptosystems

Mon, 01 Nov 1993 00:00:00 GMT

Fair Cryptosystems Micali, Silvio

A Content Routing System for Distributed Information Systems

Tue, 01 Jun 1993 00:00:00 GMT

A Content Routing System for Distributed Information Systems Sheldon, Mark A.; Duda, Andrzej; Weiss, Ron; O'Toole, James; Gifford, David K. We describe the first system that provides query based associative access to the contents of distributed information servers. Queries describe desired object attributes, and are automatically forwarded to servers that contain relevant information.

I-95 The Information Market

Sun, 01 Aug 1993 00:00:00 GMT

I-95 The Information Market

Distributed Garbage Collection in a Client-server, Transaction, Persistent Object System

Sun, 01 Aug 1993 00:00:00 GMT

Distributed Garbage Collection in a Client-server, Transaction, Persistent Object System Maheshwari, Umesh We present a design for distributed garbage collection in a object-oriented database system called Thor. Garbage collection in Thor is different from that in conventional distributed systems because Thor has a client-server architecture, in which clients fetch copies of objects from multiple servers ans run transactions.

Implementing Orthogonal Persistence: A Simple Optimization Based on Replicating Collection

Implementing Orthogonal Persistence: A Simple Optimization Based on Replicating Collection Nettels, Scott; O'Toole James

Concurrent Garbage Collection of Persistent Heaps

Tue, 01 Jun 1993 00:00:00 GMT

Concurrent Garbage Collection of Persistent Heaps O'Toole, James; Nettles, Scott; Gifford, David K. We describe the first concurrent compacting garbage collector for a persistent heap. Client threads read and write the heap in primary memory, and can independently commit or about their write operations.

Real-time Replication GC: An Implementation Report

Thu, 01 Apr 1993 00:00:00 GMT

Real-time Replication GC: An Implementation Report O'Toole, James; Nettles, Scott

Logical Disk: A Simple New Approach to Improving File System Performance

Thu, 01 Apr 1993 00:00:00 GMT

Logical Disk: A Simple New Approach to Improving File System Performance de Jonge, Wiebren; Kaashoek, M. Frans; Hsieh, Wilson C. Making a file system efficient usually requires extensive modifications. For example, making a file system log-structured requires the introduction of new data structures that are tightly coupled with the general file system code.

An Evaluation of Multiprocessor Support for Fine-grain Synchronization in Preconditioned Conjugate Gradient

Mon, 01 Feb 1993 00:00:00 GMT

An Evaluation of Multiprocessor Support for Fine-grain Synchronization in Preconditioned Conjugate Gradient Yeung, Donald This thesis explores the use of fine-grain synchronization in the preconditioned conjugate gradient (PCG) method using the modified incomplete Cholesky factorization of the coefficient matrix as a preconditioner.

The Category of Functors from State Shapes to Bottomless CPOs in Adequate for Block Structure

Fri, 01 Jan 1993 00:00:00 GMT

The Category of Functors from State Shapes to Bottomless CPOs in Adequate for Block Structure Lent, Arthur Franklin We present a programming language EoA, which embodies what Reynolds has described as the ``essence of ALGOL.'' In particular, EoA allows higher-order procedures and the declaration of block structured local variables.

A Constructive Approach to Artificial Intelligence Reexamined

Wed, 01 Jul 1992 00:00:00 GMT

A Constructive Approach to Artificial Intelligence Reexamined Ramstad, Robert Matthew Made-Up Minds: A Constructivist Approach to Artificial Intelligence, a Ph.D. thesis by Gary Drescher (MIT, Computer Science, September 1989) and a book published by MIT Press (1991) describe a learning system which controls a simulated robot and gathers information about causes and effects for various actions within the software simulated world the robot inhabits.

Family Values: A Beahvior Notion of Subtyping

Sun, 01 Aug 1993 00:00:00 GMT

Family Values: A Beahvior Notion of Subtyping Liskov, Barbara; Wing, Jeannette M.

A History of CLU

Wed, 01 Sep 1993 00:00:00 GMT

A History of CLU Liskov, Barbara H. The idea of a data abstraction has had a significant impact on the development of programming languages and on programming methodology. CLU was the first implemented programming language to provide direct linguistic support for data abstraction.

A Process Algebraic View of I/O Automata

Mon, 01 Jun 1992 00:00:00 GMT

A Process Algebraic View of I/O Automata Segala, Roberto The Input/Output Automata formalism of Lynch and Tuttle is a widely used framework for the specification and verification of concurrent algorithms. Unfortunately, it has never been provided with an algebraic characterization, a formalization which has been fundamental for the success of theories like CSP, CCP and ACP.

Concurrent Timestamping Made Simple

Tue, 01 Sep 1992 00:00:00 GMT

Concurrent Timestamping Made Simple Gawlick, Rainer Concurrent Timestamp Systems (CTSS) allow processes to temporally order concurrent events in an asynchronous shared memory system. Bounded memory constructions of a CTSS are extremely powerful tools for concurrency control, and are the basis for solutions to many coordination problems including mutual exclusion, randomized consensus, and multiwriter multireader atomic registers.

Compiler-directed Storage Reclamation Using Object Lifetime Analysis

Sun, 01 Nov 1992 00:00:00 GMT

Compiler-directed Storage Reclamation Using Object Lifetime Analysis Hicks, James Edward, Jr. Many heap-oriented languages such as Lisp and Id depend on run-time garbage collection to reclaim storage. Garbage collection can be a significant run-time expense, especially for functional languages that tend to allocate structures often.

Managing Storage for Multithreaded Computations

Tue, 01 Sep 1992 00:00:00 GMT

Managing Storage for Multithreaded Computations Blumofe, Robert D. Multithreading has become a dominant paradigm in general purpose MIMD parallel computation. To execute a multithread computation on a parallel computer, a scheduler must order and allocate threads to run on the individual processors.

Performance Assertion Checking

Tue, 01 Sep 1992 00:00:00 GMT

Performance Assertion Checking Perl, Sharon Esther Performance assertion checking is an approach to describing and monitoring the performance of complex software systems. The idea is simple: system implementors write assertions that capture their expectations for performance, the system is instrumented to collect performance data, and then the assertions are checked automatically against the data to detect violations signifying potential performance bugs.

Proceedings of the 1992 MIT Student Workshop on VLSI and Parallel Systems

Wed, 01 Jul 1992 00:00:00 GMT

Proceedings of the 1992 MIT Student Workshop on VLSI and Parallel Systems Leiserson, C.E. Proceedings of the 1992 MIT Student Workshop on VLSI and Parallel Systems. The papers in this volume were submitted to the 1992 MIT Student Workshop on VLSI and Parallel Systems. The workshop was organized by the VLSI and Parallel Systems Group at MIT to promote an interchange of ideas among the various research activities at MIT in VSLI and parallel systems. It was held on July 21, 1992 at the MIT Endicott House in Dedham, Massachusetts. Of 54 papers in this proceedings, 16 were chosen for presentation at the workshop. These papers are marked with an asterisk.

Video Coding and the Application Level Framing Protocol Architecture

Mon, 01 Jun 1992 00:00:00 GMT

Video Coding and the Application Level Framing Protocol Architecture Heybey, Andrew T. As networks and computers become faster, real time video transmission is expected to become common. Variable bit rate video coders will be used in order to take advantage of the statistical multiplexing gain and bandwidth efficiency of packet switched networks.

PIPES: Linguistic Support for Ordered Asynchronous Invocations

Wed, 01 Apr 1992 00:00:00 GMT

PIPES: Linguistic Support for Ordered Asynchronous Invocations Colbrook, Adrian; Brewer, Eric A.; Hsieh, Wilson C.; Wang, Paul; Weihl, William E. We describe pipes, a new linguistic mechanism for sequences of ordered asynchronous procedure calls in multiprocessor systems. Pipes allow a sequence of remote invocations to be performed in order, but asynchronously with respect to the calling thread.

Organization of Systems with Bussed Interconnections

Sun, 01 Mar 1992 00:00:00 GMT

Organization of Systems with Bussed Interconnections Kipnis, Shlomo This thesis explores using busses in communication architectures and control structures. First, we investigate the organization of permutation architectures with bussed interconnections. We explore how to efficiently permute data among VLSI chips in accordance with a predetermined set of permutations.

Algorithms for Exploring an Unknown Graph

Sun, 01 Mar 1992 00:00:00 GMT

Algorithms for Exploring an Unknown Graph Betke, Margrit We consider the problem of exploring an unknown strongly connected directed graph. We use the exploration model introduced by Deng and Papadimitriou [DP90]. An explorer follows the edges of an unknown graph until she has seen all the edges and vertices of the graph.

On the Sample Complexity of PAC-learning using Random and Chosen Examples

Sun, 01 Mar 1992 00:00:00 GMT

On the Sample Complexity of PAC-learning using Random and Chosen Examples Eisenberg, Bronwyn Bonnie Two protocols used for learning under the pac-learning model introduced by Valiant are learning from random examples and learning from memberships queries. Membership queries have been used to efficiently and exactly learn a concept class C that is too difficult to pac-learn using random examples.

Modeling Speaker Variability and Imposing Speaker Constrainst in Phonetic Classification

Sat, 01 Feb 1992 00:00:00 GMT

Modeling Speaker Variability and Imposing Speaker Constrainst in Phonetic Classification Niyogi, Partha This thesis deals with intra-speaker correlation analyses of speech sounds, and the possible utilization of this correlation to speech recognition. Current approaches to phonetic classification, regardless of whether they use context-dependent or -independent models, achieve classification based on locally optimum criteria.

A Distributed Data-balanced Dictionary Based on the B-link tree

Sat, 01 Feb 1992 00:00:00 GMT

A Distributed Data-balanced Dictionary Based on the B-link tree Johnson, Theodore; Colbrook, Adrian Many concurrent dictionary data structures have been proposed, but usually in the context of shared memory multiprocessors. In this paper, we present an algorithm for a concurrent distributed B-tree that can be implemented on message passing paralle

The Design and Implementation of a Parallel Persistent Object System

Sat, 01 Feb 1992 00:00:00 GMT

The Design and Implementation of a Parallel Persistent Object System Heytens, Michael L. This report describes Anga, an experimental persistent object system that we have developed that utilizes parallelism in a fundamental way to enhance performance. Parallelism is incorporated into the design of the system at all levels. We begin wit

File Systems with Multiple File Implementations

Sat, 01 Feb 1992 00:00:00 GMT

File Systems with Multiple File Implementations Stata, Raymie This thesis proposes ideas for designing file system software for the large, high-performance file server hardware we feel will be common in the middle to late nineties. In particular, the thesis examines the value and pragmatics of file systems wit

Preventing Recursion Deadlock in Concurrent Object-oriented Systems

Sat, 01 Feb 1992 00:00:00 GMT

Preventing Recursion Deadlock in Concurrent Object-oriented Systems Brewer, Eric A.; Waldspurger, Carl A. This paper presents solutions to the problem of deadlock due to recursion in concurrent object-oriented programming languages. Two language-independent, system-level mechanisms for solving this problem are proposed: a novel technique using multi-ported objects, and a named-threads scheme that borrows from previous work in distributed computing. We compare the solutions and present an analysis of their relative merits.

Full Abstraction and the Context Lemma 1

Sun, 01 Dec 1991 00:00:00 GMT

Full Abstraction and the Context Lemma 1 Jim, Trevor; Meyer, Albert R. It is impossible to add a combinator to PCF to achieve full abstraction for models such as Berry's stable domains in a way analogous to the addition of the "parallel-or" combinator that achieves full abstraction for the familiar cpo model.

Scalable Reader-writer Locks for Parallel Systems

Fri, 01 Nov 1991 00:00:00 GMT

Scalable Reader-writer Locks for Parallel Systems Hsieh, William C.; Weihl, William E. Current algorithms for reader-writer synchronization exhibit poor scalability because they do not allow readers to acquire locks independently. We describe two new algorithms for reader-writer synchronization that allow parallelism among readers during lock acquisition.

PRELUDE: A System for Portable Parallel Software

Tue, 01 Jan 1991 00:00:00 GMT

PRELUDE: A System for Portable Parallel Software Weihl, William Edward; Brewer, Eric A.; Colbrook, Adrian; Dellarocas, Chrysanthos N.; Hsieh, Wilson; Joseph, Anthony; Waldspurger, Carl; Wang, Paul This paper describes PRELUDE, a programming language and accompanying system support for writing portable MIMD parallel programs. PRELUDE supports a methodology for designing and organizing parallel programs that makes them easier to tune for particular architectures and to port to new architectures.

The Real-time Cost of Timing Uncertainty: Consensus and Failure Detection

Fri, 01 Nov 1991 00:00:00 GMT

The Real-time Cost of Timing Uncertainty: Consensus and Failure Detection Ponzio, Stephen J. In real distributed systems, processes may have only inexact information about the amount of real time needed for primitive operations such as process steps. This thesis studies the effect of this timing uncertainty on the real-time behavior of distributed systems.

Algorithms for Search Trees on Message-passing Architectures

Sun, 01 Sep 1991 00:00:00 GMT

Algorithms for Search Trees on Message-passing Architectures Colbrook, Adrian; Brewer, Eric A.; Dellarocas, Chrysanthos N.; Weihl, William E. In this paper we describe a new algorithm for maintaining a balanced search tree on a message-passing MIMD architecture; the algorithm is particularly well suited for implementation on a small number of processors.

Proteus: A High-performance Parallel-architecture Simulator

Sun, 01 Sep 1991 00:00:00 GMT

Proteus: A High-performance Parallel-architecture Simulator Brewer, Eric A.; Dellarocas, Chrysanthos N.; Colbrook, Adrian; Weihl, William E. PROTEUS is a high-performance simulator for MIMD multiprocessors. It is fast, accurate, and flexible: it is one to two orders of magnitude faster than comparable simulators, it can reproduce results from real multiprocessors, and it is easily configured to simulate a wide range of architectures.

The Use of Distinctive Features for Automatic Speech Recognition

Sun, 01 Sep 1991 00:00:00 GMT

The Use of Distinctive Features for Automatic Speech Recognition Meng, Helen Mei-Ling One of the most critical and yet unsolved problems in phonetic recognition is the transformation of the continuous speech signal to a discrete representation for accessing words in the lexicon. In order to find an efficient description of speech for recognition tasks, our research investigates to use distinctive features.

Secure Computation (Preliminary Report)

Thu, 01 Aug 1991 00:00:00 GMT

Secure Computation (Preliminary Report) Micali, Silvio; Rogaway, Phillip We define what it means for a network of communicating players to securely compute a function of privately held inputs. Intuitively, we wish to correctly compute its value in a computer manner which protects the privacy of each player's contribution, even though a powerful adversary may endeavor to disrupt this enterprise.

An Information-theoretical Approach to Studying Phoneme Collocational Constrainst

Mon, 01 Jul 1991 00:00:00 GMT

An Information-theoretical Approach to Studying Phoneme Collocational Constrainst Kassel, Robert Howard This thesis describe a lexical study of phoneme collocational constraints using a metric motivated by information theory. Phonologists have been describing the permissible combination of phonemes in the form of phonotactic rules. They have shown that these rules often can be expressed in terms of phoneme equivalence classes.

Algorithms for Approximate Graph Coloring

Sat, 01 Jun 1991 00:00:00 GMT

Algorithms for Approximate Graph Coloring Blum, Avrim

A High-performance Retargetable Simulator for Parallel Architectures

Sat, 01 Jun 1991 00:00:00 GMT

A High-performance Retargetable Simulator for Parallel Architectures Dellarocas, Chrysanthos N. The complexity of the interaction between software and hardware in MIMD machines makes experimental evaluation of parallel programs an import complement to theoretical analysis. Traditional techniques used to monitor the direct execution of programs are intrusive an d may lead to inaccurate results when applied to parallel programs.

Knowledge Representation for Supporting Decision Model Formulation in Medicine

Sat, 01 Jun 1991 00:00:00 GMT

Knowledge Representation for Supporting Decision Model Formulation in Medicine Leong, Tze-Yun Clinical decision making involves a large, complex, and ever-changing body of knowledge. Characterizing such knowledge illuminates the representational and computational requirements for automated clinical decision analysis.

The Round Complexity of Secure Protocols

Mon, 01 Apr 1991 00:00:00 GMT

The Round Complexity of Secure Protocols Rogaway, Phillip Assume we have a network of three of more players, each player in possession of some private input. The players want to compute some function of these private inputs, but in a way which protects the privacy of each participant's contribution.

Performance Tradeoffs in Multithreaded Processors

Mon, 01 Apr 1991 00:00:00 GMT

Performance Tradeoffs in Multithreaded Processors Agarwal, Anant High network latencies in large-scale multiprocessors can cause a significant drop in processor utilization. By maintaining multiple process contexts in hardware and switching among them in a few cycles, multithreaded processors can overlap computation with memory accesses and reduce processor idle time.

Randomness and Robustness in Hypercube Computation

Mon, 01 Apr 1991 00:00:00 GMT

Randomness and Robustness in Hypercube Computation Newman, Mark Joseph In this thesis we explore means by which hypercubes can compute despite faulty processors and links. We also study techniques which enable hypercubes to simulate dynamically changing networks and data structures.

Waiting Algorithms for Synchornization in Large-scale Multiprocessors

Fri, 01 Feb 1991 00:00:00 GMT

Waiting Algorithms for Synchornization in Large-scale Multiprocessors Lim, Beng-Hong; Agarwal, Anant Through analysis and experiments, this paper investigates two-phase waiting algorithms to minimize the cost of waiting for synchronization in large-scale multiprocessors. In a two-phase algorithm, a thread first waits by polling a synchronization variable.

An Evaluation of Concurrent Priority Queue Algorithms

Fri, 01 Feb 1991 00:00:00 GMT

An Evaluation of Concurrent Priority Queue Algorithms Huang, Qin The priority queue is a fundamental data structure that is used in a large variety of parallel algorithms, such as multiprocessor scheduling and parallel best-first search of state-space graphs.

The Spectral Norm of Finite Functions

Fri, 01 Feb 1991 00:00:00 GMT

The Spectral Norm of Finite Functions Bellare, Mihir In many recent results in learning and computational complexity theory which rely on Fourier analysis, the spectral norm plays a key role. An understanding of this quantity would appear to be useful in both gauging and exploiting these results, and in understanding the underlying techniques.

TCP Packet Trace Analysis

Fri, 01 Feb 1991 00:00:00 GMT

TCP Packet Trace Analysis Shepard, Timothy Jason Examination of a trace of packets collected from the network is often the only method available for diagnosing protocol performance problems in computer networks. This thesis explores the use of packet traces to diagnose performance problems of the transport protocol TCP.

Cache Coherence Protocols for Large-Scale Multiprocessors

Sat, 01 Sep 1990 00:00:00 GMT

Cache Coherence Protocols for Large-Scale Multiprocessors Chaiken, David Lars Caches have the potential to provide multiprocessors with an automatic mechanism for reducing both network traffic and average memory access latency. However, cache-based systems must address the problem of cache coherence.

An Incremental Type Inference System for the Programming Language ID

Thu, 01 Nov 1990 00:00:00 GMT

An Incremental Type Inference System for the Programming Language ID Gupta, Shail Aditya Modern computing environments strive to be robust and reliable, and at the same time, aim at providing enough flexibility to an interactive user to edit, debug, and test programs easily and efficiently.

Specification and Verification of Real-team Constraints in Coarse-grain Dataflow

Wed, 01 May 1991 00:00:00 GMT

Specification and Verification of Real-team Constraints in Coarse-grain Dataflow Henry, Dana S. We present a method for verifying real-time constraints in a distributed, coarse-grain dataflow environment starting with a program which has already been allocated onto a machine. The user specifies the timing of each module together with real-time constraints; and we verify the constraints.

On Retiming Synchronous Circuitry and Mixed-integer Optimization

Sat, 01 Sep 1990 00:00:00 GMT

On Retiming Synchronous Circuitry and Mixed-integer Optimization Papaefthymiou, Marios Christos In this paper we investigate properties of retiming, a circuit transformation which preserves the behavior of the circuit as a whole. We present an algorithm which transforms a given combinational circuit into a functionally equivalent pipelined circuit with minimum latency and clock-period no greater than a given upper bound c.

Lazy Replication: Exploiting the Semantics of Distributed Services

Sun, 01 Jul 1990 00:00:00 GMT

Lazy Replication: Exploiting the Semantics of Distributed Services Ladin, Rivka; Liskov, Barbara; Shrira, Liuba; Ghemawat, Sanjay To provide high availability for services such as mail or bulletin boards, data must be replicated. One way to guarantee consistency of replicated data is to force service operations to occur in the same order at all sites, but this approach is expensive.

Design and Implementation of a Packet Switched Routing Chip

Sat, 01 Dec 1990 00:00:00 GMT

Design and Implementation of a Packet Switched Routing Chip Joerg, Christopher Frank Monsoon is a parallel processing dataflow computer that will require a high bandwidth interconnection network. A packet switched routing chip (PaRC) is described that will be used as the basis of this network. PaRC is a 4 by 4 routing switch which has been designed and fabricated as a CMOS gate array.

On the Complexity of Computing Algebraic Functions

Sat, 01 Sep 1990 00:00:00 GMT

On the Complexity of Computing Algebraic Functions Mansour, Yishay This research addresses the problem of proving lower bounds on the complexity of algebraic computations involving the floor operation. The model of computation considered is a computation tree with the set of basic operations {+,-,*,*,[.],._ }. The constants available to the computation are 0 and 1, and every other constant needs to be generated explicitly.

Analysis of the Held-Karp Heuristic for the Traveling Salesman Problem

Fri, 01 Jun 1990 00:00:00 GMT

Analysis of the Held-Karp Heuristic for the Traveling Salesman Problem Williamson, D.P. The Held-Karp heuristic for the Traveling Salesman Problem (TSP) has in practice provided near-optimal lower bounds on the cost of solutions to the TSP. We analyze the structure of Held-Karp solutions in order to shed light on their quality.

Efficient At-most-once Messages Based on Synchronized Clocks

Sun, 01 Apr 1990 00:00:00 GMT

Efficient At-most-once Messages Based on Synchronized Clocks Liskov, Barbara; Shrira, Liuba; Wroclawski, John This paper describes a new message passing protocol that provides guaranteed detection of duplicate messages even when the receiver has no state stored for the sender.

Disconnected Actions: An Asynchronous Extensions to a Nested Atomic Action System

Mon, 01 Jan 1990 00:00:00 GMT

Disconnected Actions: An Asynchronous Extensions to a Nested Atomic Action System Ben-Zvi, Boaz Nested transactions, a generalization of atomic transactions, provide a uniform mechanism for coping with failures and obtaining concurrency within an action.

Automatic Replication for Highly Available Services

Thu, 01 Mar 1990 00:00:00 GMT

Automatic Replication for Highly Available Services Ghemawat, Sanjay Replicating various components of a system is a common technique for providing highly available services in the presence of failures. A replication scheme is a mechanism for organizing these replicas so that as a group they provide a service that has the same semantics as the original unreplicated service. Viewstamped replication is a new replication scheme for providing high availability.

Rate-based Congestion Control in Networks with Smart Links

Mon, 01 Jan 1990 00:00:00 GMT

Rate-based Congestion Control in Networks with Smart Links Heybey, Andrew Tyrrell I use a network simulator to explore rate-based congestion control in networks with "smart" links that can feed back information to tell senders to adjust their transmission rates. This method differs in a very important way from congestion control in

ML with Extended Pattern Matching and Subtypes

Tue, 01 Aug 1989 00:00:00 GMT

ML with Extended Pattern Matching and Subtypes Jategaonkar, Lalita A. We extend a fragment of the programming language ML by incorporating a more general form of record pattern matching and providing for user-declared subtypes. Together, these two enhancements may be used to support a restricted object-oriented program

Probabilistic Reasoning in the Domain of Genetic Counseling

Sun, 01 Oct 1989 00:00:00 GMT

Probabilistic Reasoning in the Domain of Genetic Counseling Harris, Nomi L. This paper describes a program, GENINFER, which uses belief networks to calculate risks of inheriting genetic disorders. GENINFER is based on Judea Pearl's [17] algorithm for fusion and propagation in probabilistic belief networks. These networks allow the effects of various pieces of information to be propagated and fused in such a way that, when equilibrium is reached, each proposition can be assigned a degree of believe consistent with the axioms of probability theory.

Should a Function Continue?

Fri, 01 Sep 1989 00:00:00 GMT

Should a Function Continue? Riecke, Jon Gary We show that two l-calculus terms can be observationally congruent (i.e., agree in all contexts) but their continuation-passing transforms may not be. We also show that two terms may be congruent in all untyped contexts but fail to be congruent in a language with call/ cc operators, and that two terms may have the same meaning in a direct semantics but in a continuation semantics.

Typechecking is Undecidable when 'Type' is a Type

Fri, 01 Dec 1989 00:00:00 GMT

Typechecking is Undecidable when 'Type' is a Type Reinhold, Mark B. A function has a dependent type when the type of its result depends upon the value of its argument. The type of all types is the type of every type, including itself. In a typed l-calculus, these two features synergize in a conceptually clean and uniform way to yield enormous expressive power at very little apparent cost. By reconstructing and analyzing a paradox due to Girard, we argue that there is no effective typechecking algorithm for such a language.

Using Cycles and Scaling in Parallel Algorithms

Tue, 01 Aug 1989 00:00:00 GMT

Using Cycles and Scaling in Parallel Algorithms Stein, Clifford We introduce the technique of decomposing an undirected graph by finding a maximal set of edge-disjoint cycles. We give a parallel algorithm to find this decomposition in O(log n) time on (m+ n)/log n processors. We then use this decomposition to to give the first efficient parallel algorithm for finding an approximation to a minimum cycle cover.

ParaTran: A Transparent, Transaction Based Runtime Mechanism for Parallel Execution of Scheme

Sat, 01 Jul 1989 00:00:00 GMT

ParaTran: A Transparent, Transaction Based Runtime Mechanism for Parallel Execution of Scheme Katz, Morry The number of applications requiring high speed symbolic computation and the performance requirements of these projects are both rapidly increasing. However, the computer science community's ability to produce high performance uniprocessor hardware is being outstripped by these needs. Therefore, we propose a unique multiprocessing solution to the high speed, symbolic computation problem. Our approach is to develop a transparent runtime mechanism for executing standard, sequential Lisp code on a multiprocessor computer.

Optimistic Concurrency Control for Nested Distributed Transactions

Thu, 01 Jun 1989 00:00:00 GMT

Optimistic Concurrency Control for Nested Distributed Transactions Gruber, Robert Edward Optimistic concurrency control techniques allow atomic transactions (or actions for short) to execute without synchronization, relying on commit-time validation to ensure serializability. Previous work in this area has focussed on single-level actions. This thesis extends previous work on optimistic concurrency control to distributed system with nested actions.

A Study of Backoff Barrier Synchronization

Thu, 01 Jun 1989 00:00:00 GMT

A Study of Backoff Barrier Synchronization Cherian, Mathews Malieakkal Shared-memory multiprocessors commonly use shared variables for synchronization. Simulations of real parallel applications show that large-scale cache-coherent multiprocessors suffer significant amounts of invalidation traffic due to synchronization. Large multiprocessors that do not cache synchronization variables are often more severely impacted.

Network Layer Protocols with Byzantine Robustness

Sat, 01 Oct 1988 00:00:00 GMT

Network Layer Protocols with Byzantine Robustness Perlman, Radia The Network Layer of a network architecture is a distributed protocol that facilitates packet delivery across multiple hops. One of its chief functions is the calculation of routes throughout the network. Traditional Network Layer protocols have addressed robustness in the face of simple failures, i.e. nodes or links becoming inoperative. This thesis examines Network Layer protocol designs that are robust in the presence in the Byzantine failures, i.e., nodes that through malice or malfunction exhibit arbitrary behavior such as corrupting, forging, or delaying routing protocol messages.

Code-mapping Policies for the Tagged-token Dataflow Architecture

Sun, 01 May 1988 00:00:00 GMT

Code-mapping Policies for the Tagged-token Dataflow Architecture Maa, Gino K. Multiprocessing seems to be the only viable way to gain significant speedup beyond that afforded by performance advances in semiconductor devices and hardware construction, which are beginning to face the limitations of physics. Although it is relatively easy to improve the "raw" computational performance of a system simply by adding more processors to it, the far more difficult task is to insure that the additional resources actually reduce a program's computing time.

A Fault-tolerant Network Kernel for Linda

Mon, 01 Aug 1988 00:00:00 GMT

A Fault-tolerant Network Kernel for Linda Xu, Andrew S. The parallel programming system Linda consists of a number of processes and a shared memory called the tuple space. In a distributed implementation of Linda, processes and the tuple space reside on different computing nodes connected by a communications network subject to a variety of node and network failures.

Viewstamped Replication for Highly Available Distributed Systems

Mon, 01 Aug 1988 00:00:00 GMT

Viewstamped Replication for Highly Available Distributed Systems Oki, Brian Masao This dissertation presents viewstamped replication, a new algorithm for the implementation of highly available computer services that continue to be usable in spite of node crashes and network partitions. Our goal is to design an efficient mechanism that makes it easy for programmers to implement these services without complicating the programming model.

FX-87 Performance Measurements: Dataflow Implementation

Thu, 01 Sep 1988 00:00:00 GMT

FX-87 Performance Measurements: Dataflow Implementation Hammel, R. Todd; Gifford, David K. We analyze how much the FX-87 static effect system can improve the execution times of five benchmark programs on a parallel graph interpreter. Three of our benchmark programs do not use side-effects (factorial, fibonacci, and polynomial division) an

Compsoing Data & Process Descriptions in the Design of Software Systems

Sun, 01 May 1988 00:00:00 GMT

Compsoing Data & Process Descriptions in the Design of Software Systems Jackson, Daniel Two paradigms are dominant in software development, the data paradigm and the process paradigm. Our contention is that relying exclusively on either is counter-productive. In the data paradigm, a system is specified as operations acting on states. The process paradigm focuses on sequences of events.

Automatic Qualitative Analysis of Ordinary Differential Equations Using Piecewise Linear Approximations

Tue, 01 Mar 1988 00:00:00 GMT

Automatic Qualitative Analysis of Ordinary Differential Equations Using Piecewise Linear Approximations Sacks, Elisha, Peretz This thesis explores automating the qualitative analysis of physical systems. Scientists and engineers model many physical systems with ordinary differential equations. They deduce the behavior of the system by analyzing the equations. Most realistic models are nonlinear, hence difficult or impossible to solve explicitly.

A High-level Signal Processing Programming Language

Tue, 01 Mar 1988 00:00:00 GMT

A High-level Signal Processing Programming Language Hicks, James Edward, Jr. The motivations for an abstract, diagrammatic signal processing language are presented along with a study of the semantics that such language should have. D-PICT, the proposed Digital Signal Processing Pictorial Language, is thoroughly described. D-PICT has a diagrammatic representation with a corresponding textual representation.

Diversity-based Inference of Finite Automata

Sun, 01 May 1988 00:00:00 GMT

Diversity-based Inference of Finite Automata Schapire, Robert Elias We present a new procedure for inferring the structure of a finite-state automaton (FSA) from its input/output behavior, using access to the automaton to perform experiments. Our procedure uses a new representation for FSA's, based on the notion of equivalence between tests.

Constructing a Highly-available Location Service for a Distributed Environment

Fri, 01 Jan 1988 00:00:00 GMT

Constructing a Highly-available Location Service for a Distributed Environment Jing-Hwa Hwang, Deborah One possible advantage a distributed system has over a centralized system is the ability to move objects from one node to another. For example, we may want to move an object if the node where it resides is overloaded. This thesis proposes to use a location service to aid in finding objects that move.

A Technique for Constructing Highly-Available Services

Fri, 01 Jan 1988 00:00:00 GMT

A Technique for Constructing Highly-Available Services Ladin, Rivka; Liskov, Barbara; Shrira, Liuba This paper describes a general method for constructing a highly available service for use in a distributed system. It gives a specific implementation of the method and proves the implementation correct. The service consists of replicas that reside at several different locations in a network. It presents its clients with a consistent view of its state, but the view may contain old information.

CALVIN: A Rule Based Expert System for Improving Arrhymia Detector Performance During Noisy ECGS

Tue, 01 Sep 1987 00:00:00 GMT

CALVIN: A Rule Based Expert System for Improving Arrhymia Detector Performance During Noisy ECGS Muldrow, Warren K. Human experts far outperform automated arrhythmia detectors in analyzing ECG data corrupted by noise and artifact. Humans make use of considerable a priori knowledge about cardiac electrophysiology and knowledge acquired from the specific ECG under analysis. R-R interval, coupling intervals of ectopic beats, and commonly occurring beat patterns observed during noise-free ECG segments form a knowledge base which is used in accurately detecting and classifying true QRS complexes in the presence of severe noise.

A Framework for Incorporating Abstraction Mechanisms into the Logic Programming Paradigm

Sat, 01 Aug 1987 00:00:00 GMT

A Framework for Incorporating Abstraction Mechanisms into the Logic Programming Paradigm Zachary, Joseph Lawrence To help make logic programming more suitable for writing large systems, we develop linguistic mechanisms that permit the organization of logic programs around abstractions. In particular, we present the design of Danali, an equational logic programming language that supports predicate and data abstraction.

Rule Based Analysis of Computer Security

Tue, 01 Mar 1988 00:00:00 GMT

Rule Based Analysis of Computer Security Baldwin, Robert W. Computers are rarely as secure as they could be. Users are lax or inconsistent in the way they configure a computer's protection system, and these user mistakes often lead to serious security holes. For example, a privileged user might accidentally make his login initialization file publicly writable and that mistake could allow ordinary users to acquire super-user privileges.

Argus Reference Manual

Sun, 01 Nov 1987 00:00:00 GMT

Argus Reference Manual Liskov, Barbara; Day, M.; Herlihy, M.; Johnson, P.; Leavens, G. Argus is an experimental language/system designed to support the construction and execution of distributed programs. Argus is intended to support only a subset of the applications that could benefit from being implemented by a distributed program. Two properties distinguish these applications: they make use of on-line data that must remain consistent in spite of concurrency and hardware failures, and they provide services under real-time constraints that are not severe.

Walter User's Manual (Version 1.0)

Tue, 01 Sep 1987 00:00:00 GMT

Walter User's Manual (Version 1.0) Gifford, David K.; Cote, Robert G.; Segal, David A. Walter is a UNIX program that provides access to databases located at MIT via the DARPA Internet. The databases provided by Walter include the full-text of the New York Times for the past 90 days. A sophisticated full-text query language is provided, and Walter uses a query routing algorithm to direct requests to the proper database server at MIT.

Clipping Service User's Manual (Version 1.2)

Tue, 01 Sep 1987 00:00:00 GMT

Clipping Service User's Manual (Version 1.2) Gifford, David K.; Cote, Robert G.; Segal, David A. The Clipping Service is a program that will send selected stories from the New York Times and other information sources to you via electronic mail. In order to use the Clipping Service, you first describe your interests to the Clipping Service in a simple full-text query language, and then mail this interest profile to the DARPA Internet mail address clip@db.lcs.mit.edu.

Boston Community Information System - 1986 Experimental Test Results

Sat, 01 Aug 1987 00:00:00 GMT

Boston Community Information System - 1986 Experimental Test Results Gifford, David K.; Heitmann, Dawn; Segal, David A.; Cote, Robert G.; Tanacea, Kendra; Burmaster, David E. This report describes the first year of an experimental test of the Boston Community Information System (Boston CommInS). The experiment implements new ideas of data communication and database design in the transmission and reception of data. The system offers the Associated Press and New York Times to participants and is provided in exchange for their monthly feedback.

KOLA: Knowledge Organization Language

Sat, 01 Oct 1988 00:00:00 GMT

KOLA: Knowledge Organization Language Jang, Yeona The focus of this research is on a representation of knowledge that captures the structure of a domain into the computational model for efficient retrieval and reasoning. With this desideratum in mind, a concept-based knowledge representation system KOLA (Knowledge Organization LAnguage ) is described.

Communication Patterns in a Symbolic Multiprocessor

Mon, 01 Jun 1987 00:00:00 GMT

Communication Patterns in a Symbolic Multiprocessor Nuth, Peter Robert An important design decision for large scale multiprocessors is the balance of processor power to communication network bandwidth. In order to evaluate different design alternatives, it is necessary to be able to predict the load imposed on the network by a programming model. This thesis quantifies that communication load for a model of parallel symbolic computing using the Multilisp language.

Dynamic Programming on Graphs with Bounded Treewidth

Mon, 01 Jun 1987 00:00:00 GMT

Dynamic Programming on Graphs with Bounded Treewidth Bodlaender, Hans L. In this paper we study the complexity of graph decision problems, restricted to the class of graphs with treewidth k, (or equivalently, the class of partial k-trees), for fixed k. We introduce two classes of graph decision problems, LCC and ECC, and subclasses C-LCC, and C-ECC. We show that each problem in LCC (or C-LCC) is solvable in polynomial (O(nc)) time, when restricted to graphs with fixed up-perbounds on the treewidth and degree; and that each problem in ECC (or C- ECC) is solvable in polynomial (O(n c)) time, when restricted to graphs with a fixed upperbound on the treewidth (with given corresponding tree-decomposition).

Synthesis of Self-timed VLSI Circuits from Graph-theoretic Specifications

Mon, 01 Jun 1987 00:00:00 GMT

Synthesis of Self-timed VLSI Circuits from Graph-theoretic Specifications Chu, Tam-Anh This thesis presents an approach for direct and efficient synthesis of self-timed (asynchronous) control circuits from formal specifications called Signal Transition Graphs (STGs). Control circuits synthesized from this graph model are speed-independent and capable of performing concurrent operation.

MAM: A Semi-automatic Debugging Tool for Distrubuted Programs

Mon, 01 Jun 1987 00:00:00 GMT

MAM: A Semi-automatic Debugging Tool for Distrubuted Programs Kolodney, Lawrence Kenneth Traditional debuggers, designed to examine single process serial programs, do not provide sufficient functionality for efficient debugging of distributed programs. There are a number of fundamental differences in the way in which a programmer understands the execution of a distributed programs, and a debugger must present data to its user in light of that fact.

Efficient Methods for Calculating Maximum Entropy Distributions

Fri, 01 May 1987 00:00:00 GMT

Efficient Methods for Calculating Maximum Entropy Distributions Goldman, Sally A. We present a new algorithm for computing the maximum entropy probability distribution satisfying a set of constraints. Unlike previous approaches, our method is integrated with the planning of data collection and tabulation. We show how adding constraints and performing the associated additional tabulations can substantially speed up computation by replacing the usual iterative techniques with a straight-forward computation.

Data Replication in Nested Transaction Systems

Fri, 01 May 1987 00:00:00 GMT

Data Replication in Nested Transaction Systems Goldman, Kenneth J. Gifford's basic Quorum Consensus algorithm for data replication is generalized to accommodate nested transactions and transaction failures (aborts). A formal description of the generalized algorithm is presented using the new Lynch-Merritt input-output automaton model for nested transaction systems.

Temporal Reasoning in Medical Expert Systems

Fri, 01 May 1987 00:00:00 GMT

Temporal Reasoning in Medical Expert Systems Kohane, Isaac S. Diseases develop and change over time. Much of the distinction between pathophysiological complexes rests on the temporal relations of their component events. Therefore, knowledge bases that fail to capture the temporal component of the course of disease omit useful diagnostic knowledge.

An Expert System for Diagnosing Gait for Cerebral Palsy Patients

Fri, 01 May 1987 00:00:00 GMT

An Expert System for Diagnosing Gait for Cerebral Palsy Patients Hirsch, David Edward Many first generation expert systems in medicine assumed that a single fault was the cause of the patient's problems. However, this is not always so and in the domain of gait analysis this is usually not the case. This work looks at an expert system for performing gait analysis on cerebral palsy patients. The system is able to handle cases where there are many interacting faults causing the patient's gait deviations.

Hierarchical Correctness Proofs for Distributed Algorithms

Wed, 01 Apr 1987 00:00:00 GMT

Hierarchical Correctness Proofs for Distributed Algorithms Lynch, Nancy A.; Tuttle, Mark S. This thesis introduces a new model for distributed computation in asynchronous networks, the input-output automaton. This simple, powerful model captures in a novel way the game-theoretical interaction between a system and its environment, and allows fundamental properties of distributed computation such as fair computation to be naturally expressed.

A Simulation Environment for Schema

Mon, 01 Dec 1986 00:00:00 GMT

A Simulation Environment for Schema St.Pierre, Margaret Ann In present day circuit design, many independent simulation tools are available for analyzing circuits at various levels of detail. This thesis presents a framework to tie these tools into the Simulation Environment in Schema, an integrated CAD system. The framework easily incorporates additional simulators, serves as a foundation upon which to build new analysis tools, and provides the ability for mixed-mode simulation.

Data Flow Computer Architecture Final Report

Thu, 01 Oct 1987 00:00:00 GMT

Data Flow Computer Architecture Final Report Dennis, Jack B. This report covers the work done by Computation Structures Group of the MIT Laboratory for Computer Science on developing models, languages, and architectures for data flow computation from 1966 to the end 1985. The work was supported by research grants and contracts from NSF, the University of California, DOE, NASA, and DARPA having periods of support as follows: Advanced Research Projects Agency (DARPA).

Remote Pipe and Procedures for Efficient Distributed Communication

Wed, 01 Oct 1986 00:00:00 GMT

Remote Pipe and Procedures for Efficient Distributed Communication Gifford, D. A new communication model for distributed systems is described that combines the advantages of remote procedure call with the efficient transfer of bulk data. Three ideas form the basis of this model. First, remote procedures are first-class values which can be freely exchanged among nodes, thus enabling a greater variety of protocols to be directly implemented in a remote procedure call framework.

Congestion Control in Routing Networks

Wed, 01 Oct 1986 00:00:00 GMT

Congestion Control in Routing Networks Chien, Andrew Andai Multistage routing networks present an attractive cost-effective method of interconnection for medium to large scale multiprocessors. Recent results concerning performance degradation in the presence of "hot spots" have raised serious questions about the robustness of previous performance estimates for these routing networks. Research to date has focused on a limited class of hot spots-those in which all the hot spot traffic is destined for the same memory address.

Logic Simulation of a Multiprocessor

Wed, 01 Oct 1986 00:00:00 GMT

Logic Simulation of a Multiprocessor Bradley, Elizabeth The performance of circuit simulators running on SISD computers is fundamentally limited by the Von Neumann bottleneck. Multiprocessors do not share this limitation. The task of solving the equations for the many parallel signal paths found in most circuits lends itself readily to concurrent computation. for both of these reasons, parallel processing is a highly promising approach to circuit simulation. This thesis explores several facets of this problem.

The Notion of Security for Probabilistic Public-key Cryptosystems

Wed, 01 Oct 1986 00:00:00 GMT

The Notion of Security for Probabilistic Public-key Cryptosystems Sloan, Robert Hal The purpose of a cryptosystem is to allow people to communicate securely over an open channel. Before one can discuss whether a cryptosystem meets this goal, however, one must first rigorously define what is meant by security. Three very different formal definitions of security for public-key cryptosystems have been proposed-two by Goldwasser and Micali and one by Yao. In this thesis, it is shown that the three definitions are essentially equivalent.

MACE: A Multiprocessing Approach to Circuit Extraction

Wed, 01 Oct 1986 00:00:00 GMT

MACE: A Multiprocessing Approach to Circuit Extraction Levitin, Samuel M.; Terman, Christopher J.; Slater, Kenneth H. The ever-increasing complexity of VLSI chips threaten to choke out all available computer power unless methods are devised to keep the CAD tasks conveniently sized. A review of the current methods of multiprocessing approaches in the domain of layout verification precedes the discussion of current work.

Long Atomic Computations

Wed, 01 Oct 1986 00:00:00 GMT

Long Atomic Computations Ng, Pui Distributed computing systems are becoming commonplace and offer interesting opportunities for new applications. In a practical system, the problems of synchronizing concurrent computations and recovering from failures must be dealt with effectively.

Replication and Reconfiguration in a Distributed Mail Repository

Wed, 01 Apr 1987 00:00:00 GMT

Replication and Reconfiguration in a Distributed Mail Repository Day, Mark S. Conventional approaches to programming produce centralized programs that run on a single computer. However, an unconventional approach can take advantage of low-cost communication and small, inexpensive computers. A distributed program provides service through programs executing at several nodes of a distributed system.

Efficient Graph Algorithms for Sequential and Parallel Computers

Sun, 01 Feb 1987 00:00:00 GMT

Efficient Graph Algorithms for Sequential and Parallel Computers Goldberg, Andrew V.

Boston Community Information System User's Manual

Thu, 01 Oct 1987 00:00:00 GMT

Boston Community Information System User's Manual Segal, David A.; Gifford, David K.; Lucassen, John M.; Henderson, James B.; Berlin, Stephen T.; Burmaster, David E. The Boston Community Information System turns your computer into a personal information assistant that monitors the news as it happens. This experiment, CommInS, tests a new way of distributing world news as it happens and features from the New York Times and the Associate Press wire service directly to personal computers via radio waves.

Compaction with Automatic Job Introduction

Sat, 01 Nov 1986 00:00:00 GMT

Compaction with Automatic Job Introduction Maley, F. Miller This thesis presents an algorithm for one-dimensional compaction of VLSI layouts. It differs from older methods in treating wires not as objects to be moved, but as constraints on the positions of other circuit components. These constraints are determined for each wiring layer using the theory of planar routing.

A Compiler for the MIT Tagged-token Dataflow Architecture

Fri, 01 Aug 1986 00:00:00 GMT

A Compiler for the MIT Tagged-token Dataflow Architecture Traub, Kenneth R. Compilation of the programming language Id Nouveau into machine code for the MIT tagged-token dataflow architecture is thoroughly described. Id Nouveau is a higher-order functional language augmented with a novel data structure facility known as I-Structures. The tagged-token dataflow architecture is a dataflow computer of the dynamic variety.

Programming Simultaneous Action Using Common Knowledge

Sun, 01 Feb 1987 00:00:00 GMT

Programming Simultaneous Action Using Common Knowledge Moses, Yoram; Tuttle, Mark R. This work applies the theory of knowledge in distributed systems to the design of efficient fault-tolerant protocols. We define a large class of problems requiring coordinated, simultaneous action in synchronous systems, and give a method of transforming specifications of such problems into protocols that are optimal in all runs: for every possible input to the system and faculty processor behavior, these protocols are guaranteed to perform the simultaneous actions as soon as any other protocol could possibly perform them.

The X Window System

Wed, 01 Oct 1986 00:00:00 GMT

The X Window System Scheifler, Robert W.; Gettys, Jim An overview of the X Window System is presented, focusing on the system substrate and the low-level facilities provided to build applications and to manage the desktop. The system provides high-performance, high-level, device-independent graphics.

Introduction to the Theory of Nested Transactions

Tue, 01 Jul 1986 00:00:00 GMT

Introduction to the Theory of Nested Transactions Lynch, Nancy A.; Merritt, Michael A new formal model is presented for studying concurrency and resiliency properties for nested transactions. The model is used to state and prove correctness of a well-known locking algorithm.

The Clock Distribution Systems of the Multiprocessor Emulation Facility

Sun, 01 Jun 1986 00:00:00 GMT

The Clock Distribution Systems of the Multiprocessor Emulation Facility Younis, Saed G. Consisting of 32 high-speed processors, the multiple processor emulation facility communicates data between its processors through the use of synchronous, high-bandwidth packet switches residing on the ports of every processor. Because of the synchronous nature of these packet switches, there was a need to design a clock distribution system that can distribute a clock signal to the 32 ports with as little clock skew as possible.

ID World: An Environment for the Development of a Dataflow Programs Written in ID

Thu, 01 May 1986 00:00:00 GMT

ID World: An Environment for the Development of a Dataflow Programs Written in ID Morais, Dinarte R. The ID WORLD involves the interfacing of a compiler, interpreter, debugger and editor mode to create an environment for the development of dataflow programs written in ID. It replaces the Tagged-Token Dataflow Architecture (TTDA) Emulator as the foundation for Multiprocessor Emulation Facility at the Laboratory for Computer Science,M.I.T.

Correctness Conditions for Highly Available Replicated Databases

Sun, 01 Jun 1986 00:00:00 GMT

Correctness Conditions for Highly Available Replicated Databases Lynch, Nancy A.; Blaustein, Barbara; Siegel, Michael Correctness conditions are given which describe some of the properties exhibited by highly available distributed database systems such as the SHARD (System for Highly Available Replicated Data) system currently being developed at Computer Corporation of America. This system allows a database application to continue operation in the face of communication failures, including network partitions.

Exploiting Parallelism in VLSI CAD

Wed, 01 Jan 1986 00:00:00 GMT

Exploiting Parallelism in VLSI CAD Marantz, Joshua David In the domain of computer science, particularly VLSI CAD, an increasing amount of engineering time is spent running compute-bound programs. Many of these programs have an intrinsic parallelism that is externally accessible. This thesis describes a novel software system that uses a small number of independent computers connected by a network to exploit the parallelism inherent in existing software, and thereby, reduce its running time.

Simulating Applicative Architectures on the Connection Machine

Sun, 01 Jun 1986 00:00:00 GMT

Simulating Applicative Architectures on the Connection Machine Kuszmaul, Bradley C. The Connection Machine (CM) is a highly parallel single instruction multiple data (SIMD) computer, which has been described as "a huge piece of hardware looking for a programming methodology.' Applicative languages, on the other hand, can be described as a programming methodology looking for a parallel computing engine.

Bounded Width Branching Programs

Sun, 01 Jun 1986 00:00:00 GMT

Bounded Width Branching Programs Barrington, David A. We examine the branching program model of computation and in particular the classes of languages which can be recognized when the width of the programs is bounded by a constant. After slightly revising the framework of definitions to sharpen analogies with other models, we prove that width 5 polynomial size branching programs can recognize exactly the parallel complexity class NC1, refuting a conjecture of Borodin et al. in [BDFP83].

Intelligent Physiologic Modeling

Tue, 01 Apr 1986 00:00:00 GMT

Intelligent Physiologic Modeling Kunstaetter, Robert This thesis describes the design and implementation of a knowledge based physiologic modeling systems (KBPMS) and a preliminary evaluation of its use as a learning resource within the context of an experimental medical curriculum--the Harvard New Pathway. KBPMS posesses combined numeric and qualitative simulation capabilities and provide explanations of its knowledge and behaviour.

A CATV-Based High-speed Packet-switching Network Design

Tue, 01 Apr 1986 00:00:00 GMT

A CATV-Based High-speed Packet-switching Network Design Feldmeier, David Charles A high-speed packet-switching data network to the home can be built on an existing, unmodified, residential cable television (CATV) system. This thesis considers the theoretical and practical technical aspects of providing such a service and suggest a possible system design.

Primitives for Real-time Animation in Three Dimensions

Tue, 01 Apr 1986 00:00:00 GMT

Primitives for Real-time Animation in Three Dimensions Chaing, Carol J. We present a general purpose imaging model which can efficiently produce computer-generated animated scenes. Displaying sophisticated graphics scenes is a computationally complex operation. Thus, an efficient imaging model is necessary for producing real-time motion.

Computation Management in a Single Address Space System

Wed, 01 Jan 1986 00:00:00 GMT

Computation Management in a Single Address Space System Gibson, James C. A multiprogramming operating system needs a mechanism to recover from the termination of one of its computations. Cleaning up, or unlinking a terminated computation from those remaining requires identifying the end of a computation, freeing resources that the computation was using, and shutting down its interfaces with other computations.

Logical Structure for Functional Languages

Sat, 01 Feb 1986 00:00:00 GMT

Logical Structure for Functional Languages Beckerle, Michael J. Functional Programming is frequently advocated as an appropriate programming discipline for parallel processing because of the difficulty of extracting parallelism from programs written in conventional sequential programming languages. Unfortunately, the use of Functional operations often implies excessive copying or unnecessary sequentiality in the access and construction of data structures.

Data Structure Management in a Data Flow Computer System

Wed, 01 May 1985 00:00:00 GMT

Data Structure Management in a Data Flow Computer System Guharoy, Bhaskar VIM is an experimental computer system being developed at MIT for supporting functional programming. The execution mechanism of the computer is based on data flow. This thesis presents mechanisms for managing data structures in this system. This thesis also develops a methodology for designing computers, which is based on successive refinement of formal models of the computer.

Remote Evaluation

Wed, 01 Jan 1986 00:00:00 GMT

Remote Evaluation Stamos, James William A new technique for computer-to-computer communication is presented that can increase the generality and performance of distributed systems. This technique, called Remote Evaluation, lets one computer send another computer a request in the form of a program. A computer that receives such a request executes the program in the request and returns the results to the sending computer.

Data Backup and Recovery in a Computer Architecture for Functional

Tue, 01 Oct 1985 00:00:00 GMT

Data Backup and Recovery in a Computer Architecture for Functional Jagannathan, Suresh The Vim computer system, an experimental project under development in the MIT/LCS Computation Structures Group, is intended to examine the efficient implementation of functional languages using the principles of data flow computation. In this thesis, we examine how to incorporate backup and recovery mechanisms into this system to guarantee that no online information is lost because of hardware malfunction.

Boston Community Information System User Manual (Version 6.0)

Tue, 01 Apr 1986 00:00:00 GMT

Boston Community Information System User Manual (Version 6.0) Lucassen, John M.; Gifford, David K.; Berlin, Stephen T.; Burmaster, David E. The Boston Community Information System turns your computer into a personal information assistant that monitors the news as it happens. This experiment, CommInS, tests a new way of distributing world news and features from the New York Times and the Associated Press (AP) wire service directly to personal computers via radio waves.

The Complexity of Graph Layour and Channel Routing for VLSI

Wed, 01 Feb 1984 00:00:00 GMT

The Complexity of Graph Layour and Channel Routing for VLSI Bhatt, Sandeep N. This thesis is motivated by the need for a clearer understanding of various issues in VLSI layout. Within a formal setting, we identify critical properties of circuits that determine the quality of their layouts.

Design and Implementation of a Distributed Program for Collaborative Editing

Sun, 01 Sep 1985 00:00:00 GMT

Design and Implementation of a Distributed Program for Collaborative Editing Seliger, R. This thesis presents the design and implementation of a distributed program for the support of multi-author collaboration on shared documents. The Collaborative Editing System, CES, provides an environment in which authors working on a document can cooperate and coordinate their individual contributions to a single document.

On Playing Well in a Sum of Games

Thu, 01 Aug 1985 00:00:00 GMT

On Playing Well in a Sum of Games Yedwab, Laura Many games are naturally described as a sum of games, e.g., nim and the endgame of Go. Let G ,...,G represent n games. Then a move in the sum G + ...+G consists of picking a component game G and making a move in G .. This thesis analyzes play in a sum of games from three different perspective: computational complexity, approximate solutions, and optimal research algorithms.

A Relative-motion Microworld

Sun, 01 Sep 1985 00:00:00 GMT

A Relative-motion Microworld Morecroft, Linda E. A relative-motion microworld has been designed to aid high-school students in understanding the concepts of relative motion and frames of reference. Relative motion and frames of reference are usually introduced in a high-school physics or mathematics course. Most students, and many teachers too, have difficulty understanding the concepts and applying them to solve problems. The traditional approach to relative motion uses vector algebra. However, vector terminology is complex and it does not allow a mental picture of what is happening to be easily built. students do not understand what it means to be in a different frame of reference and how moving objects appear within this reference frame. Most people have a much more intuitive approach to motion problems.

Equational Theories and Database Constraints

Thu, 01 Aug 1985 00:00:00 GMT

Equational Theories and Database Constraints Cosmadakis, Stavros Stylianos The implication problem for database constraints is central in the fields of automated schema design and query optimization and has been traditionally approached with resolution-based techniques. We present a novel approach to database constraints, using equations instead of Horn clauses.

A Generalized Approach to Equational Unification

Thu, 01 Aug 1985 00:00:00 GMT

A Generalized Approach to Equational Unification Yelick, Katherine Anne Given a set of equational axioms and two terms containing function symbols and variables, the equational unification problem is to find a uniform replacement of terms for the variables that makes the terms provably equal from the axioms. In the variable-only case, the two terms contain only variables and function symbols from the axioms. In the general case, the terms may contain symbols not appearing in the axioms, there may be more than on instance of a set of anxioms, and there may be more than one set of axioms.

A Program for Generating and Analyzing Term Rewriting Systems

Sat, 01 Sep 1984 00:00:00 GMT

A Program for Generating and Analyzing Term Rewriting Systems Forgaard, Randy This thesis presents new results in the use of term rewriting systems for automatic theorem proving. The design and implementation of REVE 2, a computer program that incorporates these results, is described. In addition, an introduction to the basic theory, procedures, and algorithms of term rewriting is provided, in a manner suitable for non-specialists.

Foundations of a Theory of Specification for Distributed Systems

Wed, 01 Aug 1984 00:00:00 GMT

Foundations of a Theory of Specification for Distributed Systems Stark, Eugene W. This thesis investigates a particular approach, called state-transition specification, to the problem of describing the behavior of modules in a distributed or concurrent computer system. A state-transition specification consists of: (1) a state machine, which incorporates the safety or invariance properties of the module, and (2) validity conditions on the computations of the machine, which capture the desired liveness or eventuality properties.

Routing Networks for Packet Communication Systems

Wed, 01 Aug 1984 00:00:00 GMT

Routing Networks for Packet Communication Systems Boughton, George Andrew This thesis examines the design of geographically centralized high performance packet switched networks called routing networks. Each of these networks is intended to be used to interconnect the modules of a highly parallel computer system. The design of such networks is considered in present (1984) technology where only a small number of network nodes can be placed on a single chip and in VLSI technology where a large number of nodes can be placed on a chip.

Reasoning about Preference Models

Wed, 01 May 1985 00:00:00 GMT

Reasoning about Preference Models Wellman, Michael Paul Programs that make decisions need mechanisms for representing and reasoning about the desirability of the possible consequences of their choices. This work is an exploration of preference models based on utility theory. The framework presented is distinguished by a qualitative view of preferences and a knowledge-based approach to the application of utility theory. The design for a comprehensive preference modeler is implemented in part by the Utility Reasoning Package (URP), a collection of facilities for constructing and analyzing preference models.

Generic Software for Emulating Multiporocessor Architectures

Wed, 01 May 1985 00:00:00 GMT

Generic Software for Emulating Multiporocessor Architectures Soley, Richard Mark The expense of designing, prototyping, and testing a new computer architecture (particularly non-traditional supercomputer architectures, such as the dataflow machine) is enormous. The relative inflexibility of hardware to experimental changes increases the need to fully test a new architectural idea.

Towards a Problem Solving System for Molecular Genetics

Wed, 01 May 1985 00:00:00 GMT

Towards a Problem Solving System for Molecular Genetics Koton, Phyllis A. This paper describes a program called GENEX that reasons about the behavior of bacterial operons. It is the first step towards a generalized system that will reason about genetic control mechanisms. The system is easily extensible and able to produce detailed explanations without relying on canned text. Problems in molecular genetics are complicated by uncertainty introduced when reasoning about conformations.

Some Implications of Complexity Theory on Pseudo-random Bit Generation

Tue, 01 Jan 1985 00:00:00 GMT

Some Implications of Complexity Theory on Pseudo-random Bit Generation Trilling, Stephen A recent area of interest in theoretical computer science has been in the construction of so-called pseudo-random bit generators. These generators "stretch" a short sequence of truly random bits into a longer sequence of "pseudo-random" bits. These bits are sufficiently indistinguishable from truly random bits to be useful in deterministic simulation of probabilistic computation.

Synchornizing Clocks in a Distributed System

Wed, 01 Aug 1984 00:00:00 GMT

Synchornizing Clocks in a Distributed System Lundelius, Jennifer Keeping the local times of processes in distributed system synchronized in the presence of arbitrary faults is important in many applications and is an interesting theoretical problem in its own right. In order to be practical, any algorithm to synchronize clocks must be able to deal with process failures and repairs, clock drift, and varying message delivery times, but these conditions complicate the design and analysis of algorithms.

An Approach to Functional Office Automation

Sun, 01 Apr 1984 00:00:00 GMT

An Approach to Functional Office Automation Zarmer, Craig L. Current efforts in office automation emphasize developing tools for supporting common, low-level tasks such as word processing and electronic mail. While they have a wide market, they are not very sophisticated. At the other end of the spectrum are office-specific systems, designed with complete knowledge of the office's operations. Unfortunately, such systems have a market size of one, and so are not very practical.

Parallel Simulation of Digital LSI Circuits

Fri, 01 Feb 1985 00:00:00 GMT

Parallel Simulation of Digital LSI Circuits Aronld, Jeffrey M. Integrated circuit technology has been advancing at a phenomenal rate over the last several years, and promises to continue to do so. If circuit design is to keep pace with fabrication technology, radically new approaches to computer-aided design will be necessary. One appealing approach is general purpose parallel processing. This explores the issues involved in developing a framework for circuit simulation which exploits the locality exhibited by circuit operation to achieve a high degree of parallelism.

Resource Management for the Tagged Token Dataflow Architecture

Tue, 01 Jan 1985 00:00:00 GMT

Resource Management for the Tagged Token Dataflow Architecture Culler, David E. The Tagged Token Dataflow Architecture is a multiprocessor based on the U-interpreter model of dataflow computation. It captures the essential execution mechanism of the U-interpreter precisely; operations are enabled for execution by the availability of operated data. However, computational resources in the model and the machine are viewed quite differently. This thesis addresses four major resource management issues essential to bridge the gap between the U-interpreter and the Tagged Token Dataflow Architecture.

Distributed Name Management

Fri, 01 Feb 1985 00:00:00 GMT

Distributed Name Management Sollins, Karen Rosin The problem being addressed in this research is the design of a naming facility achieving the following goals. First, two functions on names must be supported: accessing a named object, and acting as a place holder for the named object. Second, it must be possible to share those names. Third , communication of the names as well as communication by use of the names must be possible. Finally, feasibility of implementation is a goal.

Qualitative Mathematical Reasoning

Thu, 01 Nov 1984 00:00:00 GMT

Qualitative Mathematical Reasoning Sacks, Elisha Qualitative analysis is the study of abstract causal reasoning. It explores the mechanisms whereby humans analyze complex systems abstractly, while ignoring unimportant and unknown low-level details. Previous research has focused on qualitative simulation techniques, analogous to numerical simulation, that use local information about a system to predict its short-term behavior. This thesis presents a new, calculus based, type of qualitative analysis, called qualitative mathematical reasoning. It derives functional descriptions of systems and uses them to predict global behavior.

Debugging Distributed Computations in a Nest Atomic Action System

Sat, 01 Dec 1984 00:00:00 GMT

Debugging Distributed Computations in a Nest Atomic Action System Chiu, Sheng Yang Concurrent and distributed programs are hard to debug. In this thesis, we argue that structuring activities as nested atomic actions can make debugging such programs much like debugging traditional sequential programs. To support the argument, we present a method for debugging computations in the Argus language and system. Our method is applicable to other action systems since it depends only on the atomicity properties of actions.

Orphan Detection in the Argus System

Tue, 01 May 1984 00:00:00 GMT

Orphan Detection in the Argus System Walker, Edward Franklin In a distributed system, an activity running at one node can request another node to perform some service. This request results in an activity being created at the latter node to perform the requested service. The former node may then crash, destroying the activity that requested the service, but leaving behind the activity performing the service. Such surviving are known as orphans [Nelson81]. Orphans are undesirable since they waste resources and can view inconsistent data.

Using Untypes Lambda Calculus to Computer with Atoms

Wed, 01 Feb 1984 00:00:00 GMT

Using Untypes Lambda Calculus to Computer with Atoms Weiss, Paul G. Axioms and verification rules are given for typeless A -calculus with a conditional test for equality between atoms. A semantic completeness theorem is proved and a deterministic evaluator is proposed.

Partial Evaluation as a Means of Language Extensibility

Wed, 01 Aug 1984 00:00:00 GMT

Partial Evaluation as a Means of Language Extensibility Schooler, Richard An optimization technique known as partial evaluation is explored. A partial evaluator optimizes code by making use of static information about program values. Our partial evaluator is designed to optimize mainly applicative code. Un-checked assertions are used to identify applicative constructs in the input code and guide the partial evaluator. Side-effects in the input code are retained but are not optimized.

Efficient Implementation of Applicative Languages

Sun, 01 Apr 1984 00:00:00 GMT

Efficient Implementation of Applicative Languages Ackerman, William B. The analysis of parallelism in an applicative program is much easier than in a program written in a conventional statement-oriented style. This makes it possible for an optimizing compiler to prepare such a program for extremely efficient execution on a suitable enormously parallel computer. This thesis explores the transformations that must be made to achieve very high performance for numerical programs when executed on a computer that uses data flow principles in its operation.

Routing the Power and Ground Wires on a VLSI Chip

Tue, 01 May 1984 00:00:00 GMT

Routing the Power and Ground Wires on a VLSI Chip Moulton, Andrew Strout This thesis presents four new algorithms to route noncrossing power and ground trees in one metal layer of a VLSI chip. The implementation of the best algorithm forms MIT's Placement-Interconnect (PI) Project's power-ground routing phase. The input to this power-ground algorithm is a set of rectangular modules on a rectangular chip.

Type Checking in Vimval

Fri, 01 Jun 1984 00:00:00 GMT

Type Checking in Vimval Kuszmaul, Bradley C. A type system is developed for the revised version of the Val programming language (VimVal) which has the following features: (1) Type Inference: allows programs to be written with incomplete type specifications. The type checker infers the types of expressions from their context. (2) Polymorphism: allows modules to be written which operate on more than one type, performing analogous operations on different types of data. (3) higher order functions: are first class data in VIMVAL. (4) Recursive types: a type may to itself.

Coordinating Pebble Motion on Graphs, The Diameter of Permutation Groups, and Applications

Tue, 01 May 1984 00:00:00 GMT

Coordinating Pebble Motion on Graphs, The Diameter of Permutation Groups, and Applications Kornhauser, Daniel Martin The problem of memory management in totally distributed computing systems leads to the following movers' problem on graphics: Let G be a graph with n vertices with k < n pebbles number 1...,k on distinct vertices. A move consists of transferring a pebble to an adjacent unoccupied vertex. The problem is to decide whether one arrangement of the pebbles is reachable from another, and to find the shortest sequence of moves to find the rearrangement when it is possible.

Replication Methods for Abstract Data Types

Tue, 01 May 1984 00:00:00 GMT

Replication Methods for Abstract Data Types Herlihy, Maurice Peter Replication can enhance the availability of data in a distributed system. This thesis introduces a new method for managing replicated data. We propose new techniques to address four problems associated with replication: (i) the representation and manipulation of replicated data, (iii) on- the-fly reconfiguration, and (iv) enhancing availability in the presence of partitions.

A Constraint Representation and Explanation Facility for Renal Physiology

Fri, 01 Jun 1984 00:00:00 GMT

A Constraint Representation and Explanation Facility for Renal Physiology Asbell, Irwin Current research in Artificial Intelligence has yielded computer programs which have potential to augment the physician's ability to diagnose illness. The medical diagnoses programs of the first generation contain medical facts representing associations between diseases and findings. A most important step is the development of computer programs that have models of physiological processes and have the ability to derive physiological justifications of observed signs and symptoms.

An Abstract Architecture for Parallel Graph Reduction

Tue, 01 May 1984 00:00:00 GMT

An Abstract Architecture for Parallel Graph Reduction Traub, Kenneth R. An implementation technique for functional languages that has received recent attention is graph reduction, which offers opportunity for the exploitation of parallelism by multiple processors. While several proposals for parallel graph reduction machines have been made, differing terminology and approaches make these proposals difficult to compare.

Extending Binary Byzantine Agreement to Multivalued Byzantine Agreement

Sun, 01 Apr 1984 00:00:00 GMT

Extending Binary Byzantine Agreement to Multivalued Byzantine Agreement Turpin, Russell; Coan, Brian A. A binary Byzantine agreement algorithm can be extended to produce a multivalued Byzantine agreement algorithm. The resulting multivalued algorithm is cheaper than previously published algorithms when the cost of transmitting values from the multival

Specification and Implementation of Atomic Data Types

Thu, 01 Mar 1984 00:00:00 GMT

Specification and Implementation of Atomic Data Types Weihl, William Edward Maintaining the consistency of long-lived, on-line data is a difficult task, particularly in a distributed system. This dissertation focuses on atomicity as a fundamental organizational concept for such systems. It explores an approach in which

The Design and Implementation of an Online Directory Assistance System

Thu, 01 Dec 1983 00:00:00 GMT

The Design and Implementation of an Online Directory Assistance System Koile, Kimberle This thesis describes the design and implementation of an online directory assistance system called DIRSYS that was modeled after the white pages of a paper telephone book and a full-screen display editor such as Emacs. As the user begins typing a n

Cohesion in Computer Text Generation: Lexical Substitution

Sun, 01 May 1983 00:00:00 GMT

Cohesion in Computer Text Generation: Lexical Substitution Granville, Robert Alan This report describes Paul, a computer text generation system designed to create cohesive text. The device used to a achieve this cohesion is lexical substitution. Through the use of syntactic and semantic information, the system is able to determine which type of lexical substitution will provide the necessary information to generate an understandable reference, while no providing so much information that the reference is confusing or unnatural.

A Formal Model of Non-Determinate Dataflow Computation

Mon, 01 Aug 1983 00:00:00 GMT

A Formal Model of Non-Determinate Dataflow Computation Brock, Jarvis Dean Almost ten years ago, Gilles Kahn used the fixed point theory of Dana Scott to define a formal and elegant model of computation for determinate dataflow graphs, networks of determinate processes communicating asynchronously through unbounded channels.

Reliable Object Storage to Support Atomic Actions

Sun, 01 May 1983 00:00:00 GMT

Reliable Object Storage to Support Atomic Actions Oki, Brian Masao To preserve the consistency of on-line, long-lived, distributed data in the presence of concurrency and in the event of hardware failures, it is necessary to ensure atomicity and data resiliency in applications. The programming language Argus is designed to support such applications. This thesis investigates the mechanism needed to support the notion of data resiliency present in Argus.

Preliminary Report on the Larch Shared Language*

Sat, 01 Oct 1983 00:00:00 GMT

Preliminary Report on the Larch Shared Language* Guttag, John V.; Horning, J.J. Each member of the Larch family of formal specification languages has a component derived from a programming language and another component common to all programming languages. We call the former interface languages, and the latter the Larch Shared Language.

PADL - A Packet Architecture Description Language: A Preliminary Reference Manual

Sat, 01 Oct 1983 00:00:00 GMT

PADL - A Packet Architecture Description Language: A Preliminary Reference Manual Leung, Clement Kin Cho; William Y-P. PADL is a hardware description language for specifying the behavior and structure of packet communication systems. In such systems, hardware units called modules communicate by sending and receiving packets. The behavior of such a system can be specified by providing the algorithm it executes and the data structures it manipulates. On the other hand, the structure of a system is specified by giving the components or of the system and their interconnection.

A Framework for Solving VSLI Graph Layout Problems

Sat, 01 Oct 1983 00:00:00 GMT

A Framework for Solving VSLI Graph Layout Problems Bhatt, Sandeep N.; Leighton, Frank Thomson This paper introduces a new divide-and-conquer framework for VLSI graph layout. Universally close upper and lower bounds are obtained for important cost functions such as layout area and propagation delay. The framework is also effectively used to design regular and configuration layouts, to assemble large networks of processors using restructurable chips, and to configure networks around faulty processors. it is also shown how good graph partitioning heuristics may be used to develop a provably good layout strategy.

Simulation Tools for Digital LSI Design

Thu, 01 Sep 1983 00:00:00 GMT

Simulation Tools for Digital LSI Design Terman, Christopher J. This thesis proposes a timing simulator (RSIM) based on a uniquely simple transistor model. RSIM allows a designer to determine both the functional and approximate timing characteristics of a MOS network with more accuracy than gate-level simulation, and using larger circuits than are accommodated by circuit analysis programs. In RSIM, transistors are modeled as resistors; the logic states of a transistor's terminal nodes determine its effective resistance.

Dynamic Module Replacement in a Distributed Programming System

Tue, 01 Mar 1983 00:00:00 GMT

Dynamic Module Replacement in a Distributed Programming System Bloom, Toby The replacement of parts of software systems is an important aspect of programming methodology. Most of the research in this area has centered around support for modular construction and the clear separation of interface from implementation. The emphasis has been on producing easily modified static program structures.

A Multiprocessor Emulation Facility

Sat, 01 Oct 1983 00:00:00 GMT

A Multiprocessor Emulation Facility Arvind,; Dertouzos, Michael L.; Lannucci, Robert A. Interest in multiprocessor computer architectures has increased dramatically in the last ten years. However, it has become clear that, in order to effectively use multiprocessors in a general way, some fundamental changes in the model of computation are necessary. Moreover, experimentation in the field is hindered by low-performance simulation tools and high-cost hardware modeling schemes.

Creating a Computer-based Learning Environment for Physically Handicapped Children

Thu, 01 Sep 1983 00:00:00 GMT

Creating a Computer-based Learning Environment for Physically Handicapped Children Valente, Jose Armando The objective of this research is to develop a computer-based learning environment for children physically handicapped by cerebral palsy and to study several issues related to the use of this environment for diagnostic, educational, and remedial purposes. The study is motivated by the desire to better understand the intellectual and motoric deficiencies of cerebral palsied children and to use this information in the development of teaching methods to accommodate each child's particular needs.

An Argument for Soft Layering of Protocols

Mon, 01 Aug 1983 00:00:00 GMT

An Argument for Soft Layering of Protocols Cooper, Geoffrey Howard This thesis is about the efficiency of protocol layering. It examines the technique of protocol layering in an abstract way and finds two major sources of inefficiency in protocol implementations which are caused by the imposition on them of a layered structure. The conventional approach to making layered protocol implementations run efficiently--- for avoiding the sources of inefficiency discussed herein --- are all independent of the protocol specification, and thus all decrease the value of the protocol specification as a guide for implementing protocols.

A Two-tiered Approach to Specifying Programs

Sun, 01 May 1983 00:00:00 GMT

A Two-tiered Approach to Specifying Programs Marie Wing, Jeannette Current research in specifications is beginning to emphasize the practical use of formal specifications in program design. This thesis presents a specification approach, a specification language that supports that approach, and some ways to evaluate specifications written that language.

Video Games and Computer Aided Instruction

Wed, 01 Jun 1983 00:00:00 GMT

Video Games and Computer Aided Instruction Krugler, Ken This document will briefly outline the evolution of video games, discuss current video game theory, and describe a program to teach typing on the IBM Personal Computer.

Fundamental Design Problems of Distributed Systems for the Hard-real-time Environment

Wed, 01 Jun 1983 00:00:00 GMT

Fundamental Design Problems of Distributed Systems for the Hard-real-time Environment Mok, Aloysisu Ka-Lau Software designed to function in a hard-real-time environment where strict timing constraints must be met often entails implicit assumptions about a programming language and the underlying system which supports it. Programs which are logically correct, i.e., implement the intended algorithms, may not function correctly if their assumed timing characteristics are not met.

The MDL Programming Environment

Thu, 01 May 1980 00:00:00 GMT

The MDL Programming Environment Lebling, P. David

The MDL Programming Language

The MDL Programming Language Galley, S.W.; Pfister, Greg

The MDL Programming Language Primer

The MDL Programming Language Primer Dornbrook, Michael; Blank, Marc

The Impact of Layer Assignment Methods on Layout Algorithms for Integrated Circuits

Mon, 01 Aug 1983 00:00:00 GMT

The Impact of Layer Assignment Methods on Layout Algorithms for Integrated Circuits Pinter, Ron Yair Programs for integrated circuit layout at the module assembly level are typically decomposed into two phases - placement and routing. In this thesis we investigate a third phase which is often implicitly assumed - layer assignment. This thesis studies how layer assignment methodologies interact with placement and routing.

An Office Analysis and Diagnosis Methodology

Tue, 01 Mar 1983 00:00:00 GMT

An Office Analysis and Diagnosis Methodology Sutherland, Juliet With the advent of computer technology designed for use in the office, office analysis, or the process of understanding office work for the purposes of introducing technology, has become increasingly important. The Office Analysis and Diagnosis Methodology (OADM) is a tool to help the analyst gather the data required to decide how, and whether, to introduce office automation technology into a particular office, OADM is best suited for studying semi-structured offices, rather than pure processing operations or special projects. OADM is used to perform a detailed study of a single office and is not designed for use in determining the general automation needs of a large organization.

Office Analysis: Methodology and Case Studies

Tue, 01 Mar 1983 00:00:00 GMT

Office Analysis: Methodology and Case Studies Sirbu, Marvin A., Jr.; Schoichet, Sandor R.; Kunin, Jay S.; Hammer, Michael M.; Sutherland, Juliet B.; Zarmer, Craig L. The Office Analysis Methodology (OAM) is a structured methodology for understanding the current operations of an office. OAM provides guidance in interviewing techniques and approaches to establishing a positive atmosphere for possible office automation efforts. It is designed to be to learn so that people with experience in office work but little experience in analysis can easily perform a study.

On Computing Galois Groups and Its Application To Solvability by Radicals

Tue, 01 Mar 1983 00:00:00 GMT

On Computing Galois Groups and Its Application To Solvability by Radicals Laudau, Susan Eva This thesis presents a polynomial time algorithm for the basic question of Galois theory, checking the solvability by radicals of a monic irreducible polynomial over the integers. It also presents polynomial time algorithms for factoring polynomials over algebraic number fields, for computing blocks of imprimitivity of roots of a polynomial under the transitive action of the Galois group on the roots of the polynomial, and for computing intersections algebraic number fields.

On Bisecting Random Graphs

Tue, 01 Mar 1983 00:00:00 GMT

On Bisecting Random Graphs Bui, Thang Nguyen A bisection of a graph with an even number of vertices is a partition of the vertex set into two disjoint sets of equal size. Given a bisection, the number of edges having one end in each of the two subsets of the bisection is called the size of the bisection. The bisection size of a graph is the minimum size of all possible bisections of the graph.

Internal Consistency of a Distributed Transaction System with Orphan Detection

Sat, 01 Jan 1983 00:00:00 GMT

Internal Consistency of a Distributed Transaction System with Orphan Detection Goree, John A., Jr. This thesis defines a property called "view-serializability", which formalizes internal consistency for a system of nested atomic transactions. Internal consistency is a stronger condition than the usual notion of data base consistency, because it takes into account the views of transactions which will never commit. In a distributed system, local aborts of remote subactions and crashes of nodes can generate orphans: active actions which are descendants of actions that have aborted or are guaranteed to abort.

Concurrency Control for Resilient Nested Transaction

Tue, 01 Feb 1983 00:00:00 GMT

Concurrency Control for Resilient Nested Transaction Lynch, Nancy A. Concurrency control theory is extended to handle nested transactions with failures. The theory is used to present a rigorous correctness proof of a variant of Moss' locking algorithm for implementing nested transactions. The proof has an interesting structure using many levels of abstraction.

Translating Updates of Relational Database Views

Tue, 01 Feb 1983 00:00:00 GMT

Translating Updates of Relational Database Views Cosmadakis, Stavros Stylianos We study the problem of translating updates of data base views. We disambiguate a view update by requiring that a specified view compliment (i.e. a second view which contains all the data base information omitted from the given view) remains constant during the translation. We study some of the computational problems related to the application of this general methodology in the context of relational databases.

A Comparative Study of Computer-aided Clinical Diagnosis

Thu, 01 Jan 1981 00:00:00 GMT

A Comparative Study of Computer-aided Clinical Diagnosis Sherman, Howard Bruce In recent years many computer systems have been developed to assist in medical decision making. Two of these systems in particular, INTERNIST and the Present Illness Program (PIP), have been proposed as suitable for performing general medical diagnosis. However, there has been no way of comparing the performance of these two programs since the medical data used by the programs differs extensively.

Impossibility of Distributed Consensus with One Faulty Process*atio

Wed, 01 Sep 1982 00:00:00 GMT

Impossibility of Distributed Consensus with One Faulty Process*atio Fischer, Michael J.; Lynch, Nancy A.; Paterson, Michael S. The consensus problem involves an asynchronous system of processes, some of which may be unreliable. The problem is for the reliable processes to agree on a binary value. We show that every protocol for this problem has the possibility of nontermination, even with only one faulty process. By way of contrast, solutions are known for the synchronous case, the "Byzantine Generals" problem.

Multilevel Atomicity: A New Correctness Criterion for Database Concurrency Control

Sun, 01 Aug 1982 00:00:00 GMT

Multilevel Atomicity: A New Correctness Criterion for Database Concurrency Control Lynch, Nancy A. Multilevel atomicity, a new correctness criteria for database concurrency control, is defined. It weakens the usual notion of serializability by permitting controlled interleaving among transactions. It appears to be especially suitable for applications in which the set of transactions has a natural hierarchical structure based on the hierarchical structure of an organization.

An Implementation Scheme for Array Operations in Static Data Flow Computers

Sat, 01 May 1982 00:00:00 GMT

An Implementation Scheme for Array Operations in Static Data Flow Computers Guang-Rong, Gao The mapping of array operations in VAL programs on a static data flow machine with array memory is studied. The flow dependency graph is introduced as a model of array operations in VAL programs. The balancing and optimization of the flow dependency graphs is presented. The class of well-be VAL programs which can be modeled by flow dependency graphs is specified.

The Design of a Multiprocessor Development System

Wed, 01 Sep 1982 00:00:00 GMT

The Design of a Multiprocessor Development System Anderson, Thomas Lee A multiprocessor development system has been designed and a prototype system is being constructed. The system, known as Concert, is intended to support multiprocessor research efforts at M.I.T. The motivation for Concert and the project history are summarized briefly. Some intended applications are also identified.

Display Management in an Integrated Office

Fri, 01 Jan 1982 00:00:00 GMT

Display Management in an Integrated Office Rosenstein, Larry S. Advances in technology now make it possible to build office workstations that have a large amount of local computing power and high-resolution output devices. Such workstations can be used for various office applications, such as document preparation, personal databases, and electronic mail.

Efficient Modeling for Short Channel MOS Circuit Cimulation

Sun, 01 Aug 1982 00:00:00 GMT

Efficient Modeling for Short Channel MOS Circuit Cimulation Johnson, Mark Griffin Existing circuit models for short-channel MOS transistors represent a compromise between speed and ease of use. Empirical models are very fast to evaluate, but their parameters must be fitted from experimental measurements. Theoretical models require longer computation time, but they may be used to predict the performance of new, unmeasured MOS technologies since their parameters are not curve-fitted from experimental data.

Automatic Synthesis of Implementations for Abstract Data Types from Algebraic Specifications

Tue, 01 Jun 1982 00:00:00 GMT

Automatic Synthesis of Implementations for Abstract Data Types from Algebraic Specifications Srivas, Mandayam K. Algebraic specifications have been used extensively to prove properties of abstract data types and to establish the correctness of implementations of data types. This thesis explores an automatic method of synthesizing implementations for data types from their algebraic specifications.

Analysis and Specification of Office Procedures

Mon, 01 Feb 1982 00:00:00 GMT

Analysis and Specification of Office Procedures Kunuin, Jay S. Conventional approaches to "office automation" focus on the lowest common denominator of office work: typing, filing, filling in forms, etc. As a consequence, the process of office systems analysis lacks tools and techniques that address the office in terms of business functions rather than as manipulation of paper artifacts.

Layouts for the Shuffle-exchange Graph and Lower Bound Techniques for VLSI

Sun, 01 Aug 1982 00:00:00 GMT

Layouts for the Shuffle-exchange Graph and Lower Bound Techniques for VLSI Leighton, Frank Thomson The thesis is divided into two parts. In the first part, we describe and analyze several new VLSI layouts for the shuffle-exchange graph. These include:1) an asymptotically optimal, (N /log N)-area layout for the N-node shuffle-exchange graph, and 2) several practical layouts for small shuffle-exchange graphs.

Data Communications via Cable Television Networks: Technical and Policy Considerations

Sat, 01 May 1982 00:00:00 GMT

Data Communications via Cable Television Networks: Technical and Policy Considerations Estrin, Deborah Lynn Cable television networks offer peak communication data rates that are orders of magnitude greater than the telephone local loop. Although one-way television signal distribution continues to be the primary application of cable television systems, the cable television network can be used for two-way data communication.

Procedural Reflection in Programming Languages Volume I

Fri, 01 Jan 1982 00:00:00 GMT

Procedural Reflection in Programming Languages Volume I Smith, Brian Cantwell We show how a computational system can be constructed to "reason," effectively and consequentially, about its own inferential processes. The analysis proceeds in two parts. First, we consider the general question of computational semantics, rejecting traditional approaches, and arguing that the declarative and procedural aspects of computational symbols (what they stand for, and what behaviour they engender) should be analysed independently, in order that they may be coherently related.

A Computer System for Decision Analysis in Hodgkins Disease

Mon, 01 Feb 1982 00:00:00 GMT

A Computer System for Decision Analysis in Hodgkins Disease Rutherford, Cynthia, J.; Davies, Byron; Barnett, Arnold I.; Desforges, Jane F. This report draws together the diverse strands involved in developing a unique computer-based system to stage and manage Hodgkins Disease (HD). Those of us worked on the final version of this project included two hematologists, a computer scientist, and a statistician.

The Design of a Routing Service for Campus-wide Internet Transport

Fri, 01 Jan 1982 00:00:00 GMT

The Design of a Routing Service for Campus-wide Internet Transport Singh, Vineet A campus-wide network requires many subnetworks connected by gateways and it has a relatively loose administration. Modularization of network implementing is important in this environment to make efficient use of ever-improving technologies and protocols.

The Complexity of Concurrency Control for Distributed Databases

Tue, 01 Dec 1981 00:00:00 GMT

The Complexity of Concurrency Control for Distributed Databases Kanellakis, Paris C. This study is an analysis of the distributed version of data base concurrency control. It provides concrete mathematical evidence that the distributed problem is an inherently more complex task than the centralized one. The notions of transaction, concurrency, history, serializability, scheduler, etc, for centralized databases are now well-understood both from a theoretical and a practical point of view.

Derived Pairs, Overlap Closures, and Rewrite Dominoes: New Tools for Analyzing Term Rewriting Systems

Tue, 01 Dec 1981 00:00:00 GMT

Derived Pairs, Overlap Closures, and Rewrite Dominoes: New Tools for Analyzing Term Rewriting Systems Guttag, John V.; Kapur, Deepak; Musser, David R. Starting from the seminal work of Knuth and Bendix, we develop several notions useful in the study of term rewriting systems. In particular we introduce the notions of "derived pairs" and "overlap closure" and show that they are useful in analyzing sets of rewrite rules for various properties related to termination. We also introduce a new representation, based on rewrite dominoes, for rewrite rules and sequences of rewrites.

Causal Representation of Patient Illness for Electrolyte and Acid-base Diagnosis

Thu, 01 Oct 1981 00:00:00 GMT

Causal Representation of Patient Illness for Electrolyte and Acid-base Diagnosis Patil, Ramesh S. Much of the medical knowledge in the first generation Al in Medicine programs is phenomenological; that is, it describes the associations among phenomena without knowledge of the underlying causal mechanisms. Although these AIM programs provide a good first approximation to the way clinicians reason, they fail to produce clinicians reasoning based on a deaper understanding of the phenomens.

An Ease of Use Evaluation of an Integrated Editor and Formatter

Thu, 01 Jan 1981 00:00:00 GMT

An Ease of Use Evaluation of an Integrated Editor and Formatter Good, Michael Etude is an integrated text editor and formatter that was designed to be easy to learn and easy to use. To measure Etude's success in meeting these goals, twenty-one computer-naive temporary office workers were taught to use Etude in a controlled experiment. Ninety percent of the subjects were able to create and edit letters after a training period of less than two hours and twenty minutes, though they were not able to perform these tasks as quickly as they could when using a typewriter.

A Data Flow Architecture with Improved Asymptotic Performance

Wed, 01 Apr 1981 00:00:00 GMT

A Data Flow Architecture with Improved Asymptotic Performance Thomas, Robert E. Large scale integration presents a unique opportunity to design a computer compromising large numbers of small, inexpensive processors. This paper presents a design for such a machine based on the asynchronous and functional semantics of data flow.

Interactive Debigging in a Distributed Computational Envrionment

Tue, 01 Sep 1981 00:00:00 GMT

Interactive Debigging in a Distributed Computational Envrionment Schiffenbauer, Robert David

Propositional Dynamic Logic of Looping and Converse

Fri, 01 May 1981 00:00:00 GMT

Propositional Dynamic Logic of Looping and Converse Streett, Robert S. Dynamic logic [5,6,15,16] applies concepts from modal logic to a relational semantics of programs to yield various systems for reasoning about the before-after behavior of programs. Analogues to the modal logic assertions ?p (possibly p) and ?p(necessarily p) are the dynamic logic constructs p and [a]p. If a is a program and p is an assertion about the state of a computation, then ,p asserts that after executing a, p can be the case, and [a]p asserts that after executing a, p must be the case.

High Level VAL Constructs in a Static Data Flow Machine

Mon, 01 Jun 1981 00:00:00 GMT

High Level VAL Constructs in a Static Data Flow Machine Todd, Kenneth Wayne The Dennis-Misunas Form 1 Data Flow Machine can best be described as a static and scalar machine. Despite these two limiting characteristics, it is still possible to translate the whole of the functional programming language VAL into the base language of this machine. Methods for translating the various high constructs of VAL are presented which exploit the parallelism inherent in programs written in VAL mainly by pipelining through a single expression (vertical parallelism) rather than employing many copies of that same expression (horizontal parallelism), although the latter is not ruled out.

A Switch-level Simulation Model for Integrated Logic Circuits

Sun, 01 Mar 1981 00:00:00 GMT

A Switch-level Simulation Model for Integrated Logic Circuits Bryant, Randal Everitt Switch-level simulators model a metal oxide semiconductor (MOS) large scale integrated (LSI) circuits as a network of transistor "switches". They can simulate many aspects of MOS circuits which cannot be expressed in the Boolean logic gate model, such as bidrecttional pass transistors, dynamic storage, and charge sharing.

A Design Methodology for Self-time Systems

Sun, 01 Feb 1981 00:00:00 GMT

A Design Methodology for Self-time Systems Singh, Narinder Pal This thesis presents a design methodology for self-timed systems which will be extremely attractive for implementing systems in VLSI. Self-timed systems are characterized by the absence of a timing reference to which all operations are synchronized.

Application of Data Flow Architecture to Computer Music Synthesis

Sun, 01 Feb 1981 00:00:00 GMT

Application of Data Flow Architecture to Computer Music Synthesis Cesari, Carol Andrea A computer music synthesis system is the most flexible of synthesis systems. It offers a composer extensive control over the sound of his piece. A user of such a system describes his composition in some synthesis language. The computer uses this description to calculate samples of a voltage waveform that can be fed to D/A converters at a specified sampling rate.

Semiautomatic Translation of Cobol In Hibol

Wed, 01 Apr 1981 00:00:00 GMT

Semiautomatic Translation of Cobol In Hibol Faust, Gregory Gerard A severe software crisis is currently being experienced by the data processing community due to intolerable maintenance costs. A system is introduced to reduce those costs by the translation of existing COBOL software into HIBOL; a very high level language that is significantly easier to maintain.

Protecting Externally Supplied Software in Small Computers

Sun, 01 Mar 1981 00:00:00 GMT

Protecting Externally Supplied Software in Small Computers Kent, Stephen T. The increasing decentralization of computing resources and the proliferation of personal and small business computers create new problems in computer security. One such problem is the protection of externally supplied software, i.e., software supplied by other than the users/owners of these small computers. In the case of personal and small business computers, proprietary software serves as the primary example.

An Integrated Approach to Formatted Document Production

Sun, 01 Feb 1981 00:00:00 GMT

An Integrated Approach to Formatted Document Production Ilson, Richard Recent advances in printing technology have reduced the cost of typeset quality printers. Unfortunately, the production of attractively formatted documents requires typographic skill and special training on computer-based text processing systems. The principal characteristics of Etude are that it embodies substantial typographic expertise, and is based on concepts familiar to untrained users . Furthermore, Etude provides a real-time display facility that allows the results of editing and formatting operations to be seen immediately. Thus, Etude supports the entire process of producing decorously formatted documents.

Recovery of the Swallow Repository

Thu, 01 Jan 1981 00:00:00 GMT

Recovery of the Swallow Repository Arens, Gail C. This thesis presents the design of a set of recovery mechanisms for the Swallow repository. Swallow is a distributed data storage system that supports highly reliable long term storage of arbitrary sized data objects with special mechanisms for implementing multi-site atomic actions. The Swallow repository is a data storage server that keeps permanent data in write-once stable storage such as optical disk.

Producing Explanations and Justifications of Expert Consulting Programs

Thu, 01 Jan 1981 00:00:00 GMT

Producing Explanations and Justifications of Expert Consulting Programs Swartout, William R. Traditional methods for explaining programs provide explanations by converting to English the code of the program or traces of the execution of that code. While such methods can provide adequate explanations of what the program does or did, they typically cannot provide justifications of the code without resorting to canned-text explanations. That is, such systems cannot tell why what the system is doing is a reasonable thing to be doing.

Fault Tolerance in Packet Communication Computer Archiectures

Mon, 01 Dec 1980 00:00:00 GMT

Fault Tolerance in Packet Communication Computer Archiectures Leung, Clement Kin Cho It is attractive to implement a large scale parallel processing system as a self-timed hardware system with decentralized control and to improve maintainability and availability in such a system through fault tolerance. In this thesis we show how to tolerate hardware failures in a self-timed hardware system with a packet communication architecture, designed to execute parallel programs organized by data flow concepts.

Computers and People: Personal Computation

Mon, 01 Dec 1980 00:00:00 GMT

Computers and People: Personal Computation Turkle, Sherry In the January 1975 issue of Popular Electronics, MITs, short for Micro Instrumentation and Telementry System, a small computer company in Albequerque, New Mexico, announced the Altair, a computer small enough to sit on a desktop, powerful enough to support high level language programming, and that you could build for only $429.

Algorithms for Integrated Circuit Layout: An Analytic Approach

Sat, 01 Nov 1980 00:00:00 GMT

Algorithms for Integrated Circuit Layout: An Analytic Approach LaPaugh, Andrea Suzanne In this thesis, the problem of designing the layout of integrated circuits is examined. The layout of an integrated circuit specifies the position of the chip of functional components and wires interconnecting the components. We use a general model

Interprocedural Data Flow Analysis in the Presence of Pointers, Procedure Variables, and Label Variables

Wed, 01 Oct 1980 00:00:00 GMT

Interprocedural Data Flow Analysis in the Presence of Pointers, Procedure Variables, and Label Variables Weihl, William Edward The compilation of highly modular programs requires extensive interprocedural analysis in order to produce reasonable object code. Such analysis is greatly complicated when the source language contains such constructs as procedure variables and label variables.

A Survey of the Logic of Effective Definitions

Wed, 01 Oct 1980 00:00:00 GMT

A Survey of the Logic of Effective Definitions Tiuryn, J. LED, the Logic of Effective Definitions, is an extension of first order predicate calculus used for making assertions about programs. Programs are modeled as effective definitional schemes (following Friedman). Logical properties of LED and its relations to classical logics and other programming logics are surveyed.

On Memory Limitations in Natural Language Processing

Mon, 01 Sep 1980 00:00:00 GMT

On Memory Limitations in Natural Language Processing Church, Kenneth Ward This paper proposes a welcome hypothesis: a computationally simple device is sufficient for processing natural language. Traditionally it has been argued that processing natural language syntax requires very powerful machinery. Many engineers have come to this rather grim conclusion: almost all working parsers are actually Turing Machines (TM). For example, Woods specifically designed his Augmented Transition Networks (ATNs) to be Turing Equivalent.

Data Driven Loops

Fri, 01 Aug 1980 00:00:00 GMT

Data Driven Loops Ruth, Gregory R. The notion of the data driven loop arises in connection with our work in the Very High Level Language HIBOL and the automatic programming system (ProtoSystem I) that supports it. Although the concept is of general interest outside of VHLL's and automatic programming, we find it profitable to use HIBOL as a vehicle for our discussion and a means of narrowing the scope of our discussion. Therefore we first present description of the domain which HIBOL treats.

Management of Object Histories in the Swallow Repository

Fri, 01 Aug 1980 00:00:00 GMT

Management of Object Histories in the Swallow Repository Svobodova, Liba SWALLOW is an experimental distributed data storage system that provides personal computers with a uniform interface to their local data and the data stored in shared remote servers called repositories. The SWALLOW repositories provide reliable, secure reliable, secure, and efficient long-term storage for both very small and very large objects and support updating of a group of objects at one or several repositories in a single atomic action.

Simulations Amond Multidimensional Turing Machines

Fri, 01 Aug 1980 00:00:00 GMT

Simulations Amond Multidimensional Turing Machines Loui, Michael Conrad This thesis presents three independent papers: nearly optimal on-line simulations among multidimensional Turing machines, a space bound for one-tape multidimensional Turing machines, and new proofs in the pebble game.

Representation and Analysis of Real-Time Control Structures

Fri, 01 Aug 1980 00:00:00 GMT

Representation and Analysis of Real-Time Control Structures Archer, Rowland F., Jr. A new notation is introduced for representing real-time scheduling at the task and event level. These schedules are called control structures. The primary constructs included which direct the flow of control are sequencing, iteration, and preemption. Additional notation allows the representation of interrupt masking, task termination by external events, task restart as well as resumption from the point of preemption and codestripping. Algorithms are given for finding the presentation structures of a given control structure in the notation.

Safety and Optimization Transformations for Data Flow Programs

Tue, 01 Jul 1980 00:00:00 GMT

Safety and Optimization Transformations for Data Flow Programs Montz, Lynn Barbara The data flow concept of computation seeks to achieve high performance by allowing concurrent execution of instructions based on the availability of data. This thesis explores the translation of a subset of the high level languages VAL to data flow graphs. The major problem in performing this translation for the target machine. the Dennis-Misunas data flow computer, stems from the restriction that graph execution sequences place at most one value on any given are at any time. The data/acknowledge are pair transformation is introduced as a means of implementing this required operational behavior. Its effect on data flow graph operation is subsequently explored as it relates to correctness and performance.

Artwork Analysis Tool for VLSI Circuits

Sun, 01 Jun 1980 00:00:00 GMT

Artwork Analysis Tool for VLSI Circuits Baker, Clark Marshall Current methods for designing VLSI chips do not insure that the chips will perform correctly when manufactured. Because the turn around time on chip fabrication varies from a few weeks to a few months, a scheme other than "try it and see if it works" is needed. Checking of chips by hand simulation and visual inspection of check plots will not cash all of the errors. In addition, the number of transistors per chip is likely to increase from ten thousand to over a million in the next few years.This increase in complexity precludes any manual verification methods; some better method is needed.

The Complexity of Monotone Boolean Functions and an Algorithm for Finding Shortest Paths on a Graph

Sun, 01 Jun 1980 00:00:00 GMT

The Complexity of Monotone Boolean Functions and an Algorithm for Finding Shortest Paths on a Graph Bloniarz, Peter Anthony The first part of this thesis considers the complexity of Boolean functions. The main complexity measures used are the number of gates in combinational networks and the size of Boolean formulas. The case of monotone realizations, using only the operations AND and OR, of monotone functions is emphasized.

Towards a Theory for Abstract Data Types

Sun, 01 Jun 1980 00:00:00 GMT

Towards a Theory for Abstract Data Types Kapur, Deepak A rigorous framework for studying immutable data types having nondeterministic operations and operations exhibiting exceptional behavior is developed. The framework embodies the view of a data type taken in programming languages, and supports hierarchical and modular structure among data types.

Scheduling Task Systems with Resources

Thu, 01 May 1980 00:00:00 GMT

Scheduling Task Systems with Resources Lloyd, Errol Lynn Minimum execution time scheduling of task systems with resources has been the subject of several papers over the past few years. The model used for much of this work assumes that the resources in the system are continuous. That the, there is one unit of each resource, and a task may require any portion of that unit during its execution.

A Concept of Independence with Applications in Various Fields of Mathematics

Thu, 01 May 1980 00:00:00 GMT

A Concept of Independence with Applications in Various Fields of Mathematics Levin, Leonid A. We use Kolmogorov's algorithmic approach to information theory to define a concept of independence of sequences, or equivalently, the boundedness of their mutual information. This concept is applied to probability theory, intuitionistic logic, and the theory of algorithms.

Transmitting Abstract Values in Messages

Thu, 01 May 1980 00:00:00 GMT

Transmitting Abstract Values in Messages Herlihy, Maurice Peter This thesis develops primitives for a programming language intended for use in a distributed computer system where individual nodes may have different hardware or software configurations. Our primitives are presented as extensions to the CLU language.

Automatic Extension of an Augmented Transition Network Grammar for Morse Code Conversations

Tue, 01 Apr 1980 00:00:00 GMT

Automatic Extension of an Augmented Transition Network Grammar for Morse Code Conversations Kaiser, Gail E. This report describes a 'learning program' that acquires much of the knowledge required by a parsing system that processes conversations in a 'natural' language akin to ham-radio jargon. The learning program derives information from example sentence taken from transcripts of actual conversations, and uses this knowledge to extend the 'core' augmented transition network (ATN) grammar.

The Complexity of the Maximum Network Flow Problem

Sat, 01 Mar 1980 00:00:00 GMT

The Complexity of the Maximum Network Flow Problem Baratz, Alan Edward This thesis deals with the computational complexity of the maximum network flow problem. We first introduce the basic concepts and fundamental theorems upon which the study of "max-flow" has been built. We then trace the development of max-flow algorithms from the original "labeling algorithm" of Ford and Fulkerson, through a recent 0 (V-E-log 2 V) algorithm due to Galil and Naamad.

Automatic Verification of Serializers

Sat, 01 Mar 1980 00:00:00 GMT

Automatic Verification of Serializers Atkinson, Russ R. This thesis is concerned with the problem of controlling concurrent access to shared data. A language construct is proposed to enforce such control; a specification language is defined to describe the formal requirements of such control; and verification techniques are given to prove that instances of the construct satisfy their specifications.

An Abstract Implementation for a Generalized Data Flow Language

Tue, 01 Jan 1980 00:00:00 GMT

An Abstract Implementation for a Generalized Data Flow Language Weng, Kung-Song In this thesis we are concerned with issues arising from the need to achieve concurrency of operation with a computation on a large scale. Several factors contribute toward increasing interest in systems capable of exploiting the concurrency of computation.

Incomprehensible Computer Systems: Knowledge Without Wisdom

Tue, 01 Jan 1980 00:00:00 GMT

Incomprehensible Computer Systems: Knowledge Without Wisdom Rosenberg, Ronni Lynne An analysis of the incomprehensibility of large, complex computer systems is made. The thesis is that there is a strong relationship between system incomprehensibility and the necessity to trust computer systems. A cogent definition of incomprehensibility in computer system is established, with common themes drawn from interdisciplinary literature dealing with computers and society.

CLU Reference Manual

Mon, 01 Oct 1979 00:00:00 GMT

CLU Reference Manual Liskov, Barbara; Atkinson, Russ R.; Bloom, Toby; Moss, J. Eliot B.; Schaffert, Craig; Scheifler, Bob; Snyder, Alan This document serves both as an introduction to CLU and as a language reference manual. Sections 1 through 4 present an overview of the language. These sections highlight the essential features of CLU, and discuss how CLU differs from other, more conventional, languages.

Toward a Computational Theory of Indirect Speech Acts

Mon, 01 Oct 1979 00:00:00 GMT

Toward a Computational Theory of Indirect Speech Acts Brown, Gretchen P. The variety of surface forms that may be used to convey a given speech act pose a major problem in modeling task-oriented (and other) dialogues. Many such forms are so-called indirect speech acts, that is, surface form does not correspond to the (or one) intended speech act.

Abstract Model Specifications for Data Abstractions

Sun, 01 Jul 1979 00:00:00 GMT

Abstract Model Specifications for Data Abstractions B_rzin_, Valdis Andris A data abstraction introduces a data type with a hidden representation. Specifications of data abstractions are required to allow the data to be described and used without reference to the underlying representation. There are two main approaches to specifying data abstractions, the abstract model approach and the axiomatic approach.

Denotational Semantics of Determinate and Non-Determinate Data Flow Programs

Sun, 01 Jul 1979 00:00:00 GMT

Denotational Semantics of Determinate and Non-Determinate Data Flow Programs Kosinski, Paul Roman Among its other characteristics, a programming language should be conducive to writing modular program's, be able to express parallelism and non-determinate behavior, and it should have a cleanly formalizable semantics. Data flow programming languages have all these characteristics and are especially amenable to mathematization of their semantics in the denotational style of Scott and Strachey.

Copying Complex Structures in a Distributed System

Sun, 01 Jul 1979 00:00:00 GMT

Copying Complex Structures in a Distributed System Sollins, Karen Rosin This thesis presents a model of a distributed system. The universe of objects in the distributed system is divided into mutually exclusive sets, each set corresponding to a context. This model allows naming beyond the context boundaries, but limits communication across such boundaries to message passing only.

VAL- A Value-oriented Algorithmic Language: Preliminary Reference Manual

Fri, 01 Jun 1979 00:00:00 GMT

VAL- A Value-oriented Algorithmic Language: Preliminary Reference Manual Ackerman, William B.; Dennis, Jack B. The programming language VAL (Value-Oriented Algorithmic Language) is designed for expressing algorithms for execution on computers capable of highly concurrent operation. More specifically, the application area to be supported is numerical computation which strains the limits of high performance machines, and primary targets for translation of VAL programs are data driven machines of the form under development by the Computation Structures Group of the MIT Laboratory for Computer Science for high performance numerical computation.

Storage and Access Costs for Implementations of Variable - Length lists

Sun, 01 Apr 1979 00:00:00 GMT

Storage and Access Costs for Implementations of Variable - Length lists Brown, Donna Jean Consider a machine with a cellular memory used to store a list X , where X is a finite alphabet and i N. We investigate the machine representation of such a list and the implementation of common list operations such as determining the i th element and adding or deleting an element.

Analysis of the Simple Code for Dataflow Computation

Tue, 01 May 1979 00:00:00 GMT

Analysis of the Simple Code for Dataflow Computation Myers, John M. We analyze a problem in hydrodynamics from the standpoint of computation on a data flow compute that is not yet fully specified, with the objectives of helping to further specify the computer and helping to develop VAL as its source language. Lawrence Liver,ore Laboratory supplied the algorithm for hydrodynamics, including heat flow, as a 1749-line FORTRAN code called SIMPLE.

Distributed Computer Systems: Structure and Semantics

Sun, 01 Apr 1979 00:00:00 GMT

Distributed Computer Systems: Structure and Semantics Svobodova, Liba; Liskov, Barbara; Clark, David D This report describes an ongoing project in the area of design of distributed systems. The goal is to develop an effective programming system that will support well-structured design implementation, maintenance and control of distributed processing applications.

Probabilistic Algorithm in Finite Fields

Mon, 01 Jan 1979 00:00:00 GMT

Probabilistic Algorithm in Finite Fields Rabin, Michael O. We present probabilistic algorithms for the problems of finding an irreducible polynomial of degree n over a finite field, finding roots of a polynomial and factoring the polynomial into its irreducible factors over a finite field. All of these problems are of importance in algebraic coding theory, algebraic symbol manipulation, and number theory.

Digitalized Signatures and Public-key Functions as Intractable as Factorization

Mon, 01 Jan 1979 00:00:00 GMT

Digitalized Signatures and Public-key Functions as Intractable as Factorization Rabin, Michael O. We introduce a new class of public-key functions involving a number n = p.q having two large prime factors. As usual, the key n is public, while p and q are the private key used by the issuer for production of signatures and function inversion. These functions can be used for all the applications involving public-key functions proposed by Diffie and Hellman [ 2 ], including digitalized signatures.

Synchronization Mechanism for Modular Programming Language

Sun, 01 Apr 1979 00:00:00 GMT

Synchronization Mechanism for Modular Programming Language Bloom, Toby Any programming language that supports concurrency needs a synchronization construct with which to express access control for shared resources. This thesis examines synchronization constructs from the standpoint of language design for reliable software. The criteria a synchronization mechanism must satisfy to support construction of reliable, easily maintainable concurrent software are defined.

Serializability of Concurrent Data Base Updates

Thu, 01 Mar 1979 00:00:00 GMT

Serializability of Concurrent Data Base Updates Papadimitriou, Christos H. A sequence of interleaved user transactions in a data base system may not be serializable, i.e., equivalent to some sequential execution of the individual transactions. Using a simple transaction model we show that recognizing the transaction histories which are serializable is an NP-complete problem.

A Machine Architecture to Support an Object-Oriented Language

Thu, 01 Mar 1979 00:00:00 GMT

A Machine Architecture to Support an Object-Oriented Language Snyder, Alan In object-oriented languages (e.g., LISP, Simula, and CLU), all (or most) data objects used by a program are implicitly allocated from a free-storage area and are accessed via fixed-size references. The storage for an object is automatically reclaimed (garbage collected) when the object is no longer accessible to the program.

A Microcomputer Network Simulation System

Thu, 01 Feb 1979 00:00:00 GMT

A Microcomputer Network Simulation System Krizan, Brock Collins The design, development and use of cost-effective computer networks require information about system behavior given a variety of network structures and operational policies. Because computer networks are complex systems whose behavior is generally not intuitively understood, there is a need for system analysis tools to provide a wide range of performance information.

Robust Concurrency Control for a Distriuted Information System

Sun, 01 Jan 1978 00:00:00 GMT

Robust Concurrency Control for a Distriuted Information System Montgomery, Warren A.

Naming and Synchornization in a Decentralized Computer System

Sun, 01 Oct 1978 00:00:00 GMT

Naming and Synchornization in a Decentralized Computer System Reed, David Patrick In this dissertation a new approach to the synchronization of accesses to shared data objects is developed. Traditional approaches to the synchronization problems of shared data accessed by concurrently running computations have relied on mutual exclusion--the ability of one computation to stop the execution of other computations that might access or change shared data accessed by that computation.

Real-time Control Structures for Block Diagram Schemata

Tue, 01 Aug 1978 00:00:00 GMT

Real-time Control Structures for Block Diagram Schemata Teixeira, Thomas Joseph Block diagram schemata model computation systems in the context of an external environment. The environment imposes various constraints on the real-time performance of any implementation of a block diagram schema. The model is used to provide precise definitions of real-time performance. The portion of the implementation that affects the real-time performance is called the control structure.

Synthesis of Synchronization Code for Data Abstractions

Sat, 01 Jul 1978 00:00:00 GMT

Synthesis of Synchronization Code for Data Abstractions Laventhal, Mark Steven Synchronization code is necessary to control shared access of an abstract data object in a parallel-processing environment. This thesis explores an approach in which a synchronization property can be specified in a high-level nonprocedural language, and an implementation for the specified property can be synthesized algorithmically.

A Formalization of the State Machine Specification Technique

Sat, 01 Jul 1978 00:00:00 GMT

A Formalization of the State Machine Specification Technique Principato, Robert N., Jr. This thesis develops the state machine specification technique, a formal specification technique for data abstractions based on Parnas' work on specifying software modules.

A Denotational Semantics of CLU

Mon, 01 May 1978 00:00:00 GMT

A Denotational Semantics of CLU Scheifler, Robert W. A denotational semantics of CLU, an object-oriented language supporting data abstractions, is presented. The definition is based on Scott's lattice-theoretic approach to the theory of computation. Modules, the basic unit of compilation, are represented in terms of a set of recursively defined domains called the abstract syntax.

Logics of Programs: Axiomatics and Descriptive Power

Mon, 01 May 1978 00:00:00 GMT

Logics of Programs: Axiomatics and Descriptive Power Harel, David This thesis is concerned with the development of mathematical tools for reasoning about computer programs. The approach is to design and investigate the properties of various dynamic logics with an emphasis on useful expressive power and adequate proof theory.

The Specification of Code Generation Algorithms

Sat, 01 Apr 1978 00:00:00 GMT

The Specification of Code Generation Algorithms Terman, Christopher J. This thesis addresses the problem of automatically constructing the code generation phrase of a compiler from a specification of the source language and target machine. A framework for such a specification is presented in which information about language and machine dependent semantics in incorporated as a set of transformation on an internal representation of the source language program.

Actor Systems for Real-time Computation

Wed, 01 Mar 1978 00:00:00 GMT

Actor Systems for Real-time Computation Baker, Henry Givens, Jr. Actor theory was invented by Hewitt and collaborators as a synthesis of many of the ideas from the high-level languages LISP, GEDANKEN, SMALLTALK, SIMULA-67, and others. Actor theory consists of a group of active objects called Actors, which communicate by passing messages to one another.

Final Report of the Multics Kernal Design Project

Wed, 01 Mar 1978 00:00:00 GMT

Final Report of the Multics Kernal Design Project Schroeder, Michael D.; Clark, David D; Saltzer, Jerome H.; Wells, D.H. We describe a plan to create an auditable version of Multics. The engineering experiments of that plan are now complete. Type extension as a design discipline has been demonstrated feasible, even for the internal workings of an operating system, where many subtle intermodule dependencies were discovered and controlled.

On Time-space Classes and Their Relation to the Theory of Real Addition

Wed, 01 Mar 1978 00:00:00 GMT

On Time-space Classes and Their Relation to the Theory of Real Addition Bruss, Anna R. A new lower bound on the computational complexity of the theory of real addition and several related theories is established: any decision procedure for these theories requires either space n2 or nondeterministic time 2en2 for some constant E> 0 and infinitely many n.

Actors and Continuous Functionals

Wed, 01 Feb 1978 00:00:00 GMT

Actors and Continuous Functionals Hewitt, Carl; Baker, Henry Givens, Jr. This paper presents precise versions of some "laws" that must be satisfied by computations involving communicating parallel processes. The laws take the form of stating plausible restrictions on the histories of computations that are physically realizable .

Attribute Partitioning in a Self-adaptive Relational Data Base Systems

Sun, 01 Jan 1978 00:00:00 GMT

Attribute Partitioning in a Self-adaptive Relational Data Base Systems Niamir, Bahram One technique that is sometimes employed to enhance the performance of a data base management system is known as attribute partitioning. This is the process of dividing the attributes of a file into subfiles that are stored separately. By storing together those attributes that are frequently requested together by transactions, and by separating those that are not, attribute partitioning can reduce the number of pages that must be transferred from secondary storage to primary memory in order to process a transaction.

Specifications and Verification Techniques for Parallel Programs Based on Message Passing Semantics

Sun, 01 Jan 1978 00:00:00 GMT

Specifications and Verification Techniques for Parallel Programs Based on Message Passing Semantics Yonezawa, Akinori This thesis presents formal specification and verification techniques for both serial and parallel programs written in SIMULA-like object oriented languages. These techniques are based on the notion of states of individual objects which are defined uniformly in serial and parallel computations. They can specify and verify the behavior of data and procedural objects in multi-process environments, thus overcoming some of the difficulties in dealing with parallelism which characterized previous work on formal specifications for abstract data types.

Abstract Data Types in Stack Based Languages

Wed, 01 Feb 1978 00:00:00 GMT

Abstract Data Types in Stack Based Languages Moss, J. Eliot B. Abstract data types are the basis of an emerging methodology of computer programming. The only existing languages supporting abstract data types directly, CLU and Simula, both require compacting garbage collection, and thus they are not suitable for many applications. This thesis presents the design of a new language incorporating abstract data types; the language requires only a run-time stack, and not garbage collection.

Formal Specifications for Packet Communication Systems

Tue, 01 Nov 1977 00:00:00 GMT

Formal Specifications for Packet Communication Systems Ellis, David J. One of the most difficult tasks facing computer scientists is that of designing systems and making sure that they perform their intended functions correctly. As computer systems have grown in size and complexity, the problems of system design and verification have become increasingly acute.

Simulation of Packet Communication Architecture Computer Systems

Tue, 01 Nov 1977 00:00:00 GMT

Simulation of Packet Communication Architecture Computer Systems Bryant, Randal R. Simulations of computer systems have traditionally been performed on a single sequential computer, even if the system to be simulated contains a number of components which operate concurrently.

A Structure Memory for Data Flow Computers

Mon, 01 Aug 1977 00:00:00 GMT

A Structure Memory for Data Flow Computers Ackerman, William B. A data flow computer is one which achieves enormous concurrency of instruction execution through a machine architecture that acts directly on a data dependency graph of the program.

Deadlock Detection in Computer Networks

Thu, 01 Sep 1977 00:00:00 GMT

Deadlock Detection in Computer Networks Goldman, Barry The problem of detecting process deadlocks is common to transaction oriented computer systems which allow data sharing. Several good algorithms exist for detecting process deadlocks in a single location facility. However, the deadlock detection problem becomes more complex in a geographically distributed computer network due to the fact that all the information needed to detect a deadlock is not necessarily available in a single node, and communications delays may lead to synchronization problems in getting an accurate view of the network state.

Facilitating Interprocess Communication in a Heterogeneous Network Environment

Fri, 01 Jul 1977 00:00:00 GMT

Facilitating Interprocess Communication in a Heterogeneous Network Environment Levine, Paul H. Passing information among processors with different internal data formatting schemes has proven to be a major complication to computer networking efforts. Data format translation is necessary to support information exchange in a heterogeneous network environment. Three strategies for performing this translation for communications between a message sender translation by an intermediate translator, and the use of a standard intermediate format.

A Framework for Processing Dialogue

Wed, 01 Jun 1977 00:00:00 GMT

A Framework for Processing Dialogue Brown, Gretchen P. This report describes a framework for handling mixed-initiative English dialogue in a console session environment, with emphasis on recognition. Within this framework, both linguistic and non-linguistic activities are modelled by structures called methods, which are a declarative form of procedural knowledge. Our design focus on units of linguistic activity larger than the speech act, so that the pragmatic and semantic context of an utterance can be used to guide its interpretation. also important is the treatment of indirect speech acts,e.g., the different ways to ask a question, give a command, etc.

The Complexity of the Finite Containment Problem for Petri Nets

Wed, 01 Jun 1977 00:00:00 GMT

The Complexity of the Finite Containment Problem for Petri Nets Mayr, Ernst Wilhelm If the reachability set of a Petri net (or, equivalently, vector addition system) is finite it can be effectively constructed. Furthermore, the finiteness is decidable. Thus, the containment and equality problem for finite reachability sets become solvable. We investigate the complexity of decision procedures for these problems and show by reducing a bounded version of Hilbert's Tenth Problem to the finite containment problem that these two problems are extremely hard, that, in fact, the complexity of each decision procedure exceeds any primitive recursive function infinitely often.

A Simple and Flexible System Initialization Mechanism

Sun, 01 May 1977 00:00:00 GMT

A Simple and Flexible System Initialization Mechanism Luniewski, Allen W. This thesis presents an approach to system initialization which is simple and easy to understand and, at the same time, is versatile in the face of configuration changes. This thesis considers initialization of a layered system. The initialization mechanism is built upon three key concepts: existence of a minimal configuration, a core image of the system and dynamic reconfiguration.

Non-discretionary Access Control for Decentralized Computing Systems

Sun, 01 May 1977 00:00:00 GMT

Non-discretionary Access Control for Decentralized Computing Systems Karger, Paul A. This thesis examines the issues relating to non-discretionary access controls for decentralized computing systems. Decentralization changes the basic character of a computing system from a set of processes referencing a data base to a set of processes sending and receiving messages. Because massages must be acknowledge, operations that were read-only in a centralized system become read-write operations.

A Layered Virtual Memory Manager

Sun, 01 May 1977 00:00:00 GMT

A Layered Virtual Memory Manager Mason, Andrew Halstead This thesis presents a specification for the Multics virtual memory manager. The virtual memory manager is that part of the operating system which coordinates the usage of physical memory and which manages the bindings between logical memory and physical memory. In the case of Multics, physical memory is composed of fixed-length blocks called frames and logical memory consists of segments, representing sets of frames.

A Digitalis Therapy Advisor with Explanations

Tue, 01 Feb 1977 00:00:00 GMT

A Digitalis Therapy Advisor with Explanations Swartout, William R. This thesis describes the English explanation facility of the OWL Digitalis Advisor, a program designed to advise physicians regarding digitalis therapy. The program is written in OWL, an English-based computer language being developed at MIT. The system can explain, in English, both the methods it uses and how those methods were applied during a particular session. In addition, the program can explain how it acquires information and tell the user how it deals with that information either in general or during a particular session.

A Robust Environment for Program Development

Tue, 01 Feb 1977 00:00:00 GMT

A Robust Environment for Program Development Goldberg, Harold J. This thesis examines the problems of debugging and preservation of the user programming environment and proposes a scheme by which the program development environment can be protected. Typically, designers of timeshared or multiprogrammed computer systems only consider inter-user interference as a source of problems and do not worry about what users do in their own environments. Thus, users can, by writing incorrect programs, cause destruction of the programming environment and personal data bases.

A Case Study of Intermodule Dependencies in a Virtual Memory Subsystem

Wed, 01 Dec 1976 00:00:00 GMT

A Case Study of Intermodule Dependencies in a Virtual Memory Subsystem Hunt, Douglas H. A problem currently confronting computer scientists is to develop a method for the production of large software systems that are easy to understand and certify. The most promising methods involve decomposing a system into small modules in such a way that there are few intermodule dependencies.

Coordination of Parallel Processes in the Actor Model of Computation

Wed, 01 Dec 1976 00:00:00 GMT

Coordination of Parallel Processes in the Actor Model of Computation Goodman, Nathan Two algorithms for the mutual exclusion problem are described and proven to operate correctly. The algorithms are unique in that they use very simple synchronization primitives yet are fair and retain their fairness even if the number of parallel processes in the computer system increases unboundedly over time. One of the algorithms uses simple cells of read/write storage as the primitive; the algorithm is similar to the classic algorithms for this problem proposed by Dijkstra and Knuth, but is generalized to handle an arbitrary number of processes.

A Multi-process Design of Paging System

Wed, 01 Dec 1976 00:00:00 GMT

A Multi-process Design of Paging System Huber, Andrew R. This thesis presents a design for a paging system that may be used to implement a virtual memory on a large scale, demand paged computer utility. A model for such a computer system with a multi-level, hierarchical memory system is presented. The functional requirements of a paging system for such a model are discussed, with emphasis on the parallelism inherent in the algorithms used to implement the memory management functions.

The Logic of Systems

Wed, 01 Dec 1976 00:00:00 GMT

The Logic of Systems Furtek, Frederick Curtis We present a theory about the logical relationships associated with system behavior. The rules governing the behavior of a system are expressed by a Petri net. A set of assumptions about the modeling of a system permit us to separate system behavior into two components, what we refer to as information and control.

Diagnostic Planning and Cancer Management

Wed, 01 Sep 1976 00:00:00 GMT

Diagnostic Planning and Cancer Management Safran, Charles; Desforges, Jane F.; Tsichlis, Philip N. This report describes a computer system for evaluating patients with Hodgkin's disease which has been developed by Clinical Decision Making Group (CDMG) at MIT Laboratory for Computer Science in conjunction with the Blood Research Laboratory of the New England Medical Center Hospitals and Department of Hematology, Tufts University School of Medicine (T-NEMC.H.). This system uses decision theoretic techniques to aid in the formulation of a diagnostic plan for cancer patient.

Semantical Considerations on Floyd-Hoare Logic

Wed, 01 Sep 1976 00:00:00 GMT

Semantical Considerations on Floyd-Hoare Logic Pratt, Vaughan R. This paper deals with logics of programs. The objective is to formalize a notion of program description and to give both plausible (semantic) and effective (syntactic) criteria for the notion of truth of a description. A novel feature of this treatment is the development of the mathematics underlying Floyed-Hoare axiom systems independently of such systems. Our directions that such research might take are also considered.

Using Type Extension to Organize Virtual Memory Mechanisms

Wed, 01 Sep 1976 00:00:00 GMT

Using Type Extension to Organize Virtual Memory Mechanisms Janson, Philippe Arnaud Much effort is currently being devoted to producing systems that are easy to understand, to verify and to develop. The general methodology for designing such a system consists of decomposing it into a structured set of modules so that the modules can be understood, verified and developed individually, and so that the understanding/ verification of the system can be derived from the understanding/ verification of its modules. while many of the mechanisms in a computer system have been decomposed successfully into a structured set of modules, no technique has been proposed to organize the virtual memory mechanism of a system in such a way.

Index Selection in a Self-Adaptive Relational Data Base Management System

Wed, 01 Sep 1976 00:00:00 GMT

Index Selection in a Self-Adaptive Relational Data Base Management System Chan, Arvola Y. The development of large integrated data bases that support a variety of applications in an enterprise promises to be one of the most important data processing activities of the next decade. The effective utilization o such data bases depends on the ability of data base management systems to cope with the evolution of data base applications.

High Level Expression of Semantic Integrity Specifications in a Relational Data Base System

Wed, 01 Sep 1976 00:00:00 GMT

High Level Expression of Semantic Integrity Specifications in a Relational Data Base System McLeod, Dennis J. The "semantic integrity" of a data base is said to be violated when the data base ceases to represent a legitimate configuration of the application environment it is intended to model. In the context of the relational data model, it is possible to identify multiple levels of semantic integrity information: (1) the description of the domains of the data base, as abstract sets of atomic data values (domain definition), (2) the specification of the fundamental structure of the data (relation structure specification), (3) the definition of the abstract operations which are meaningful in terms of the application environment (structured operations), and (4) the expression of additional semantic information not contained in the structure of the relations nor in the identities of their underlying domains (relation constraints).

Processor Multiplexing in a Layered Operating Systems

Thu, 01 Jul 1976 00:00:00 GMT

Processor Multiplexing in a Layered Operating Systems Reed, David Patrick This thesis presents a simply structured design for the implementation of process in a kernel-structured operating system. The design provides a minimal mechanism for the support of two distinct classes of processes found in the computer system -- those which are part of kernel operating system itself, and those used to execute user-specified computations.

A Secure and Flexible Model of Process Initiation for a Computer Utility

Tue, 01 Jun 1976 00:00:00 GMT

A Secure and Flexible Model of Process Initiation for a Computer Utility Montgomery, Warren Alan This thesis demonstrates that the amount of protected, privileged code related to process initiation in a computer utility can be greatly reduced by making process creation unprivileged. The creation of processes can be controlled by the standard mechanism for controlling entry to a domain, which forces a new process to begin execution at a controlled location.

Encryption-based Protection Protocols for Interactive User-computer Communication

Tue, 01 Jun 1976 00:00:00 GMT

Encryption-based Protection Protocols for Interactive User-computer Communication Kent, Stephen T. This thesis develops a complete set of protocols, which utilize a block cipher, e.g., the NBS data encryption standard, for protection interactive user-computer communication over physically unsecured channels. The use of the block cipher protects against disclosure of message contents to an intruder, and the protocols provide for the detection of message stream modification and dental of message service by an intruder.

Decidability Questions for Petri Nets

Tue, 01 Jun 1976 00:00:00 GMT

Decidability Questions for Petri Nets Hack, Michel Henri Théodore An understanding of the mathematical properties of Petri Nets is essential when one wishes to use Petri Nets as an abstract model for concurrent systems. The decidability of various problems which arise in this context is an important aspect of this question. The fact that these problems also arise in the context of other mathematical theories, such as commutative, closure under linear relations, Matrix Context-Free grammars, or Weak Counter Automata, provides further motivation.

A Program for the Design of Procurement Systems

Sat, 01 May 1976 00:00:00 GMT

A Program for the Design of Procurement Systems Bosyj, Michael Computer technology has had a limited success in producing useful business applications. Management systems seldom meet users' requirements, are often inappropriate to an application, and are frequently abandoned. But why? Business lacks expertise in the application of computers .

Petri Net Language

Mon, 01 Mar 1976 00:00:00 GMT

Petri Net Language Hack, Michel Henri Théodore In a labeled Petri Net we assign symbols from an alphabet to some or all the transitions of a Petri Net. To each firing sequence of such a Labeled Petri Net corresponds to a string over the alphabet. We study the languages obtained in this way by all firing sequences of a Petri Net, or by all firing sequences which reach a given final marking.

Some Data Base Applications of Constraint Expressions

Sun, 01 Feb 1976 00:00:00 GMT

Some Data Base Applications of Constraint Expressions Grossman, Richard Weaver This report presents a novel network-like representation for information, called "constraint expressions" (CE). CE makes use of some of the knowledge-representation techniques developed by Artificial Intelligence research.

A Preliminary Study in Computer-aided Legal Analysis

Sat, 01 Nov 1975 00:00:00 GMT

A Preliminary Study in Computer-aided Legal Analysis Meldman, Jeffrey A. This paper describes the prototype for a computer system that can perform a simple kind of legal analysis. The system user, who is presumed to be a lawyer, describes to the system a hypothetical set of facts. The system determines the extent to which these facts fall within certain legal doctrines (by syllogism), or near to these doctrines (by analogy).

Minimizing the Naming Facilities Requiring Protection in a Computing Utility

Mon, 01 Sep 1975 00:00:00 GMT

Minimizing the Naming Facilities Requiring Protection in a Computing Utility Bratt, Richard Glenn This thesis examines the various mechanisms for naming the information objects stored in a general-purpose computing utility, and isolates a basic set of naming facilities that must be protected to assure complete control over user interaction and that allow desired interactions among users to occur in a natural way. Minimizing the protected naming facilities consistent with functional objective of controlled, but natural, user interaction contribute to defining a security kernel for a general-purpose computing utility.

Mechanization of Temporal Knowledge

Mon, 01 Sep 1975 00:00:00 GMT

Mechanization of Temporal Knowledge Kahn, Kenneth M. The design and implementation of a collection of computer programs knowledgeable about time "in general," called the time specialist, is described. The thesis that this time specialist can be placed in the service of larger more general problem solvers is demonstrated for two examples, medical diagnosis and the understanding of a time-travel story.

Semantic of Communication Parallel Processes

Mon, 01 Sep 1975 00:00:00 GMT

Semantic of Communication Parallel Processes Greif, Irene Gloria The thesis of this dissertation is that an understanding of the ordering constraints that are introduced among events of parallel process is essential to the understanding of synchronization and that therefore any language for specifying synchronization of parallel processes should be based on a theory of such orderings. While it is possible to write specifications for systems communicating parallel processes by reference to the time ordering of some global clock external to the system, such specifications cannot be as useful as ones which are in terms of orderings derivable within the system.

Strategy Selection in Medical Diagnosis

Mon, 01 Sep 1975 00:00:00 GMT

Strategy Selection in Medical Diagnosis Miller, Peter B. The recorded, verbal problem-solving behavior of doctors performing the diagnostic task of taking a present illness was analyzed in this research. The goal of the analysis was to discover that data-acquisition strategies were used by the doctors to accomplish the task. A model called the strategy frame model was created to describe the strategies that were found and to provide a mechanism for the selection of a strategy.

Equivalence Problems for Monadic Schemas

Sun, 01 Jun 1975 00:00:00 GMT

Equivalence Problems for Monadic Schemas Qualitz, Joseph E. A class of monadic program schemas is defined. This class, called iteration schemas, consists of schemas whose programs comprise assignment statements, conditional statements, and iteration statements. These schemas are shown to correspond to program schemas which are structured, and are shown to be strictly less "powerful" than monadic program schemas.

Automatic Test, Configuration and Repair of Cellular Arrays

Sun, 01 Jun 1975 00:00:00 GMT

Automatic Test, Configuration and Repair of Cellular Arrays Manning, Frank B. A cellular array is an iterative array of identical information processing machines, cells. The arrays discussed are rectangular arrays of programmable logic, in which information stored in a working cell tells the cell how to behave. No signal line connects more than a few cells. A loading mechanism in each cell allows a computer directly connected to one cell to load any good cell that is not walled off by flawed cells.

A Portable Compiler for the Language C

Thu, 01 May 1975 00:00:00 GMT

A Portable Compiler for the Language C Snyder, Alan This paper describes the implementation of a compiler for the language C. The compiler has been designed to be able to be capable of producing assembly-language code for most register-oriented machines with only minor recoding.

Program Restructuring for Virtual Memory Systems

Sat, 01 Mar 1975 00:00:00 GMT

Program Restructuring for Virtual Memory Systems Johnson, Jerry William The problem area addressed in this report is program restructuring, a method of reordering the relocatable sectors of a program in its address space to increase the locality of the programs reference behavior, thereby reducing the number of page fetches require for execution in a virtual memory system.

A Formalization and Correctness Proof of the CGOL Language System

Sat, 01 Mar 1975 00:00:00 GMT

A Formalization and Correctness Proof of the CGOL Language System VanDeVanter, Michael Lee In many important ways the design and implementation of programming languages are hindered rather than helped by BNF. We present an alternative meta-language based on the work of Pratt which retains much of the effective power of BNF but is more convenient for designer, implementer, and user alike. Its amenability to formal treatment is demonstrated by a rigorous correctness proof of a simple implementation.

The Computational Complexity of Some Logical Theories

Sat, 01 Feb 1975 00:00:00 GMT

The Computational Complexity of Some Logical Theories Rackoff, Charles Weill Upper and lower bounds on the inherent computational complexity of the decision problem for a number of logical theories are established. A general form of Ehrenfeucht game technique for deciding theories is developed which involves analyzing the expressive power of formulas with given quantifier depth. The method allows one to decide the truth of sentences by limiting quantifiers to range over finite sets.

A Digitalis Therapy Advisory

Wed, 01 Jan 1975 00:00:00 GMT

A Digitalis Therapy Advisory Silverman, Howard The physician administering digitalis makes use of the full richness of the clinical setting to form his/her impressions and decide on a therapeutic program. The weakness of existing programs which formulate digitalis dosage regimens lies in their inability to use all of the clinical data available-both quantitative. and qualitative. This report describes the construction of a computer system which formulates digitalis dosage regimens and which adjusts this regimen by interpreting the patient's response to the original dosage regimen.

Some Problems in German in English Machine Translation

Sun, 01 Dec 1974 00:00:00 GMT

Some Problems in German in English Machine Translation Brown, Gretchen P. This paper discusses some problems in the machine translation of natural language, in particular, for translation from German into English. An implementation of some parts of the translating process has been built. The system consists of a German interpretive grammar, to take in German text and output a set of semantic representation, and a generator, to produce English sentences from single semantic representations.

Naming and Protection in Extendable Operating Systems

Fri, 01 Nov 1974 00:00:00 GMT

Naming and Protection in Extendable Operating Systems Redell, David D. The properties of capability-based extendible operating systems are described, and various aspects of such systems are discussed, with emphasis on the conflict between free distribution of access privileges and later revocation of those privileges.

Nondeterministic Time and Space Complexity Classes

Sun, 01 Sep 1974 00:00:00 GMT

Nondeterministic Time and Space Complexity Classes Seiferas, Joel Irvin The marginal utility of the Turing machine computational resources running time and storage space are studied. A technique is developed which, unlike diagonalization, applies equally well to nondeterministic and deterministic automata. For f, g time or space bounding functions with f (n+1) small compared to g(n), it is shown that, in terms of word length n, there are languages which are accepted by Turing machines operating within time or space g(n) but which are accepted by no Turing machine operating within time or space f(n). The proof involves use of the recursion theorem together with "padding" or "translational" techniques of formal language theory.

Functional Domains of Applicative Languages

Sun, 01 Sep 1974 00:00:00 GMT

Functional Domains of Applicative Languages Ward, Stephen A. The expressive power of a particular applicative language may be characterized by the set of abstract functions directly representable in that language. The common FUNARG and applicative order problems are scrutinized in this way, and the effects of these weaknesses are related to the inexpressibility of classes of functions.

Semantics of Data Structures and References

Thu, 01 Aug 1974 00:00:00 GMT

Semantics of Data Structures and References Ellis, David J. Each programming language that handles data structures has its own set of rules for working with them. Notions such as assignment and construction of structures values appear in a huge number of different and complicated versions. This thesis presents a methodology which provides a common basis for describing ways in which programming languages deal with data structures and reference to them.

Removing the Dynamic Linker from the Security Kernel of a Computing Utility

Sat, 01 Jun 1974 00:00:00 GMT

Removing the Dynamic Linker from the Security Kernel of a Computing Utility Jason, Philippe Arnaud In order to enforce the security of the information stored in a computing utility, it is necessary to certify that the protection mechanism is correctly implemented so that there exist no uncontrolled access path to the stored information.

Mathematical Logic for Computer Scientists

Sat, 01 Jun 1974 00:00:00 GMT

Mathematical Logic for Computer Scientists Levin, Michael This book is an introductory course in mathematical logic covering basic topics in quantification theory and recursive function theory, and is intended for the reader who is interested in artificial intelligence, computer linguistics, and other related areas. The text is theoretical, but organized with implementation in mind.

Using Interactive Graphics in Simulating the Hospital Emergency Room

Wed, 01 May 1974 00:00:00 GMT

Using Interactive Graphics in Simulating the Hospital Emergency Room Weissberg, Richard W. The hospital emergency room is a complex system having many interrelated factors contributing to its operation. The emergency room administrator has limited control over certain of these factors: numbers of beds, nurses, doctors, x-ray units; for example. Other factors such as patient arrival rates and demands made upon available resources are largely uncontrollable.

The Computer Utility as a Marketplace for Computer Service

Wed, 01 May 1974 00:00:00 GMT

The Computer Utility as a Marketplace for Computer Service Frankston, Robert Mm. Computers are unique in their ability to be programmed for a wide variety of applications. This is in contrast with hardware dedicated to specific tasks such as the telephone system. Because of its flexibility, a computer system can support, concurrently, many diverse services that do not require dedicated hardware.

An Experimental Analysis of Program Reference Patterns in the Multics Virtual Memory

Wed, 01 May 1974 00:00:00 GMT

An Experimental Analysis of Program Reference Patterns in the Multics Virtual Memory Greenberg, Bernard Stewart This thesis reports the design, conducting, and results of an experiment intended to measure the paging rate of a virtual memory computer system as a function of paging memory size. This experiment, conducted on the Multics computer system at MIT, a large interactive computer utility serving an academic community, sought to predict paging rates for paging memory sizes larger than existing memory at the time.

A Model-debugging System

Tue, 01 Jan 1974 00:00:00 GMT

A Model-debugging System Mark, William S.

Verification of Programs Operating on Structured Data

Fri, 01 Mar 1974 00:00:00 GMT

Verification of Programs Operating on Structured Data Laventhal, Mark Steven The major method for verifying the correctness of computer program is the inductive assertion approach. This approach has been limited in the past by the lack of techniques for handling data structures. In particular, there has been a need for concepts with which to describe structured data during intermediate and final stages of a computation.

Introduction to Multics

Fri, 01 Feb 1974 00:00:00 GMT

Introduction to Multics Saltzer, Jerome H. The Multics project was begun in 1964 by the Computer Systems Research group of M.I.T. Project MAC. The goal was to create a prototype of a computer utility. In 1965, the project became a cooperative venture of M.I.T. Project MAC, the General Electric Company Computer Department (now Honeywell Information Systems Inc. ) and Bell Telephone Laboratories. In 1969, at the end of the research phase of the project, Bell Telephone Laboratories ended its active involvement.

On Lower Bounds for Selection Problems

Fri, 01 Mar 1974 00:00:00 GMT

On Lower Bounds for Selection Problems Yao, Foong Frances Let V i (n) be the minimum number of binary comparisons that are required to determine the i-th largest of n elements drawn from a totally ordered set. In this thesis we use adversary strategies to prove lower bounds on V i (n). For i = 3, our lower bounds determine V 3(n) precisely for infinitely many values of n,and determine V 3(n) to within 2 for all n.

Analysis of Asynchronous Concurrent Systems by Timed Petri Nets

Fri, 01 Feb 1974 00:00:00 GMT

Analysis of Asynchronous Concurrent Systems by Timed Petri Nets Ramchandani, Chander This thesis is concerned with the modeling and performance analysis of systems which consist of concurrently acting components, an example of which is an asynchronous pipelined processor. The work is divided into two parts. In the first part, a suitable model is developed for describing the structure of asynchronous concurrent systems. In conventional automata theory, the finite-state machine model is used to describe the behavior of systems; the problem with this is that a large number of states results when practical systems are modelled.

An Abstract Model of a Research Institute: Simple Automatic Programming Approach

Fri, 01 Mar 1974 00:00:00 GMT

An Abstract Model of a Research Institute: Simple Automatic Programming Approach Briabrin, Victor A problem of knowledge representation is considered in terms of designing a model for a simple sociological structure. A version of the access language is proposed which is based on three kind of expressions accepted by the system - constructors, specificators and requests. In addition, some topics concerned with model implementation and extension are discussed.

An Input/Output Architecture for Virtual Memory Computer Systems

Tue, 01 Jan 1974 00:00:00 GMT

An Input/Output Architecture for Virtual Memory Computer Systems Clark, David D In many large systems today, input/output is not performed directly by the user, but is done interpretively by the system for him, which causes additional overhead and also restricts the user to whatever algorithms the system has implemented. Many causes contribute to this involvement of the system in user input/output, including the need to enforce protection requirements, the inability to provide adequate response to control signals from devices, and the difficulty of running devices in a virtual environment, especially a virtual memory.

Backup and Recovery of On-line Information in a Computer Utility

Tue, 01 Jan 1974 00:00:00 GMT

Backup and Recovery of On-line Information in a Computer Utility Stern, Jerry A. This thesis describes a design for an automatic backup mechanism to be incorporated in a computer utility for the protection of on-line information against accidental or malicious destruction. This protection is achieved by preserving on magnetic tape recent copies of all items of information known to the online system. In the event of a system failure, file system damage is automatically assessed and missing information is recovered from backup storage.

Packet Communication

Sat, 01 Dec 1973 00:00:00 GMT

Packet Communication Metcalfe, Robert Melancton This report develops a theory of packet communication; it analyzes users of computers in digital communication systems and examines structures for organizing computers in highly communicative environments. Various examples from existing computer networks, including the ARPA Computer Network and the ALOHA System, are used to motivate and substantiate analysis of (1) store-and-forward packet communication, (2) broadcast packet communication, and (3) distributed interprocess communication.

On Reducibility Among Combinatorial Problems

Sat, 01 Dec 1973 00:00:00 GMT

On Reducibility Among Combinatorial Problems Herrmann, Paul Peter A large class of combinatorial problems have been shown by Cook and Karp to be computationally equivalent to within a polynomial. We exhibit some new problems in this class, and provide simpler proofs for some of the known reductions.

Productivity in Parallel Computational Schemata

Sat, 01 Dec 1973 00:00:00 GMT

Productivity in Parallel Computational Schemata Linderman, John P. A general model for parallel computation is developed in three parts. One part, the data flow graph, describes how actors which transform and test values are connected to the locations in a finite memory. Another part, an interpretation, supplies information about the contents of memory and the detailed nature of the transformations and tests.

Complexity Classes of Recursive Functions

Fri, 01 Jun 1973 00:00:00 GMT

Complexity Classes of Recursive Functions Moll, Robert An honest function is one whose size honestly reflects its computation time. In 1969 Meyer and McCreight proved the "honesty theorem," which says that for every t, the t-computable functions are the same as the t'computable functions for some honest honest t'.

Storage Hierarchy Systems

Sun, 01 Apr 1973 00:00:00 GMT

Storage Hierarchy Systems Madnick, Stuart E. The relationship between page size, program behavior, and page fetch frequency in storage hierarchy systems is formalized and analyzed. It is proven that there exist cyclic program reference patterns that can cause page fetch frequency to increase significantly if the page used is decreased (e.g., reduced by half).

The Emptiness and Complementation Problems for Automata on Infinite Trees

Mon, 01 Jan 1973 00:00:00 GMT

The Emptiness and Complementation Problems for Automata on Infinite Trees Rackoff, Charles Weill In [6] Rabin defines Automata on Infinite Trees, and the body of that paper is concerned with proving two theorems about these automata. The result we consider in the first chapter says that there exists an effective procedure to determine, given an automaton on infinite trees, whether or not it accepts anything at all. We present a new decision procedure which is much simpler than Rabin's since we do not use an induction argument as he does.

An Analysis of Sorting Networks

Sun, 01 Oct 1972 00:00:00 GMT

An Analysis of Sorting Networks Smith, Burton J. Comparators which sort two numbers can be interconnected to form networks which sort n numbers for any n. The input and output characteristics of comparator networks are analyzed from several different points of view.

Cooperation of Mutually Suspicious Subsystems in a Computer Utility

Fri, 01 Sep 1972 00:00:00 GMT

Cooperation of Mutually Suspicious Subsystems in a Computer Utility Schroeder, Michael D. This thesis describes practical protection mechanisms that allow mutually suspicious subsystems to cooperate in a single computation and still be protected from one another. The mechanisms are based on the division of a computation into independent domains of access privilege, each of which may encapsulate a protected subsystem. The central component of the mechanisms is a hardware processor that automatically enforces the access constraints associated with a multidomain computation implemented as a single execution point in a segmented virtual memory.

Finite Tree Automata and W-Automata

Fri, 01 Sep 1972 00:00:00 GMT

Finite Tree Automata and W-Automata Hossley, Robert Chapter I is a survey of finite automata as acceptors of finite labeled trees. Chapter II is a survey of finite automata as acceptors of infinite strings on a finite alphabet. Among the automata models considered in Chapter II are those used by McNaughton, Buchi, and Landweber. In Chapter II we also consider several new automata models based on a notion of a run of a finite automataton on an infinite string suggested by Professor A.R. Meyer in private communication. We show that these new models are all equivalent to various previously formulated models.

On the Design and Specification of a Common Base Lanaguage

Thu, 01 Jun 1972 00:00:00 GMT

On the Design and Specification of a Common Base Lanaguage Dennis, Jack B. This is the report on the work of the Computational Structures Group of Project MAC toward the design and specification of a common base language for programs and information structures. We envision that the meanings of programs expressed in practical source languages will be defined by rules of translation into the base language.

Further Results on Hierarchies of Canonic Systems

Thu, 01 Jun 1972 00:00:00 GMT

Further Results on Hierarchies of Canonic Systems Mandl, Robert This thesis outlines a new way of presenting the theory of canonic systems, including a distinction (for mathodic reasons) between simple canonic systems and general canonic systems, and proves a series of results on hierarchies of canonic systems. After a brief summary of Doyle's results on a partial hierarchy of canonic systems, a new hierarchy is developed (Chapter II) which relates the general canonic systems not only to all 4 types of formal grammars defined by Chomsky but also to any class shown (Chapter III) that all attempts to define a mathematical system which exactly corresponds to the recursive sets are necessarily fruitless.

Relativization of the Theory of Computational Complexity

Thu, 01 Jun 1972 00:00:00 GMT

Relativization of the Theory of Computational Complexity Lynch, Nancy A. Blum's machine-independent treatment of the complexity of partial recursive functions is extended to relative algorithms (as represented by Turing machines with oracles). We prove relativizations of several results of Blum complexity theory, such as the compression theorem. A recursive relatedness theorem is proved, showing that any two relative complexity measures are related by fixed recursive function. This theorem allows us to obtain proofs of results for all measures from proofs for a particular measure.

The Complexity of Finite Functions

Wed, 01 Mar 1972 00:00:00 GMT

The Complexity of Finite Functions Vilfan, Bostjan Lower bounds on the length of formulas for finite functions are obtained from a generalization of a theorem of Specker. Let f: (0,1,...,d-1) [0,1,...,d-1] be a function which can be represented by a formula of length < c.n. For any m, if n is sufficiently large, there is a restriction f': {0,1,...,d-1}m > {0,...,d-1} of f which, is representable by special class of formulas called homogeneous e-complexes.

Autonomous, Synchronous Counters Constructed Only of J-K Flip-flops

Mon, 01 May 1972 00:00:00 GMT

Autonomous, Synchronous Counters Constructed Only of J-K Flip-flops Manning, Frank This report describes research into some properties of autonomous, synchronous counters constructed with only the simplest form of J-K Flip-Flop. The research revolved around a system with a special-purpose digital machine and a general-purpose computer. The special-purpose searched through all the possible counters constructed of five or fewer J-K Flip-Flops for all counters with a period equal to that specified by th input to the system.

Essays in Algebraic Simplification

Sat, 01 Apr 1972 00:00:00 GMT

Essays in Algebraic Simplification Fateman, Richard J. This thesis consists of essays on several aspects of the problem of algebraic simplification by computer. Since simplification is at the core of most algebraic manipulations, efficient and effective simplification procedures are essential to building useful computer systems for non-numerical mathematics. Efficiency is attained through carefully designed and engineered algorithms, heuristics,and data types, while effectiveness is assured through theoretical considerations.

Analysis of Production Schemata by Petri Nets

Tue, 01 Feb 1972 00:00:00 GMT

Analysis of Production Schemata by Petri Nets Hack, Michel Henri Théodore Petri nets provide a powerful graphical tool for representing and analyzing complex concurrent systems. Properties such as hang-up freeness, determinacy, conflict, concurrency and dependency, can be represented and studied. The precise relationship between structural and behavioral properties, and between local and global properties is not well-understood for the most general class of Petri Nets.

Induction in Proofs about Programs

Tue, 01 Feb 1972 00:00:00 GMT

Induction in Proofs about Programs Greif, Irene Gloria Four methods for proving equivalence of programs by induction are described and compared. They are recursion induction, structural induction, mu-rule induction, and truncation induction. McCarthy's formalism for conditional expressions as function definitions is used and reinterpreted in view of Park's work on results on results in lattice theory as related to proofs about programs. The possible application of this work to automatic program verification is commented upon.

Evaluation of Definite Integrals by Symbolic Manipulation

Wed, 01 Sep 1971 00:00:00 GMT

Evaluation of Definite Integrals by Symbolic Manipulation Wang, Paul S. A heuristic computer program for the evaluation of real definite integrals of elementary functions is described. This program, called WANDERER (WANg's DEfinite integRal EvaluatoR), evaluates many proper and improper integrals. The improper integrals may have a finite or infinite range of integration. Evaluation by contour integration and residue theory is among the methods used. A program called DELIMITER (DEfinitive LIMITEvaluatoR) is used for the limit computations needed in evaluating some definite integrals.

Cost Analysis of Debugging Systems

Wed, 01 Sep 1971 00:00:00 GMT

Cost Analysis of Debugging Systems Lester, Bruce P. A general method is presented for performing cost analysis of interactive debugging systems. The method is based on an abstract model of program execution. This model is derived from the interpreter used in the Vienna method of semantic definition of PL/I. A brief discussion of the overall operation and significance of Vienna interpreter is included.

Primary Access Control in Large-scale Time-shared Decision Systems

Thu, 01 Jul 1971 00:00:00 GMT

Primary Access Control in Large-scale Time-shared Decision Systems Owens, Richard C., Jr. The computer differs from other tools in that it presently does not provide its users with a working environment transparent to their desires; in particular, current computer systems do not support adequate mechanisms for controlled sharing of sensitive information. Four primary dimensions of the access control problem are identified. They are: 1) the physical level at which to apply control; 2) the fineness of distinction applied to the term ""access"" 3) the meaning of the term "user identification",and 4) the degree of sophistication employed in automatically assigned restrictions to new data files.

Bounds on Information Retrieval Efficiency in Static File Structures

Tue, 01 Jun 1971 00:00:00 GMT

Bounds on Information Retrieval Efficiency in Static File Structures Welch, Terry A. This research addresses the problem of file organization for efficient information retrieval when each file item may be accessed through any one of a large number of identification keys. The emphasis is on library problems, namely large, low-update, directory-oriented files, but other types of files are discussed.

Dynamic Reconfiguration in a Modular Computer System

Tue, 01 Jun 1971 00:00:00 GMT

Dynamic Reconfiguration in a Modular Computer System Schell, Roger R. This thesis presents an orderly design approach for dynamically changing the configuration of constituent physical units in a modular computer system. Dynamic reconfiguration contributes to high system availability by allowing preventative maintenance, development of new operating systems, and changes in system capacity on a non-interference basis.

Automatic Creation of a Code Generator from a Machine Description

Sat, 01 May 1971 00:00:00 GMT

Automatic Creation of a Code Generator from a Machine Description Miller, Perry L. This paper studies some of the problems involved in attaining machine independence for a code generator, similar to the language independence and the token independence attained by automatic parsing and automatic lexical systems. In particular, the paper examines the logic involved in two areas of code generation: computation and data reference.

Computer Analysis of Visual Properties of Curved Objects

Sat, 01 May 1971 00:00:00 GMT

Computer Analysis of Visual Properties of Curved Objects Krakauer, Lawrence J. A method is presented for the visual analysis of objects by computer. It is particularly well suited for opaque objects with smoothly curved surfaces. The method extracts information about the object's surface properties, including measures of its specularity, texture, and regularity. It also aids in determining the object's shape.

Information Processing and Transmission in Cellular Automata

Fri, 01 Jan 1971 00:00:00 GMT

Information Processing and Transmission in Cellular Automata Banks, Edwin R. A cellular automaton is an iterative array of very simple identical information processing machines called cells. Each cell can communicate with neighboring cells. At discrete moments of time the cells can change from one state to another as a function of the states of the cell and its neighbors. Thus on a global basis, the collection of cells is characterized by some type of behavior.

Shape from Shading: A Method for Obtaining the Shape of a Smooth Opaque Object From One View

Sun, 01 Nov 1970 00:00:00 GMT

Shape from Shading: A Method for Obtaining the Shape of a Smooth Opaque Object From One View Horn, Berthold K. P. A method will be described for finding the shape of a smooth opaque object from a monocular image, given a knowledge of the surface photometry, the position of the light-source and certain auxiliary information to resolve ambiguities. This method is complementary to the use of stereoscopy which relies on matching up sharp detail and will fail on smooth objects. Until now the image processing of a single views has been restricted to objects which can meaningfully be considered two-dimensional or bounded by plane surfaces.

Design Strategies for File Systems

Thu, 01 Oct 1970 00:00:00 GMT

Design Strategies for File Systems Madnick, S.E. This thesis describes a methodology for the analysis and synthesis of modern general purpose file systems. The two basic concepts developed are (1) establishment of a uniform representation of a file's structure in the form of virtual memory or segmentation and (2) determination of a hierarchy of logical transformations within a file system.

Complexity Measures for Language Recognition by Canonic Systems

Thu, 01 Oct 1970 00:00:00 GMT

Complexity Measures for Language Recognition by Canonic Systems Haggerty, Joseph P. A canonic system C is a specification of a recursively enumerable set, such as a set of strings over a finite alphabet. From this description C, it is possible to generate a system C , called a proof measure function, which is an indication of the complexity of the language defined. For certain simple but important classes of canonic system, algebraic bounds on these functions can be derived from the structure of the system.

Deadlock-free Sharing of Resources in Asynchornous Systems

Tue, 01 Sep 1970 00:00:00 GMT

Deadlock-free Sharing of Resources in Asynchornous Systems Hebalkar, Prakash G. Whenever resources are shared among several activities that hoard resources, the activities can attain a state of deadlock in which, for lack of resources, none of the activities can proceed. Deadlocks can be prevented by coordination of the sharing. efficient running of the activities under such coordination requires knowledge of the patterns of use of resources by the activities.

Integral Convex Polyhedra and an Approach to Integralization

Sat, 01 Aug 1970 00:00:00 GMT

Integral Convex Polyhedra and an Approach to Integralization Edelberg, Murray Many combinatorial optimization problems may be formulated as integer linear programming problems - that is, problems of the form: given a convex polyhedron P contained in the non-negative orthant of n-dimensional space, find a integer point in P which maximizes (or minimizes) a given linear objective function. Well known linear programming methods would suffice to solve such a problem if: (i) P is an integral convex polyhedron, or (ii) P is transformed into the integral convex polyhedron that is the convex hull of the set of integer points in P, a process which is called integralization.

Computer Recognition of Prismatic Solids

Sat, 01 Aug 1970 00:00:00 GMT

Computer Recognition of Prismatic Solids Griffith, Arnold Koons An investigation is made into the problem of constructing a model of the appearance to an optical input device of scenes consisting of plane-faced geometric solids. The goal is to study algorithms which find the real straight edges in the scenes, taking into account smooth variations in intensity over faces of the solids, blurring of edges and noise.

Coordination of Asynchronous Event

Mon, 01 Jun 1970 00:00:00 GMT

Coordination of Asynchronous Event Patil, Suhas Shrikrishna The way activity in a system proceeds is that events occur as a result of some conditions and lead to some new conditions which make other events possible. Often it is necessary to coordinate such events to ensure proper behavior. Coordination nets for representing such coordinations and physically realizable structures for enforcing such coordinations are presented. These structures are modular and can be mechanically derived from the coordination nets. Coordination involved in concurrent management of resources are also discussed.

A Computer-controlled Graphical Display Processor

Mon, 01 Jun 1970 00:00:00 GMT

A Computer-controlled Graphical Display Processor Fiasconaro, James Gerard A cathode-ray tube, (CRT), is frequently employed to display text and drawings generated by a digital computer. Unfortunately, all of the commercially available CRT display systems are either very expensive or have limited dynamic capability resulting from the use of some form of storage-type CRT. A need exists to develop a low-cost, relatively sophisticated display compute-generated pictures.

Generalized Organization of Large Data Bases: A Set-theoretic Approach to Relations

Mon, 01 Jun 1970 00:00:00 GMT

Generalized Organization of Large Data Bases: A Set-theoretic Approach to Relations Fillat, Andrew Irwin; Kraning, Leslie Alan Problems inherent in representation and manipulation of large data bases are discussed. Data management is considered as the manipulation of relationships among elements of a data base. A detailed analogy introduces concepts embodied in a data management system. Set theory is used to describe a model for data-bases, and operations suitable for manipulation of relations are defined.

Economies of Scale in Computer Use: Initial Test and Implication A for the Computer Utility

Mon, 01 Jun 1970 00:00:00 GMT

Economies of Scale in Computer Use: Initial Test and Implication A for the Computer Utility Selwyn, Lee L. This study is concerned with the existence of economies of scale in the production of data processing and other computing services, and the possible regulatory and public policy implications of such economies. The rapid development of the technology of computation since the Second World War has raised many questions as to the supervision by public authorities of the use and progress of this technology.

Controlled Information Sharing in a Computer Utility

Wed, 01 Oct 1969 00:00:00 GMT

Controlled Information Sharing in a Computer Utility Vanderbilt, Dean H. A computer utility is envisioned as a large, multi-access computer system providing its users with the ability to store information and share its use with other system users. This thesis considers the nature of information sharing and how a computer utility can provide facilities allowing such sharing to take place in a controlled manner.

Recognition of Translators Invariants* By Iterative Arrays

Wed, 01 Oct 1969 00:00:00 GMT

Recognition of Translators Invariants* By Iterative Arrays Beyer, Wendel Terry A study is made of the recognition and transformation of figures by iterative arrays of finite state automata. A figure is a finite rectangular two-dimensional array of symbols. The iterative arrays considered are also finite, rectangular and two-dimensional.

Practical Translators for LR(k) Languages

Wed, 01 Oct 1969 00:00:00 GMT

Practical Translators for LR(k) Languages Deremer, Franklin Lewis A context-free syntactical translator (CFST) is a machine which defines a translation from one context-free language to another. A transduction grammar is a formal system based on a context-free grammar and it specifies a context-free syntactical translation. A simple suffix transduction grammar based on a context-free grammar which is LR(k) specifies a translation which can be defined by a deterministic push-down automation (DPDA).

A Graph Model for Parallel Computations

Mon, 01 Sep 1969 00:00:00 GMT

A Graph Model for Parallel Computations Rodrigues, Jorge E. This report presents a computational model called program graphs which makes possible a precise description of parallel computations of arbitrary complexity on non-structured data. In the model, the computation steps are represented by the nodes of a directed graph whose links represent elements of storage and transmission of data and /or control information.

Case Study in Interactive Graphics Programming: A Circuit Drawing and Editing Program for Use with A Storage-tube Display Terminal

Wed, 01 Oct 1969 00:00:00 GMT

Case Study in Interactive Graphics Programming: A Circuit Drawing and Editing Program for Use with A Storage-tube Display Terminal Brackett, J.; Hammer, M.M.; Thornhill, D. The concepts involved in building and manipulating a data structure through graphical interaction are presented, using the drawing and editing of electrical circuits as a vehicle. The circuit drawings program was designed to operate on an ARDS storage-tube display terminal attached to the M.I.T. Project MAC IBM 7094 Compatible Time-Sharing System.

EPS: An Interactive System for Solving Elliptic Boundary-Value Problems with Facilities for Data Manipulation

Sun, 01 Jun 1969 00:00:00 GMT

EPS: An Interactive System for Solving Elliptic Boundary-Value Problems with Facilities for Data Manipulation Tillman, Coyt C., Jr. This appendix for the author's forthcoming thesis, "On-Line Solution of Elliptic Boundary-Value Problems," is a user's guide for EPS. EPS solves two-dimensional boundary-value problems for elliptic systems of second-order partial differential equations. It also has general-purpose capabilities which permit the on-line definition and execution of arbitrary numerical procedures.

Interactive Computer-mediated Animation

Sun, 01 Jun 1969 00:00:00 GMT

Interactive Computer-mediated Animation Baeker, Ronald M. The use of interactive computer graphics in the construction of animated visual displays is investigated. The dissertation presents a process called interactive computer-mediated animation, in which dynamic displays are constructed by utilizing direct console commands, algorithms, free-hand sketches, and real-time actions. The resulting "movie" can then be immediately viewed and altered.

A Formal System for Defining the Syntax and Semantics of Computer Languages

Tue, 01 Apr 1969 00:00:00 GMT

A Formal System for Defining the Syntax and Semantics of Computer Languages Ledgard, Henry Francis The thesis of this dissertation is that formal definitions of the syntax and semantics of computer languages are needed. This dissertation investigates two candidates for formally defining computer languages: (1) the formalism of canonical systems for defining the syntax of a computer language and its translation into a target language for defining the semantics of a computer language.

Computer Recognition of Three-Dimensional Objects in a Visual Scene

Sun, 01 Dec 1968 00:00:00 GMT

Computer Recognition of Three-Dimensional Objects in a Visual Scene Guzman-Arenas, Aldolfo Methods are presented 1) to partition or decompose a visual scene into the bodies forming it; (2) to position these bodies in three-dimensional space, by combining two scenes that make a stereoscopic pair; 3) to find the regions or zones of a visual scene that belong to its background, (4) to carry out the isolation of objects in (1) when the input has inaccuracies.

A Simulator of Multiple Interactive Users to Drive a Time-shared Computer System

Wed, 01 Jan 1969 00:00:00 GMT

A Simulator of Multiple Interactive Users to Drive a Time-shared Computer System Greenbaum, Howard Jacques In the construction and maintenance of a time-shared computer system the need arises for a tool which can provide a controlled, repeatable environment for the purpose of making performance measurements. This thesis describes the use of a small second computer to simulate the actions of multiple interactive users over individual communication lines. Each simulated user exhibits responses similar to those of a "normal" interactive user.

Lambda Calculus Models of Programming Languages

Sun, 01 Dec 1968 00:00:00 GMT

Lambda Calculus Models of Programming Languages Morris, James H. Two aspects of programming languages, recursive definitions and type declarations are analyzed in detail. Church's -calculus is used as a model of a programming language for purposes of the analysis. The main result on recursion is an analogue to Kleene's first recursion theorem: If A= FA for any A-expressions A and F, then A is an extension of YF in the sense that if E[YE], any expressions containing YF, has a normal form then E[F] =E {A]. Y is Curry's paradoxical combinator. The result is shown to be invariant for many different versions of Y.

An Integrated Hardware-software Systems for Computer Graphics in Time-sharing

Sun, 01 Dec 1968 00:00:00 GMT

An Integrated Hardware-software Systems for Computer Graphics in Time-sharing Thornhill, D.E.; Stotz, R.H.; Ross, D.T.; Ward, J.E. This report describes the ESL Display Console and its associated user-oriented software systems developed by the M.I.T. Computer-Aided Design Project with Project MAC. Console facilities include hardware projection of three-dimensional line drawings, automatic light pen tracking, and a flexible set of knob, switch, and push-button inputs. The console is attached to the Project MAC IBM 7094 Compatible Time-Sharing System either directly or through a PDP-7 Computer.

Implementing Multi-process Primitives in a Multiplexed Computer System

Fri, 01 Nov 1968 00:00:00 GMT

Implementing Multi-process Primitives in a Multiplexed Computer System Rappaport, Robert Lee In any computer system primitive functions are needed to control the actions of processes in the system. This thesis discusses a set of six such process control primitives which are sufficient to solve many of the problems involved in parallel processing as well as in the efficient multiplexing of system resources among the many processes in a system.

The Graph Display as an Aid in the Monitoring of a Time-shared Computer System

Tue, 01 Oct 1968 00:00:00 GMT

The Graph Display as an Aid in the Monitoring of a Time-shared Computer System Grochow, Jerrold Marvin The problem of dynamic observation of the state of a time-shared computer system is investigated. The Graphical Display Monitoring System was developed as a medium for this experimental work. It is an integrated system for creating graphic displays, dynamically retrieving data from Multics Time-Sharing Systems supervisor data bases, and on-line viewing of this data viewing of this data via the graphics displays.

Absenetee Computations in a Multiple-access Computer System

Thu, 01 Aug 1968 00:00:00 GMT

Absenetee Computations in a Multiple-access Computer System Deital, H.M. in multiple-access computer systems, emphasis is placed upon serving several interactive users simultaneously. However, many computations do not require user interaction, and user may therefore want to run these computations 'absentee' (or, user not present). A mechanism is presented which provides for the handling of absentee computations in a multiple-access computer system.

CARPS, A Program Which Solves Calculus Word Problems

Mon, 01 Jul 1968 00:00:00 GMT

CARPS, A Program Which Solves Calculus Word Problems Charniak, Eugene A program was written to solve calculus word problems. The program CARPS (Calculus Rate Problem Solver), is restricted to rate problems. The overall plan of the program is similar to Bobrow's STUDENT, the primary difference being the introduction of "structures" as the internal model in CARPS. Structures are stored internally as trees. Each structures is designed to hold the information gathered about one object.

Resource Allocation in Multiprocess Computer Systems

Wed, 01 May 1968 00:00:00 GMT

Resource Allocation in Multiprocess Computer Systems Denning, Peter James The dynamic allocation for limited processor and main memory resources among members of a user community is investigated as a supply-and-demand problem. The work is divided into four phases. First phase is the construction of the working set model for program behavior. This model is based on locality, the concept that, during any interval of execution, a program favors a subset of its information; a computation's working set is a dynamic measure of this set of favored information. A working set storage management policy is one that allocates processors to a computation if and only if there is enough uncommitted space in main memory to contain its working set.

Incremental Simulation on a Time-shared Computer

Mon, 01 Jan 1968 00:00:00 GMT

Incremental Simulation on a Time-shared Computer Jones, Malcolm Murray This thesis describes a system which allows simulation models to be built and tested incrementally. It is called OPS-4 and is specifically designed to operate in the environment of the Multics system. It represents a major expansion and improvement of the OPS-3 system implemented in CTSS and also includes many features adapted from other current simulation systems.

Symbolic Integration

Fri, 01 Dec 1967 00:00:00 GMT

Symbolic Integration Moses, Joel SIN and SOLDIER are heuristic programs written in LISP which solve symbolic integration problems. SIN (Symbolic INtegrator) solves indefinite integration problems at the difficulty approaching those in the larger integral tables. SIN contains several more methods than are used in the previous symbolic integration program SAINT, and solves most of the problems attempted by SAINT in less than one second. SOLDIER (SOLution of Ordinary Differential Equations Routine) solves first order, first degree ordinary differential equations at the level of a good college sophomore and at an average of about five seconds per problem attempted.

A Canonic Translator

Wed, 01 Nov 1967 00:00:00 GMT

A Canonic Translator Alsop, Joseph Wright An algorithm to recognize and translate sets of character strings specified by canonic system is presented. The ability of canonic systems to define the context sensitive features of strings and to specify their translation allows the algorithm to recognize and translate real computer languages. It is also applicable in other languages systems.

On the Simulation of Dynamic Systems with Lumped Parameters and Time Delays

Sun, 01 Jan 1967 00:00:00 GMT

On the Simulation of Dynamic Systems with Lumped Parameters and Time Delays Leal-Cantu, Nestor

A System for Computer-aided Diagnosis

Fri, 01 Sep 1967 00:00:00 GMT

A System for Computer-aided Diagnosis Gorry, Gregory Anthony This thesis describes a model diagnostic problem and a computer program designed to deal with this problem. The model diagnostic problem is an abstract problem. A major contention of this thesis, however, is that this problem subsumes the principal feature of a number of ostensibly different real diagnostic problems including certain problems of medical diagnosis and the diagnosis of machine failures. A second major contention of this thesis is that strategies for the solution of the model diagnostic problem can be formulated in terms sufficiently explicit to permit their incorporation in a computer program.

Program Analysis by Digital Computer

Tue, 01 Aug 1967 00:00:00 GMT

Program Analysis by Digital Computer Wilde, Daniel Underwood A comparison of the properties of non-modifying and self-modifying programs leads to the definition of independent and dependent instructions. Because non-modifying programs contain only independent instructions, such programs can be analyzed by a straight forward, two -step analysis procedure. First, the program control flow is detected; second, that control flow is used to determine the program data flow or data processing. However, self-modifying programs can also contain dependent instructions, and the program control flows and data flows exhibit cyclic interaction.

Design and Implementation of a Table-Driven Compiler System

Sat, 01 Jul 1967 00:00:00 GMT

Design and Implementation of a Table-Driven Compiler System Liu, Chung L.; Change, Gabriel D.; Marks, Richard E. Our goal is to provide users of the table-driven compiler system with an environment within which they can freely design and produce their compilers. The primary design criterion is generality so that the users can define a large class of input languages oriented toward any kind of problem-solving purposes, and can also define a large class of object programs to be executed on different computer systems. Therefore, in our system we do not limit the users to specific ways of doing syntactic analysis, or doing storage allocation, or producing binary programs of a specific format for a particular computer system. What we provide are mechanisms that are general enough for whichever way a user desires to build his compiler.

Surfaces for Computer-aided Design of Space Forms

Thu, 01 Jun 1967 00:00:00 GMT

Surfaces for Computer-aided Design of Space Forms Coons, Steven A. The design of airplanes, ships, automobiles, and so-called ""sculptured parts"" involves the design, delineation, and mathematical description of bounding surfaces. A method is described which makes possible the description of free-form doubly curved surfaces of a very general kind. An extension of these ideas to hyper-surfaces in higher dimensional spaces is also indicated.

On-line Analysis for Social Scientists

Mon, 01 May 1967 00:00:00 GMT

On-line Analysis for Social Scientists Miller, James R. A library of computer routines has been compiled to facilitate the analysis of social science research data. Many of these routines are designed to test statistical hypotheses. All routines are operated on-line and permit conversational interaction between the user and a time-shared computer. Input data are typed directly into the computer through a teletype console. Explicit typing directions and error diagnostics, where appropriate, are printed out by each routine to guide the input process. Analyses are executed immediately, and computed results are printed out in typical publication language.

Syntax-based Analytic Reading of Musical Scores

Sat, 01 Apr 1967 00:00:00 GMT

Syntax-based Analytic Reading of Musical Scores Forte, Allen As part of a larger research project in musical structure, a program has been written which ""reads"" scores encoded in an input language isomorphic to music notation. The program is believed to be the first of its kind. From a small number of parsing rules the program derives complex configurations, each of which is associated with a set of reference points in a numerical representation of a time-comtinuum. The logical structure of the program is such that all and only the defined classes of events are represented in the output.

A Low-cost Output Terminal for Time-shared Computers

Wed, 01 Mar 1967 00:00:00 GMT

A Low-cost Output Terminal for Time-shared Computers Rosenburg, Ronald C.; Kennedy, Daniel W.; Humphrey, Roger A. This report describes a low-cost remote terminal to provide switch-form output from a time-shared digital computer. The terminal consists of a modified model 35 KSR teletype and a local memory unit. The unit is independent of any particular computer, and is easy to test and maintain. The states of the memory control and memory words are observable directly by indicator lights.

Some Aspects of Pattrn Recognition by Computer

Wed, 01 Feb 1967 00:00:00 GMT

Some Aspects of Pattrn Recognition by Computer Guzman-Arenas, Adolfo A computer may gather a lot of information from its environment in an optical or graphical manner. A scene, as seen for instance from a TV camera or a picture, can be transformed into a symbolic description of points and lines or surfaces. This thesis describes several programs, written in the language CONVERT, for the analysis of such descriptions in order to recognize, differentiate and identify desired objects or classes of objects in the scene. Examples are given in each case.

An On-line System for Algebraic Manipulation

Thu, 01 Dec 1966 00:00:00 GMT

An On-line System for Algebraic Manipulation Fenichel, Robert R. This thesis describes an approach to the problem of programming a computer for algebraic manipulation. The motivating threads of the work are first picked up in Chapter I. To test the descriptive intuitions urged normatively in Chapter I, an experimental system was actually implemented. This system is described in Chapter II and in the Appendices.

Computer Design for Asynchronously Reproducible Multiprocessing

Tue, 01 Nov 1966 00:00:00 GMT

Computer Design for Asynchronously Reproducible Multiprocessing Van Horn, Earl C. A concept is presented for designing either a computing system, or a programming language system, so that the following problem is avoided: during a multiprocess computation in which several processes communicate, and in which the relative timing of the processes is arbitrary, the output produced by the computation might not be a function of only the initial computation state, i.e., of only the inputs and initial program of the computation.

ADEPT: A Heuristic Program for Proving Theorems of Group Theory

Sat, 01 Oct 1966 00:00:00 GMT

ADEPT: A Heuristic Program for Proving Theorems of Group Theory Norton, Lewis Mark A computer program, named ADEPT (A Distinctly Empirical Prover of Theorems), has been written which proves theorems taken from the abstract theory of groups. Its organization is basically heuristic, incorporating many of the techniques of the human mathematician in a "natural" way. This program has proved almost 100 theorems, as well as serving as a vehicle for testing and evaluating special-purpose heuristics.

Pilot: A Step Towards Man-Computer Symbiosis

Thu, 01 Sep 1966 00:00:00 GMT

Pilot: A Step Towards Man-Computer Symbiosis Teitelman, Warren PILOT is a programming system constructed in LISP. It is designed to facilitate the development of programs by easing the familiar sequence: write some code, run the program, make some changes, write some more code, run the program again, etc. As a program becomes more complex, making theses changes becomes harder and harder because the implications of changes are harder to anticipate.

Models and Data Structures for Digital Logic Simulation

Mon, 01 Aug 1966 00:00:00 GMT

Models and Data Structures for Digital Logic Simulation Smith, Donald Leigh A digital logic simulation system is proposed for design verification. Logic to be simulated is specified with a high level register transfer design language, and the simulation system operates on-line on a large time-shared computer. The problem of selecting adequate circuit and signal models for this purpose is considered. models are proposed with sufficient timing detail to allow the simulation system to detect timing errors which currently are found by manual checking or prototype.

Traffic Control in a Multiplexed Computer System

Fri, 01 Jul 1966 00:00:00 GMT

Traffic Control in a Multiplexed Computer System Saltzer, Jerome H. This thesis describes a scheme for processor multiplexing in a multiple user, multiple processor computer system. The scheme is based upon a distributed supervisor which may be different for different users. The processor multiplexing method provides smooth inter-process communication, treatment of input/output control as a special case of inter-process communication, and provision for a user to specify parallel processing or simultaneous input/output without interrupt logic.

Search Procedures Based on Measures of Relatedness Between Documents

Wed, 01 Jun 1966 00:00:00 GMT

Search Procedures Based on Measures of Relatedness Between Documents Ivie, Evan Leon In this thesis a new type of information retrieval system is suggested which utilizes data of the type generated by the users of the system instead of data generated by indexers. The theoretical model on which the system is based consists of three basic elements. The first element is measure of the relatedness between document-pairs. It is derived from information theory.

Input/Output in Time-shared, Segmented, Multiprocessor Systems

Wed, 01 Jun 1966 00:00:00 GMT

Input/Output in Time-shared, Segmented, Multiprocessor Systems Smith, Arthur Anshel After introducing and defining the concepts of time-sharing, segmentation, and multiprocessing, two classes of systems incorporating these are introduced. Both classes use associative memories, as 'look behind' devices to speed the operation of addressing the segment memory, with the distinction between classes being the location of the associative memory.

OCAS - On-line Cryptanalytic Aid System

Sun, 01 May 1966 00:00:00 GMT

OCAS - On-line Cryptanalytic Aid System Edwards, Daniel James Deficiencies of various programming languages for dealing with quantities frequently encountered in cryptanalysis of simple cipher systems will be discussed. A programming system is proposed which will permit a cryptanalyst to write and debug programs to aid in he solution of cryptograms or cryptographic systems. The basic elements of the proposed programming system are discussed in detail. They include: 1) a programming language to handle both algebraic quantities and character strings, 2) a display generator to permit quick specification of a display frame containing both alphanumeric strings and numerical data for an on-line CRT display device, and 2) an on-line program to control operation of the system and in debugging programs written in the proposed language.

Design of a Low-cost Character Generator for Remote Computer Displays

Tue, 01 Mar 1966 00:00:00 GMT

Design of a Low-cost Character Generator for Remote Computer Displays Cheek, Thomas Burrell A requirement exists for a low-cost remote display terminal with alphanumeric and line-drawing capabilities for use with time-shared computer systems. This thesis, conducted as part of the overall remote display design project, was undertaken to investigate novel approaches to character generation, with the goal of drastically reducing present-day costs for such devices. A survey of existing devices and character generation techniques was carried out, and a design approach was chosen which takes advantage of mass-fabrication techniques. This includes using a five-by-seven dot matrix raster and a resistor array "read-only" character memory for the 96 printable symbols of the Revised Proposed ASCII Code. Circuits designed, included a dot matrix generator and a register array memory with selection logic sense amplifiers, and a shift register output buffer. An experimental character generator with an eight-word memory was built, largely using integrated circuits and was found to work as desired. It is concluded that the design approach will yield a character generator that is of low enough cost to find wide use in remote computer terminals.

Investigation of an Analog Technique to Decrease Pentracking Time in Computer Display

Tue, 01 Mar 1966 00:00:00 GMT

Investigation of an Analog Technique to Decrease Pentracking Time in Computer Display Stratton, William David Many modern digital computer systems contain cathode-ray tube display equipment to facilitate man-machine communications. Through the use of a display and a light-sensitive pen, graphical material can be directly inserted into the computer by using the pen to control the position of the electron beam at the face of the CRT-a process called pen tracking. Beam position is continually sampled by the computer, permitting continuous display of the material being sketched. In present digital pen-tracking techniques, a tracking pattern (usually a cross) with a substantial number of points is generated on the face of the CRT and the binary response of the pen to the individual points of the pattern is employed to calculate pen position. The large number of pattern points, and the phosphor decay time associated with each, yield a typical tracking cycle of 500 to 1000 microseconds. Since the cycle must be repeated about 100 times per second, 5 to 10 percent of display time is consumed. To reduce the time required by the tracking operation, an analog technique employing a four-point tracking pattern is proposed in this study, in which the amplitude response of the pen to corresponding pairs of points is used to determine the position of the pen relative to the center of the pattern. To study the method, one channel of the proposed two-channel analog tracking system was designed, constructed, and coupled to the horizontal channel of a high-speed computer display console. To avoid the phosphor-decay limitation, an experimental "Beam" pen capable of detecting the electron beam rather than the phosphor luminescence is employed. The system included a pattern generator, sample-and-hold gates, difference amplifier, envelope detector and noise filter, and a threshold-logic analog-to-digital converter. The time required to generate the tracking pattern and develop the binary equivalent of the horizontal distance separating pen and pattern center is only 25 microseconds. Tracking is generally satisfactory, but some anomalies were noted, apparently due to the characteristics of the experimental pen being used. It is concluded that the analog technique is feasible for improving the speed of pen tracking, but recommended that further studies be made of the limitations inherent in the method.

MAP: A System for On-line Mathematical Analysis

Sat, 01 Jan 1966 00:00:00 GMT

MAP: A System for On-line Mathematical Analysis Kaplow, Roy; Strong, Stephen; Brackett, John This manual describes a computer suitable for use on the time-sharing facility at the M.I.T. Computation Center or at Project MAC. Designated for direct computer access through a remote console, the system replaces the normal procedures of programming with a question and answer interchange between the user (hereinafter called U) and the computer (hereinafter called C). The system is intended for the solution of mathematical problems. It should be usable by a person with no knowledge of computers or programming and little knowledge of numerical analysis. Within its range of capabilities, it should be as efficient as are the normal means of computer access for the more sophisticated user. The system establishes a "conversation" between U and C with an electric typewriter as the means of communication. U can give information to C and can ask it certain questions. C can answer those questions if it is given enough information. C can also ask questions and can therefore request any missing information. In addition, C can explain procedures to U in order to help the latter transmit the required information in a proper form. U, therefore, only needs to know a few basic rules, such as how to phrase his questions and how to name and tabulate his data.

Programming Semantics for Multiprogrammed Computations

Wed, 01 Dec 1965 00:00:00 GMT

Programming Semantics for Multiprogrammed Computations Dennis, Jack B.; Van Jhorn, Earl C. The semantics are defined for a number of meta-instructions which perform operations essential to the writing of programs in multiprogrammed computer systems. These meta-instructions relate to parallel procession, protection of separate computations, program debugging, and the sharing among users of memory segments and other computing objects, the names of which are hierarchically structured. The language sophistication contemplated is midway between an assembly language and an advanced algebraic language.

The Priority Problem

Mon, 01 Nov 1965 00:00:00 GMT

The Priority Problem Greenberger, Martin Priority decisions arise whenever limited facilities must be apportioned among competitive demands for service. Broadly viewed, even the familiar first-come-first served discipline is a priority rule. It favors the longest-waiting user, and guards against excessive delays. Other priority rules, such as shortest-job-next, are keyed instead to considerations of operating efficiency. Urgency of request is still another common consideration. Since these considerations often conflict, the priority rule serves as mediator. Use of a common cost measure can help effect this mediation, as results from recent job-shop simulations illustrate. A priority operation of contemporary interest is scheduling a time-shared computer among its concurrent users. Service requirements are not known in advance of execution. To keep response times short for small requests, service intervals are partitioned and segments are served separately in round-robin fashion. A mathematical analysis pinpoints the tradeoff between overhead and discrimination implicit in this procedure, and allows alternate strategies to be costed. Extensions of the simple round-robin procedure are suggested, the objectives of time-sharing are reviewed, and implications are drawn for the design of future priority and pricing systems.

Queueing Models for File Memory Operation

Fri, 01 Oct 1965 00:00:00 GMT

Queueing Models for File Memory Operation Denning, Peter James A model for the auxiliary memory function of a segmented, multiprocessor, time-shared computer system is set up. A drum system in particular is discussed, although no loss of generality is implied by limiting the discussion to drums. Particular attention is given to the queue of requests waiting for drum use. It is shown that a shortest access time first queue discipline is the most efficient, with the access time being defined as the time required for the drum to be positioned, and is measured from the finish of service of the last request to the beginning of the data transfer for the present request. A detailed study of the shortest access time queue is made, giving the minimum access time probability distribution, equations for the number in the queue, and equations for the wait in the queue. Simulations were used to verify these equations; the results are discussed. Finally, a general Markov Model for Queues is discussed in an Appendix.

Calculaid: An On-line System for Algebraic Computation and Analysis

Wed, 01 Sep 1965 00:00:00 GMT

Calculaid: An On-line System for Algebraic Computation and Analysis Wantman, Mayer Elihu OPS is an on-line system developed by M. Greenberger et al. at Project MAC. The present work provides a powerful and simple way to perform numerical manipulations and calculations within OPS. The program package is called CALCULAID. A method of executing algebraic assignment statements, of which MAD and FORTRAN assignments are a subset, is provided. When this assignment-statement ability is coupled with other features of the OPS system, such as unconditional transfers, general conditionals, and array and function declarations, most of the ability of a compiler language is provided. Because the programs written in OPS are executed interpretively, OPS-3 programs can be changed and re-run immediately, without being compiled. The other elements of CALCULAID are a program for creating multiple linear regression models, rank-ordering and counting data, and finding roots to polynomial equations in one unknown. The applications of CALCULAID to the analysis of a round-robin scheduling model and to a process-control problem are discussed, and conclusions regarding the suitability of running computational programs in an interpretive mode are drawn.

A Heuristic Approach to Alternate Routing in a Job Shop

Tue, 01 Jun 1965 00:00:00 GMT

A Heuristic Approach to Alternate Routing in a Job Shop Russo, F.J. The research reported here investigates the use of heuristics for selecting from several alternate routes resulting from partially ordered tasks in a job shop order file. The experimental vehicle employed was digital simulation. The concept of the "Alternate string" has been developed to generalize the existence of partially ordered operations. That term is defined as a concatenation of operations that can be performed in any order, with the additional specification that all within the string can be attempted. The presence of alternate strings with two or more member gives rise to the alternate routing problem, whose solution is approached by heuristic methods. Choosing from among several alternate routes constitutes a three level decision problem. At the lowest level, routes can be chosen when the order enters the shop. This is equivalent to fixed routing. At a higher level, alternates can be selected at the time of transition from one work station to another. The third decision level occurs at operation time, when one of the alternate operations is placed on a machine. Heuristics were tested at the latter two levels. There were two prior assertions that this thesis set out to prove. The first was that alternate routing at the highest decision level would produce significant reductions in the mean tardiness of orders completed past their designated due dates, the improvement being both relative to fixed routing and to alternate routing heuristics implemented at lower decision levels. Secondly, the contention was made that the improvement would be as such a magnitude that on-line, real-time systems become economically justifiable as a means of mitigating the attendant control problems caused by non-deterministic paths through the queuing network. The methodology employed here was to conduct two passes of simulated shop runs. The first, with two artificially high levels of alternate incidence, tested the efficiency of five different alternate routing heuristics in reducing mean tardiness. The second pass consisted of runs with the best heuristic developed during the first experimental phase applied to a realistic length and frequency of alternate strings. The results of the experiments strongly support the assertions made at the outset of the thesis. The performance characteristics of the different heuristics are discussed at length. In addition, some implications are drawn of the computational nature of alternate routing and the difficulties encountered in implementing alternate routing heuristics at operation time.

An Analysis of Time-Shared Computer Systems

Tue, 01 Jun 1965 00:00:00 GMT

An Analysis of Time-Shared Computer Systems Scherr, Allan Lee Some of the aspects of the operation of time-shared, interactive computer systems are analyzed. The emphasis is on the reaction of hardware systems to the demands that its users make upon it. Simply shared systems and their users in order to be able to predict the performance of the two operating together. Portions of this problem include the specification and measurement of user characteristics, the development and verification of both simulation and mathematical models for time-shared systems, and the specification and measurement of performance metrics for such systems. The user and some of the performance measurements were made on Project MAC's "Compatible Time-Sharing System" (CTSS). First, simulation models are used to study the effects of changing small details in the operation of CTS-like systems. Then, a continuous-time Markov process model is derived to predict the performance of a broad class of systems. Throughout, the CTSS data are used as a basis for comparison with model predictions. In order to be able to take measurements and to build models, many definitions of commonly used time-shared system terminology are made precise.

Time Sharing on a Multiconsole Computer

Mon, 01 Mar 1965 00:00:00 GMT

Time Sharing on a Multiconsole Computer Samuel, Arthur L. After a brief historical review and a description of the three basic types for time-sharing systems, the general purpose time-sharing system as exemplified by the M.I.T. CTSS system is described in general terms, with particular attention to the way the system looks to the user.

CTSS Technical Notes

Mon, 01 Mar 1965 00:00:00 GMT

CTSS Technical Notes Saltzer, Jerome H. This report is a technical description of the 7094 Compatible Time-Sharing System in use at Project MAC and the M.I.T. Computation Center. It is designed to acquaint a system programmer with the techniques of construction which were used in this particular time-sharing system. Separate chapters discuss the overall supervisor program flow: console message input and output: the scheduling and storage algorithms: and a thumbnail sketch is given of each of the subroutines which make up the supervisor program. This report was prepared with the aid of the compatible time-sharing system and the TYPSET and RUNOFF commands.

Use of CTSS in a Teaching Environment

Sun, 01 Nov 1964 00:00:00 GMT

Use of CTSS in a Teaching Environment Roos, Daniel Computer time-sharing offers many interesting possibilities for use in teaching computer technology. It might be expected that with proper hardware and software, students using time-sharing as a teaching machine could acquire proficiency in the fundamentals of programming more easily than using batch-processing. To test this hypothesis, the M.I.T. Department of Civil Engineering divided a freshman programming class so that half the students used batch-processing methods, and half used the Project MAC time-sharing system to do the same work. This paper describes the experiment and its tentative results.

A New Methodology for Computer Simulation

Thu, 01 Oct 1964 00:00:00 GMT

A New Methodology for Computer Simulation Greenberger, Martin Computer simulation is a cooperative venture between researcher and information processor, but the processor's role customarily begins too late. The researcher can benefit substantially by bringing the computer up into the earlier, creative phases of the simulation process. An on-line computer system that makes this possible is described.

The MAC System: A Progress Report

Thu, 01 Oct 1964 00:00:00 GMT

The MAC System: A Progress Report Fano, Robert M. The notion of machine-aided cognition implies an intimate collaboration between a human user and a computer in a real-time dialogue on the solution of a problem, in which the two parties contribute their best capabilities. In order for this intimate collaboration to be possible, a computer system is needed that can serve simultaneously a large number of people, and that is easily accessible to them, both physically and intellectually. The present MAC System is a first step toward this goal. The purpose of this paper is to present a brief description of the current system, to report on the experience gained from its operation, and to indicate directions along which future developments are like to proceed.

Program Structure in a Multi-access Computer

Fri, 01 May 1964 00:00:00 GMT

Program Structure in a Multi-access Computer Dennis, Jack B. A multi-access computer (MAC) system consists of processing units and directly addressable main memory in which procedure information is interpreted as sequences of operations on data, a system of terminal devices through which users may communicate with procedures operating for them, and mass memory where procedures and data may be held when not required for immediate reference. One fundamental attraction of the MAC concept is the increased productivity of "computer catalyzed research" that results from close man-machine interaction. Another attraction is wealth of data and procedures that are accessible to a large user community through the file memory of a MAC system. In this report thoughts are developed which form an adequate model of program structure. These concepts have grown out of many discussions with colleges in Project MAC, and our experience to date in the design and operation of multi-access computer systems.

The OPS-1 Manual

Fri, 01 May 1964 00:00:00 GMT

The OPS-1 Manual Greenberger, Martin The recent attainment and continuing development of personally accessible computer facilities have opened another chapter in the use of machines by man. A number of current research efforts, including Project MAC at M.I.T., are designing new conceptual systems to adapt the emerging technology to a wide range of human activity. Activities relating to management are the concern of a trial system at Project MAC called OPS-1. The OPS-1 system and the experiment that launched it are described in this manual. {AD 604-681}

OPL-I An Open Ended Programming System Within CTSS

Wed, 01 Jan 1964 00:00:00 GMT

OPL-I An Open Ended Programming System Within CTSS Weizenbaum, Joseph OPL-1, an incremental programming system presently operating with CTSS, permits the user to augment both his program and his data base during widely separated successive sessions at his terminal. Facilities are provided which make it possible for the user to operate on his already established data base both by means of built-in operators and in terms of operators (functions) which the user has previously defined in the language of the system. Underlying the system is a powerful list processing scheme embedded in FORTRAN (SLIP). The machinery of this fundamental language drives the system and is also largely available to the user. The data base generated by the user is therefore a set of list structures (trees), and most of the operators available to him are list processing operators. Data structures with considerably complex inter-relational properties may therefore be treated quite directly.

Stress: A Problem-oriented Language for Structural Engineering

Fri, 01 Jul 1966 00:00:00 GMT

Stress: A Problem-oriented Language for Structural Engineering Biggs, John M.; Logcher, Robert D. STRESS is a general purpose programming system for the analysis of structures. Compared to most other structural programs it has three distinguishing characteristics: (1) The input language is that of the structural engineer which makes possible direct communication between the engineer and the machine; (2) The system is capable of analyzing a wide variety of structural types and loading conditions thus permitting industrial use on a routine basis; and (3) The design process is expedited by the fact that modifications of the original structure for alternate designs can be easily executed. This last capability is most effective when STRESS is used in the time-sharing mode. These features combine to provide a system which not only reduces the effort required for structural analysis but, more significantly, enhances the designer's ability to evolve an efficient structure.

CARPS, A Program Which Solves Calculus Word Problems

CARPS, A Program Which Solves Calculus Word Problems Charniak, Eugene

Verbal and Graphical Language for the AED System: A Progress Report

Fri, 01 May 1964 00:00:00 GMT

Verbal and Graphical Language for the AED System: A Progress Report Ross, Douglas T.; Feldman, Clarence G. For Computer-Aided Design, use of time-sharing a single language which can take either verbal or graphical form is required. This paper describes how a single language processing technique, which is in turn a special application of more general concepts concerning the step-by-step growth and processing of large structures of interrelated elements, can efficiently process both language forms in the same manner. Illustrations of the concepts involved are also drawn from the methods used in the AED-O Compiler, an efficient ALGOL-60-based compiler used in Computer-Aided Design work, which is available as a public command in the Project MAC CTSS.

System Requirements for Multiple -Access, Time-shared Computers

Fri, 01 May 1964 00:00:00 GMT

System Requirements for Multiple -Access, Time-shared Computers Corbató, Fernando J. It is now clear that it is possible to create a general-purpose time-shared multiple access system on most contemporary computers. However, it is equally clear that none of the existent computers are well designed for multiple access systems. At present, good service to a few dozen simultaneous users is considered state-of-the-art. Discussions include: clocks, memory protection and supervisor mode, program relocation and common subroutines which expose the reader to the difficulties encountered with contemporary machines when multiple user multiple-processor systems are considered.

SIR: A Computer Program for Semantic Information Retrieval

Mon, 01 Jun 1964 00:00:00 GMT

SIR: A Computer Program for Semantic Information Retrieval Raphael, Bertram SIR is a computer system, programmed in the LISP language, which accepts information and answers questions expressed in a restricted form of English. This system demonstrates what can reasonably be called an ability to "understand" semantic information. SIR's semantic and deductive ability is based on the construction of an internal model, which uses word associations and property lists, for the relational information normally conveyed in conversational statements. A format-matching procedure extracts semantic content from English sentences. If an input sentence is declarative, the system adds appropriate information to the model. If an input sentence is a question, the system searches the model until it either finds the answer or determines why it cannot find the answer. In all cases SIR reports its conclusions. The system has some capacity to recognize exceptions to general rules, resolve certain semantic ambiguities, and modify its model structure in order to save computer memory space. Judging from its conversational ability, SIR is more "intelligent" than any existing question-answering system. The author describes how this ability was developed and how the basic features of SIR compare with those of other systems. The working system, SIR , is a first step toward intelligent machine communication. The author proposes a next step by describing how to construct a more general system which is less complex and yet more powerful than SIR . This proposed system contains a generalized version of the SIR model, a formal logical system called SIR1 , and a computer program for testing the truth of SIR1 statements with respect to the generalized model by using partial proof procedures in the predicate calculus. The thesis also describes the formal properties of SIR1 and how they relate to the logical structure of SIR .

Natural Language Input for a Computer Problem Solving System

Tue, 01 Sep 1964 00:00:00 GMT

Natural Language Input for a Computer Problem Solving System Bobrow, Daniel .G The STUDENT problem solving system, programmed in LISP, accepts as input a comfortable but restricted subset of English which can express a wide variety of algebra story problems. STUDENT finds the solution to a large class of these problems. STUDENT can utilize a store of global information not specific to any one problem, and may make assumptions about the interpretation of ambiguities in the wording of the problem being solved. If it uses such information, or makes any assumptions, STUDENT communicates this fact to the user. The thesis includes a summary of other English language question-answering systems. All these systems, and STUDENT are evaluated according to four standard criteria. The linguistic analysis in STUDENT is a first approximation to the analytic portion of a semantic theory of discourse outlined in the thesis. STUDENT finds the set of kernel sentences which are the base of the input discourse, and transforms this sequence of kernel sentences into a set of simultaneous equations which form the semantic base of the Student system. STUDENT then tries to solve this set of equations for the values of requested unknowns. If it is successful it gives the answers in English. If not, STUDENT asks the user for more information, and indicates the nature of the desired information. The STUDENT system is a first step toward natural language communication with computers. Further work on the semantic theory proposed should result in much more sophisticated systems.

Execution Model Enforcement Via Program Shepherding

Thu, 01 May 2003 00:00:00 GMT

Execution Model Enforcement Via Program Shepherding Kiriansky, Vladimir; Bruening, Derek; Amarasinghe, Saman Nearly all security attacks have one thing in common: they coerce the target program into performing actions that it was never intended to perform. In short, they violate the program's execution model. The execution model encompasses the Application Binary Interface (ABI), higher-level specifications from the program's source programming language, and components specific to the program --- for example, which values a particular function pointer may take. If this execution model were enforced, and only program actions that the programmer intended were allowed, a majority of current security holes would be closed. In this paper, we employ program shepherding[26] to enforce a program's execution model. Program shepherding monitors control flow in order to enforce a security policy. We use static and dynamic analyses to automatically build a custom security policy for a target program which specifies the program's execution model. We have implemented our analyses in the DynamoRIO [5] runtime code modification system. The resulting system imposes minimal or no performance overhead, operates on unmodified native binaries, and requires no special hardware or operating system support. Our static analyses require source code access but not recompilation. The analysis process requires no user interaction, but is able to build a strict enough policy to prevent all deviations from the program's control flow graph and nearly all violations of the calling convention, greatly reducing the possibility of an unintended program action.

Scalable Packet Classification Using Bit Vector Aggregating and Folding

Tue, 01 Apr 2003 00:00:00 GMT

Scalable Packet Classification Using Bit Vector Aggregating and Folding Li, Ji; Liu, Haiyang; Sollins, Karen Packet classification is a central function for a number of network applications, such as routing and firewalls. Most existing algorithms for packet classification scale poorly in either time or space when the database size grows. The scalable algorithm Aggregated Bit Vector (ABV) is an improvement on the Lucent bit vector scheme (BV), but has some limitations. Our algorithm, Aggregated and Folded Bit Vector (AFBV), seeks to reduce false matches while keeping the benefits of bit vector aggregation and avoiding rule rearrangement. It combines bit vector aggregation and folding to achieve this goal. Experiments showed that our algorithm outperforms both the BV and ABV schemes in synthetically generated databases.

Stream Algorithms and Architecture

Sat, 01 Mar 2003 00:00:00 GMT

Stream Algorithms and Architecture Henry, Hoffman; Strumpen, Volker; Agarwal, Anant Wire-exposed, programmable microarchitectures including Trips [11]], Smart Memories [8], and Raw [13] offer an opportunity to schedule instruction execution and data movement explicitly. This paper proposes stream algorithms, which, along with a decoupled systolic architecture, provide an excellent match for the physical and technological constraints of single-chip tiles architectures. Stream algorithms enable programmed systolic computations for different problem sizes, without incurring the cost of memory accesses. To that end, we decouple memory accesses from computation and move the memory accesses off the critical path. By structuring computations in systolic phases, and deferring memory accesses to dedicated memory processors, stream algorithms can solve many regular problems with varying sizes on a constant-sized tiled array. Contrary to common sense, the compute efficiency of stream algorithms increases as we increase the number of processing elements. In particular, we show that the compute efficiency of stream algorithms can approach 100% asymptotically, that is for large numbers of processors and appropriate problem size.

A Theoretical and Practical Approach to Instruction Scheduling on Spatial Architectures

Sun, 01 Dec 2002 00:00:00 GMT

A Theoretical and Practical Approach to Instruction Scheduling on Spatial Architectures Mirrokni, Vahab S.; Lee, Walter; Karger, David; Amarasinghe, Saman This paper studies the problem of instruction assignment and scheduling on spatial architectures. Spatial architectures are architectures whose resources are organized in clusters, with non-zero communication delays between the clusters. On these architectures, instruction scheduling include both space scheduling, where instructions are mapped to clusters, and the traditional time scheduling. This paper considers the problem from both the theoretical and practical perspectives. It presents two integer linear program formulations with known performance bounds. We also present an 8-approximation algorithm for constant m and constant communication delays. Then, we introduce three heuristic algorithms based on list scheduling. Then we study a layer partitioning method. Our final algorithm is a combination of layer partitioning and the third heuristic. Two of the better algorithms are evaluated on the Raw machine. Results show that they are competitive with previously published results; for scientfici codes, our heuristics can perform an average of 25% better.

pStore: A Secure Peer-to-Peer Backup System

Tue, 01 Oct 2002 00:00:00 GMT

pStore: A Secure Peer-to-Peer Backup System Batten, Christopher; Barr, Kenneth; Saraf, Arvind; Trepetin, Stanley In an effort to combine research in peer-to-peer systems with techniques for incremental backup systems, we propose pStore: a secure distributed backup system based on an adaptive peer-to-peer network. pStore exploits unused personal hard drive space attached to the Internet to provide the distributed redundancy needed for reliable and effective data backup. Experiments on a 30 node network show that 95% of the files in a 13 MB dataset can be retrieved even when 7 of the nodes have failed. On top of this reliability, pStore includes support for file encryption, versioning, and secure sharing. Its custom versioning system permits arbitrary version retrieval similar to CVS. pStore provides this functionality at less than 10% of the network bandwidth and requires 85% less storage capacity than simpler local tape backup schemes for a representative workload.

Offline Authentication of Untrusted Storage

Thu, 01 Aug 2002 00:00:00 GMT

Offline Authentication of Untrusted Storage Clarke, Dwaine; Gassend, Blaise; Suh, G. Edward; van Dijk, Marten; Devadas, Srinivas We extend the offline memory correctness checking scheme presented by Blum et. al [BEG+91], by using incremental cryptography, to detect attacks by an active adversary. We also introduce a hybrid o_ine-online checking scheme designed for untrusted storages in file systems and databases. Previous work [GSC+02] [FKM00] [MVS00] describe systems in which Merkle trees are used to verify the authenticity of data stored on untrusted storage. The Merkle trees [Mer79] are used to check, after each operation, whether the storage performed correctly. The offline and hybrid checkers are designed for checking sequences of operations on an untrusted storage and, in the common case, require only a constant overhead on the number of accesses to the storage, as compared to the logarithmic overhead incurred by online Merkle tree schemes

Phased Computation Graphs in the Polyhedral Model

Thu, 01 Aug 2002 00:00:00 GMT

Phased Computation Graphs in the Polyhedral Model Thies, William; Lin, Jasper; Amarasinghe, Saman We present a translation scheme that allows a broad class of dataflow graphs to be considered under the optimization framework of the polyhedral model. The input to our analysis is a Phased Computation Graph, which we define as a generalization of the most widely used dataflow representations, including synchronous dataflow, cyclo-static dataflow, and computation graphs. The output of our analysis is a System of Affine Recurrence Equations (SARE) that exactly captures the data dependencies between the nodes of the original graph. Using the SARE representation, one can apply many techniques from the scientific community that are new to the DSP domain. For example, we propose simple optimizations such as node splitting, decimation propagation, and stead-state invariant code motion that leverage the fine-grained dependence information of the SARE to perform novel transformations on a stream graph. We also propose ways in which the polyhedral model can offer new approaches to classic problems of the DSP community, such as minimizing buffer size, code size, and optimizing the schedule.

A Collision Model for Randomized Routing In Fat-Tree Networks

Mon, 01 Jul 2002 00:00:00 GMT

A Collision Model for Randomized Routing In Fat-Tree Networks Strumpen, Volker; Krishnamurthy, Arvind

How to Build Scalable On-Chip ILP Networks for a Decentralized Architecture

Sat, 01 Apr 2000 00:00:00 GMT

How to Build Scalable On-Chip ILP Networks for a Decentralized Architecture Taylor, Michael Bedford; Lee, Walter; Frank, Matthew; Amarasinghe, Saman; Agarwal, Anant The era of billion transistors-on-a-chip is creating a completely different set of design constraints, forcing radically new microprocessor archiecture designs. This paper examines a few of the possible microarchitectures that are capable of obtaining scalable ILP performance. First, we observe that the network that interconnects the processing elements is the critical design point in the microarchitecture. Next, we characterize four fundamental properties that have to be satisfied by the interconnection network. Next, we provide case studies of two different networks that satisfy these properties. Finally, a detailed evaluation of these networks is presented to highlight the scalability and performance of these microarchitectures. We show that by using compile time information, we can build simpler networks and use them efficiently.

A Stream Compiler for Communication-Exposed Architectures

Fri, 01 Mar 2002 00:00:00 GMT

A Stream Compiler for Communication-Exposed Architectures Gordon, Michael; Thies, William; Karczmarek, Michael; Wong, Jeremy; Hoffmann, Henry; Maze, David; Amarasinghe, Saman With the increasing miniturization of transistors, wire delays are becoming a dominant factor in microprocessor performance. To address this issue, a number of emerging architectures contain replicated processing units with software-exposed communication between one unit and another (e.g., Raw, iWarp, SmartMemories). However, for their use to be widespread, it will be necessary to develop compiler technology that enables a portable, high-level language to execute efficiently across a range of wire-exposed architectures. In this paper, we describe our compiler for StreamIt: a high-level, architecture-independent language for streaming applications. We focus on our backend for the Raw processor. Though StreamIt exposes the parallelism and communication patterns of stream programs, much analysis is needed to adapt a stream program to a parallel stream processor. We describe fission and fusion transformations that can be used to adjust the granularity of a stream graph, a layout algorithm for mapping a stream graph to a given network topology, and a scheduling algorithm for generating a fine-grained static communication pattern for each computational element. We have implemented a fully functional compiler that parallelizes StreamIt applications for Raw, including several load-balancing optimizations. Using the cycle-accurate Raw simulator, we demonstrate that these optimizations can improve performance by up to 145%. We consider this work to be a first step towards a portable programming model for communication-exposed architectures.

A Note on the Stability Requirements of Adaptive Virtual Queue

Fri, 01 Feb 2002 00:00:00 GMT

A Note on the Stability Requirements of Adaptive Virtual Queue Katabi, Dina; Blake, Charles Choosing the correct value for the parameters of an Active Queue Management (AQM) scheme is a well-known hard problem. The Adaptive Virtual Queue (AVQ) attempts at solving this problem by using stability requirements to devise a rule for setting its parameter. This memo shows that the AVQ rule for setting its parameter is impractical for many real-life situations.

Secure Execution Via Program Shepherding

Fri, 01 Feb 2002 00:00:00 GMT

Secure Execution Via Program Shepherding Kiriansky, Vladimir; Bruening, Derek; Amarasinghe, Saman We introduce program shepherding, a method for monitoring control flow transfers during program execution to enforce a security policy. Shepherding ensures that malicious code masquerading as data is never executed, thwarting a large class of security attacks. Shepherding can also enforce entry points as the only way to execute shared library code. Furthermore, shepherding guarantees that sandboxing checks around any type of program operation will never be bypassed. We have implemented these capabilities efficiently in a runtime system with minimal or no performance penalties. This system operates on unmodified native binaries, requires no special hardware or operating system support, and runs on existing IA-32 machines.

Efficient View-Dependent Sampling of Visual Hulls

Fri, 01 Feb 2002 00:00:00 GMT

Efficient View-Dependent Sampling of Visual Hulls Matusik, Wojciech; Buehler, Chris; McMillan, Leonard In this paper we present an efficient algorithm for sampling visual hulls. Our algorithm computers exact points and normals on the surface of visual hull instead of a more traditional volumetric representation. The main feature that distinguishes our algorithm from previous ones is that it allows for sampling along arbitrary viewing rays with no loss of efficiency. Using this property, we adaptively sample visual hulls to minimize the number of samples needed to attain a given fidelity. In our experiments, the number of samples can typically be reduced by an order of magnitude, resulting in a corresponding performance increase over previous algorithms.

An Efficient Visual Hull Computation Algorithm

Fri, 01 Feb 2002 00:00:00 GMT

An Efficient Visual Hull Computation Algorithm Matusik, Wojciech; Buehler, Chris; McMillan, Leonard; Gortler, Steven J. In this paper we describe an efficient algorithm for computing the visual hull of an object. This problem is equivalent to computing the intersection of generalized cones. The naïve visual hull computation algorithm requires intersecting 3D polyhedra. We exploit the special structure of generalized cone polyhedra and show how to reduce this computation to a set of intersections in 2D. Moreover, we describe how the 2D intersections can be carried out efficiently.

StreamIT: A Complier for Streaming Applications

Fri, 01 Feb 2002 00:00:00 GMT

StreamIT: A Complier for Streaming Applications Thies, William F.; Karczmarek, Michael; Gordon, Michael; Maze, David; Wong, Jeremy; Hoffmann, Henry; Brown, Matthew; Amarasinghe, Saman Streaming programs represent an increasingly important and widespread class of applications that holds unprecedented opportunitie sfor high-impact compiler technology. Unlike sequential programs with obscured dependence information and complex communication patterns, a stream program is naturally written as a set of concurrent filters with regular steady-state communication. The StreamIt language aims to provide a natural, high-level syntax that improves programmer productivity in the streaming domain. At the same time, the language imposes a hierarchical structure on the stream graph that enables novel representations and optimizations within the StreamIt compiler. We define the "stream dependence function," a fundamental relationship between the input channels of two filters in a stream graph. We also describe a suite of stream optimizations, a denotational semantics for validating these optimizations, and a novel phased scheduling algorithm for stream graphs. In addition, we have implemented a prototype of the StreamIt optimizing compiler that is showing promising results.

Techniques for Increasing and Detecting Memory Alignment

Thu, 01 Nov 2001 00:00:00 GMT

Techniques for Increasing and Detecting Memory Alignment Larsen, Samuel; Witchel, Emmett; Amarasinghe, Saman Memory alignment is an important property in memory system performance. Extraction of alignment information at compile-time enables the possibility for new classes of program optimization. In this paper, we present methods for increasing and detecting the alignment of memory references in a program. Our transformations and analyses do not require interprocedural analysis and introduce almost no overhead. As a result, they can be incorporated into real compilation systems. On average, our techniques are able to achieve a five-fold increase in the number of dynamically aligned memory references. We are then able to detect 94% of these operations. This success is invaluable in providing performance gains in a range of different areas. When alignment information is incorporated into a vectorizing compiler, we can increase the performance of a G4 AltiVec processor by more than a factor of two. Using the same methods, we are able to reduce energy consumption in a data cache by as much as 35%.

StreaMIT: A Language for Streaming Applications

Wed, 01 Aug 2001 00:00:00 GMT

StreaMIT: A Language for Streaming Applications Thies, William F.; Karczmarek, Michael; Amarasinghe, Saman We characterize high-performance streaming applications as a new and distinct domain of programs that is becoming increasingly important. The StreaMIT language provides novel high-level representations to improve programmer productivity and program robustness within the streaming domain. At the same time, the StreaMIT compiler aims to improve the performance of streaming applications via stream-specific analyses and optimizations. In this paper, we motivate, describe and justify the language features of StreaMIT, which include: a structured model of streams, a messaging system for control, a re-initialization mechanism, and a natural textual syntax. We also present a means of reasoning about time in terms of "information flow": a concept that we believe is fundamental to the streaming domain. Using this concept, we give a formal semantics for StreaMIT's messaging system, as well as a simple algorithm for detecting deadlock and buffer overlow.

A Software Framework for Supporting General Purpose Applications on RAW Computation Fabrics

Sun, 01 Jul 2001 00:00:00 GMT

A Software Framework for Supporting General Purpose Applications on RAW Computation Fabrics Frank, Matthew; Lee, Walter; Amarasinghe, Saman This paper presents SUDS (Software Un-Do Systems), a data speculation system for Raw processors. SUDS manages specultation in software. Thekey to managing speculation in software is to use the compiler to minimize the number of data items that need to be managed in runtime. Managing speculation in software enables Raw processors to achieve good performance on integer applications without sacrificing chip area for speculation hardware. This additional area can instead be devoted to additional computer resources, improving the performance of dense matrix and media applications.

Persona: A Contextualized and Personalized Web Search

Tue, 01 May 2001 00:00:00 GMT

Persona: A Contextualized and Personalized Web Search Tanudjaja, Francisco; Mui, Lik Recent advances in graph-based search techniques derived from Kleinberg's work [1] have been impressive. This paper further improves the graph-based search algorithm in two dimensions. Firstly, variants of Kleinberg's techniques do not take into account the semantics of the query string nor of the nodes being searched. As a result, polysemy of query words cannot be resolved. This paper presents an interactive query scheme utilizing the simple web ontology provided by the Open Directory Project to resolve meanings of a user query. Secondly, we extend a recently proposed personalized version of the Kleinberg algorithm [3]. Simulation results are presented to illustrate the sensitivity of our technique. We outline the implementation of our algorithm in the Persona personalized web search system.

Ratings in Distributed Systems: A Bayesian Approach

Tue, 01 May 2001 00:00:00 GMT

Ratings in Distributed Systems: A Bayesian Approach Mui, Lik; Mohtashemi, Mojdeh; Ang, Cheewee; Szolovits, Peter; Halberstadt, Ari For distributed systems at large and e-commerce systems in particular, ratings play an increasingly important role. Rating confer reputation measures about sources. This paper reports our formalization of the rating process. This paper argues that rating shuold be context- and individual- dependent quantities. In contrast to existing rating systems in many e-commerce or developer sites, our approach makes use of personalized and contextualized ratings for assessing source reputation. Our approach is based on a Bayesian probabilistic framework.

Three Round Zero-Knowledge Using a Proof of Knowledge Assumption

Sun, 01 Apr 2001 00:00:00 GMT

Three Round Zero-Knowledge Using a Proof of Knowledge Assumption Lepinski, Matthew; Micali, Silvio We provide a proof of knowledge assumption that allows us to construct a three round zero-knowledge proof system for any language in NP.

Mutually Independent Commitment

Sun, 01 Apr 2001 00:00:00 GMT

Mutually Independent Commitment Liskov, Moses; Lysyanskeya, Anna; Micali, Silvio; Reyzin, Leonid; Smith, Adam We describe a new kind of commitment scheme in which two parties commit to values in a commitment stage, at the end of which we are assured that the values they have committed to cannot be correlated to one another. We call this new primitive mutually independent commitments. We present three mutually independent commitment schemes which handle single bit commitments, and which are computationally hiding and perfecting binding.

Forward-Secure Signatures with Optimal Signing and Verifying

Sun, 01 Apr 2001 00:00:00 GMT

Forward-Secure Signatures with Optimal Signing and Verifying Itkis, Gene; Reyzin, Leonid Ordinary digital signatures have an inherent weakness: if the secret key is leaked, then all signatures, even the ones generated before the leak, are no longer trustworthy. Forward-secure digital signatures were recently proposed to address this weakness: they ensure that past signatures remain secure even if the current secret key is leaked. We propose the first forward-secure signature scheme for which both signing and verifying are as efficient as for one of the most efficient ordinary signature schemes (Guillou-Quisquater): each requiring just two modular exponentiations with a short exponent. All previously proposed forward-secure signature schemes took significantly longer to sign and verify than ordinary signature schemees. Our scheme requires only fractional increases to the sizes of keys and signatures, and no additional public storage. Like the underlying Guillou-Quisquater scheme, our scheme is provably secure in the random oracle model.

A Unified Framework for Schedule and Storage Optimization

Wed, 01 Nov 2000 00:00:00 GMT

A Unified Framework for Schedule and Storage Optimization Thies, William F.; Viven, Frederic; Sheldon, Jeffery W.; Amarasinghe, Saman We present a unified mathematical framework for analyzing the tradeoffs between parallelism and storage allocation within a parallelizing compiler. Using this framework, we show how to find the best storage mapping for a given schedule, the best schedule for a given storage mapping, and the best storage mapping that is valid for all legal schedules. Our techniques combines affine scheduling techniques with occupancy vector analysis, and incorporates general affine dependencies across statements and loop nests. We formulate the constraints imposed by the data dependencies and the storage mapping as a set of linear inequalities, and apply numerical programming techniques to efficiently solve for the best occupancy vector. We consider out method to be a first step towards automating a procedure that finds the optimal tradeoff between parallelism and storage space.

Availability Study of Dynamic Voting Algorithms

Wed, 01 Nov 2000 00:00:00 GMT

Availability Study of Dynamic Voting Algorithms Ingols, Kyle; Keidar, Idit Fault tolerant distributed systems often select a primary component to allow a subset of the processes to function when failures occur. The dynamic voting paradigm defines rules for selecting the primary component adaptively: when a partition occurs, if a majority of the previous primary component is connected, a new and possibly smaller primary is chosen. Several studies have shown that dynamic voting leads to more available solutions than other paradigms for maintaining a primary component. However, these studies have assumed that every attempt made by the algorithm to form a new primary component terminates successfully. Unfortunately, in real systems, this is not always the case: a change in connectivity can interrupt the algorithm whiel it is still attempting to form a new primary component; in such cases, algorithms typically block until processes can resolve the outcome of the interrupted attempt. This paper uses simulations to evaluate the effect of interruptions on the availability of dynamic voting algorithms. We study four dynamic voting algorithms, and identify two important characteristics that impact an algorithm's availability in runs with frequent connectivity changes. First, we show that the number of communication rounds exchanged in an algorithm plays a significant role in the availability achieved, especially in the degradation of availability as connectivity changes become more frequent. Second, we show that the number of processes that need to be present in order to resolve past attempts impacts the availability, especially during long runs with numerous connectivity changes.

A General Framework for Highly Available Services based on Group Communication

Wed, 01 Nov 2000 00:00:00 GMT

A General Framework for Highly Available Services based on Group Communication Fekete, Alan; Keidar, Idit We present a general framework for building highly available services. The framework uses group communication to coordinate a collection of servers. Our framework is configurable, in that one can adjust parameters such as the number of servers and the extent to which they are synchronized. We analyze the scenarios that can lead to the service availability being temporarily comprised, and we discuss the tradeoffs that govern the choice of parameters.

Concurrent/Resettable Zero-Knowledge Protocols for NP in the Public Key Model

Tue, 01 Aug 2000 00:00:00 GMT

Concurrent/Resettable Zero-Knowledge Protocols for NP in the Public Key Model Micali, Silvio; Reyzin, Leonid We propose a four-round protocol for concurrent and resettable zero-knowledge arguments for any langauge in NP, assuming the verifier has a pre-registered public-key. We also propose a three-round protocol with an additional timing assumption.

A New Self-Play Experiment in Computer Chess

Mon, 01 May 2000 00:00:00 GMT

A New Self-Play Experiment in Computer Chess Heinz, Ernst This paper presents the results of a new self-play experiment in computer chess. It is the _x000C_rst such experiment ever to feature search depths beyond 9 plies and thousands of games for every single match. Overall, we executed 17,150 self-play games (1,050{3,000 per match) in one \\calibration" match and seven \\depth X+1 , X" handicap matches at _x000C_xed iteration depths ranging from 5{12 plies. For the experiment to be realistic and independently repeatable, we relied on a state-of-the-art commercial contestant: Fritz 6 , one of the strongest modern chess pro- grams available. The main result of our new experimentis thatit shows the existence of diminishing returns for additional search in computer chess self-play by Fritz 6 with 95% statistical con_x000C_dence. The dimin- ishing returns manifest themselves by declining rates of won games and reversely increasing rates of drawn games for the deeper searching pro- gram versions. The rate of lost games, however, remains quite steady for the whole depth range of 5{12 plies.

Systematic Testing of Multithreaded Programs

Mon, 01 May 2000 00:00:00 GMT

Systematic Testing of Multithreaded Programs Bruening, Derek; Chapin, John We present a practical testing algorithm called ExitBlock that systematically and deterministically finds program errors resulting from unintended timing dependencies. ExitBlock executes a program or a portion of a program on a given input multiple times, enumerating meaningful schedules in order to cover all program behaviors.

Softspec: Software-based Speculative Parallelism

Sat, 01 Apr 2000 00:00:00 GMT

Softspec: Software-based Speculative Parallelism Bruering, Derek; Devabhaktuni, Srikrishna; Amarasinghe, Saman We present Softspec, a technique for parallelizing sequential applications using only simple software mechanisms, requiring no complex program analysis or hardware support. Softspec parallelizes loops whose memory references are stride-predictable.

An Information Theoretic Approach for Shared Bottleneck Inference Based on End-to-end Measurements

Wed, 01 Mar 2000 00:00:00 GMT

An Information Theoretic Approach for Shared Bottleneck Inference Based on End-to-end Measurements Katabi, Dina; Bazzi, Issam; Yang, Xiaowei Recent years have marked a growing interest in studying Internet path characteristics. However, most of the currently available tools to an end system to perform such measurements are slow inaccurate and generate an excessive amount of probing traffic. This paper introduces entropy as a novel and efficient metric for discovering Internet path characteristics based on data collected by an end system. In particular, the paper presents an entropy-based technique that enables an end system to cluster flows it receives according to their shared bottleneck. Our mechanism relies solely on information extracted from the packets' inter-arrivals at the receiver. It does not generate any probing traffic and can use data extracted from both TCP and UDP flows. Moreover, it requires only a small number of packets from each flow, which makes it useful for short-lived flows. We report the result of running the algorithm on simulated data and Internet traffic.

Proving Correctness of a Distributed Shared Memory Implementation

Fri, 01 Jan 1999 00:00:00 GMT

Proving Correctness of a Distributed Shared Memory Implementation Castro, Miquel DiSOM [3,4,2] is a distributed shared memory system that offers users an atomic collection of memory cells provided they satisfy certain well-formedness conditions. This report proves the correctness of DiSOM. The system partitions memory into a set of objects and implicitly associates a read-write lock with each object. Users synchronize accesses to these objects-write implementation guarantees progress and the usual read-write lock exclusions conditions.

Bitwidth Analysis with Application to Silicon Compilation

Mon, 01 Nov 1999 00:00:00 GMT

Bitwidth Analysis with Application to Silicon Compilation Stephenson, Mark; Babb, Jonathan; Amarasinghe, Saman In this paper introduces Bitwise, a compiler that minimizes the bitwidth - the number of bits used to represent each operand - for both integers and pointers in a program. By propagating static information both forward and backward in the program dataflow graph, Bitwise frees in cases where the compiler can determine bitwidths automatically. We find a rich opportunity for bitwidth reductionin modern multimedia and streaming application workloads. For new architectures that support sub-word quantities, we expect that our bitwidth reductions will save power and increase processor performance.

Exploiting Superword Level Parallelism with Multimedia Instruction Sets

Mon, 01 Nov 1999 00:00:00 GMT

Exploiting Superword Level Parallelism with Multimedia Instruction Sets Larsen, Samuel; Amarasinghe, Saman Increasing focus on multimedia applications has prompted the addition of multimedia extensions to most existing general-purpose microprocessors. This added functionality comes primarily in the addition of short SIMD instructions. Unfortunately, access to these instructions is limited to in-line assembly and library calls. Some researchers have proposed using vector compilers as a means of exploiting multimedia instructions. Although vectorization technology is well understood, it is inherently complex and fragile. In addition, it is incapable of locating SIMD-style parallelism within a basic block. In this paper we introduce the concept of Superword Level Parallelism (SLP), a novel way of viewing parallelism in multimedia applications. We believe SLP is fundamentally different from the loop-level parallelism exploited by traditional vector processing, and therefore warrants a different method for extracting it. We have developed a simple and robust compiler technique for detecting SLP that targets basic blocks rather than loop nests. As with techniques designed to extract ILP, ours is able to exploit parallelism both across loop iterations and within badic blocks. The result is an algorithm that provides excellent performance in several application domains. Experiments on scientific and multimedia benchmarks have yielded average performance improvements of 84%, and range as high as 253%.

Strength Reduction of Integer Division and Modulo Operations

Mon, 01 Nov 1999 00:00:00 GMT

Strength Reduction of Integer Division and Modulo Operations Amarasinghe, Saman; Lee, Walter; Greenwald, Ben Integer division, modulo, and remainder operations are expressive and useful operations. They are logical candidates to express complex data accesses such as the wrap-around behavior in queues using ring buffers, array address calculations in data distribution, and cache locality compiler-optimizations. Experienced application programmers, however, avoid them because they are slow. Furthermore, while advances in both hardware in both hardware and software have improved the performance of many parts of a program, few are applicable to division and modulo operations. This trend makes these operations increasingly detrimental to program performance.

A Correctness Proof for a Practical Byzantine-Fault-Tolerant Replication Algorithm

Tue, 01 Jun 1999 00:00:00 GMT

A Correctness Proof for a Practical Byzantine-Fault-Tolerant Replication Algorithm Castro, Miguel We have developed a practical algorithm for state-machine replication [7,11] that tolerates Byzantine faults. The algorithm is described in [4]. It offers a strong safety property - it implements a linearizable [5] object such that all operations invoked on the object execute atomically despite Byzantine failures and concurrency. Unlike previous algorithms [11, 10, 6], ours works correctly in asynchronous systems like the Internet, and it incorporates important optimizations that enable it to outperform previous systems by more than on order of magnitude [4].

The MASC Composable Computing Infrastructure for Intelligent Environments

The MASC Composable Computing Infrastructure for Intelligent Environments Shatterjee, Sandeep; Devadas, Srinivas We present a system architecture and framework for creating rapidly deployable intelligent environments. The rapid pace of innovation of computer hardware and intelligent systems software leads to uncertainty that deters manufacturers from adopting a single processor, network, or software environment for placement into their products. The MASC Composable Computing infrastructure addresses these issues by providing an upgradable hardware and software infrastructure that supports rapid development and deployment, as well as simple and economical maintenance of intelligent environmental systems.

Authenticated Byzantine Fault Tolerance Without Public-Key Cryptography

Tue, 01 Jun 1999 00:00:00 GMT

Authenticated Byzantine Fault Tolerance Without Public-Key Cryptography Castro, Miguel; Liskov, Barbara We have developed a practical state-machine replication algorithm that tolerates Byzantine faults: it works correctly in asynchronous systems like the Internet and it incorporates several optimizations that improve the response time of previous algorithms by more than an order of magnitude.

Can Statistical Zero Knowledge be made Non-interactive? or On the Relationship of SZK and NISZK

Wed, 01 Sep 1999 00:00:00 GMT

Can Statistical Zero Knowledge be made Non-interactive? or On the Relationship of SZK and NISZK Goldreich, Oded; Sahai, Amit; Vadhan, Salil We extend the study of non-interactive statistical zero-knowledge proofs. Our main focus is to compare the class NISZK of problems possessing such non-interactive proofs to the class SZK of problems possessing interactive statistical zero-knowledge proofs. Along these lines, we first show that if statistical zero knowledge is non-trivial then so is non-interactive statistical zero knowledge, where by non-trivial we mean that the class includes problems which are not solvable in probabilistic polynommial-time. (The hypothesis holds under various assumptions, such as the intractability of the Discrete Logarithm Problem.) Furthermore, we show that if NISZK is closed under complement, then in fact SZK = NISZk, i.e. all statistical zero-knowledge proofs can be made non-interactive. The main tools in our analysis are two promise problems that are natural restrictions of promise problems known to be complete for SZK. We show that these restricted problems are in fact completer for NISZK and use this relationship to derive our results comparing the two classes. The two problems refer to the statistical difference, and difference in entropy, respectively, of a given distribution from the uniform one. We also consider a weak form of NISZK, in which only requires that for every inverse polynomial 1/p(n), there exists a simulator which achieves simulator deviation 1/p(n), and show that this weak form of NISZK actually equals NISZK.

A Client-Server Oriented Algorithm for Virtually Synchronous Group Membership in WANs

Tue, 01 Jun 1999 00:00:00 GMT

A Client-Server Oriented Algorithm for Virtually Synchronous Group Membership in WANs Keidar, Idit; Sussman, Jeremy; Marzullo, Keith; Dolev, Danny We describe a novel scalable group membership algorithm designed for wide area networks(WANs.) Our membership service does not evolve from existing LAN-oriented membership services; it was designed explicitly for WANs.

NAIVE - Network Aware Internet Video Encoding

Thu, 01 Apr 1999 00:00:00 GMT

NAIVE - Network Aware Internet Video Encoding Briceno, Hector; Gortler, Steven J.; McMillan, Leonard The distribution of digital video content over computer networks has become commonplace. Unfortunately, most digital video encoding standards do not degrade gracefully in the face of packet losses, which often occur in a bursty fashion. We propose an new video encoding system that scales well with respect to the network's performance and degrades gracefully under packet loss.

Dynamic Load Balancing with Group Communication

Fri, 01 Oct 1999 00:00:00 GMT

Dynamic Load Balancing with Group Communication Dolev, Shlomi; Segala, Roberto; Shvartsman, Alexander A. This work considers the problem of efficiently performing a set of tasks using a network of processors in the setting where the network is subject to dynamic reconfigurations, including partitions and merges. A key challenge for this setting is the implementation of dynamic load balancing that reduces the number of tasks that are performed redundantly because of the reconfigurations. We explore new approaches for load balancing in dynamic networks that can be employed by applications using a group communication service. The group communication services that we consider include a membership service (establishing new groups to reflect dynamic changes) but does not include maintenance of a primary component. For the n-processor, n-task load balancing problem defined in this work, the following specific results are obtained. For the case of fully dynamic changes including fragmentation and merges we show that the termination time of any on-line task assignment algorithm is greater than the termination time of an off-line task assignment algorithm by a factor greater than n/12. We present a load balancing algorithmthat guarantees completion of all tasks in all fragments caused by partitions with work O(n + f ÔøΩ n) in the presence of f fragmentation failures. We develop an effective scheduling strategy for minimizing the task execution redundancy and we prove that our strategy provides each of the n processors with a schedule of ?(n1/3) tasks such that at most one task is performed redundantly by any two processors.

Complexity Results for Single Machine Distance Constrained Scheduling Problems

Sun, 01 Nov 1998 00:00:00 GMT

Complexity Results for Single Machine Distance Constrained Scheduling Problems Engels, Daniel W.; Karger, David; Devadas, Srinivas Scheduling problems that involve timing constraints between tasks occur often in machine shop scheduling (e.g., job shop scheduling problems) and code scheduling during software compilation for pipelined processors (e.g., multiprocessor sequencing and scheduling problems).

Extracting all the Randomness from a Weakly Random Source

Sat, 01 Aug 1998 00:00:00 GMT

Extracting all the Randomness from a Weakly Random Source Vadhan, Salil In this paper, we give two explicit constructions of extractors, both of which work for a source of any min-entropy on strings of length n. The first extracts any constant fraction of the min-entropy using O(log^2 n) additional random bits. The second extracts all the min-entropy using O(log 3 n) additional random bits. Both constructions use fewer truly random bits than any previous. construction which works for all min-entropies ans extracts a constant fraction of the min-entropy.

Local Rules Modeling of Nucleation-Limited Virus Capsid Assembly

Sat, 01 Aug 1998 00:00:00 GMT

Local Rules Modeling of Nucleation-Limited Virus Capsid Assembly Schwartz, Russell; Prevelige, Peter E.; Berger, Bonnie We describe an application of computer modeling to the study of the kinetics of virus capsid (protein shell) assembly. We examine two proposed models of the source of nucleation-limited growth, an observed growth pattern in which initiation of new capsids occurs significantly more slowly than subunit addition onto initiated capsids.

Maps: a Compiler-Managed Memory System for RAW Machines

Wed, 01 Jul 1998 00:00:00 GMT

Maps: a Compiler-Managed Memory System for RAW Machines Barua, Rajeev; Lee, Walter; Amarasinghe, Saman; Agarwal, Anant Microprocessors of the next decade and beyond will be built using VLSI chips employing billions of transistors. In this generation of microprocessors, achieving a high level of parallelism at a reasonable clock speed will require full distribution of mac.

Indolent Closure Creation

Mon, 01 Jun 1998 00:00:00 GMT

Indolent Closure Creation Strumpen, Volker A closure is a representation of a thread in memory, ready to be executed. The goal of this work is to create portable closures that can be transferred across binary incompatible architectures. Consequently, indolent closures are software-implemented, and rely on a copy mechanism which allows for potential data representation conversion on-the-fly.

More on Proofs of Knowledge

Fri, 01 May 1998 00:00:00 GMT

More on Proofs of Knowledge Halevi, Shai; Micali, Silvio The notion of proofs of knowledge is central to cryptographhic protocols, and many definitions for it have been proposed. In this work we explore a different facet of this notion, not addressed by prior definitions. Specifically, prior definitions concentrate on capturing the properties of the verifier, and do not pay much attention to the properties of the prover. Our new definition is strictly stronger than previous ones, and captures new and desirable properies. In particular, it guarantees prover feasibility, that is, it guarantees that the time spent by the prover in a proof of knowledge is comparable to that it spends in an "extraction" of this knowledge. Our definition also enables one to consider meaningfully the case of a single, specific prover.

Copmutationally Sound Proofs

Sat, 01 Jan 2000 00:00:00 GMT

Copmutationally Sound Proofs Micali, Silvio This paper puts forward a new notion of a proof based on computational complexity and explores its implications for computation at large. Computationally sound proofs provide, in a novel and meaningful framework, answer to old and new questions in complexity theory. In particular, given a random oracle or a new complexity assumption, they enable us to 1. prove that verifying is easier than deciding for all theorems; 2. provides a quite effective way to prove membership in computationally hard languages (such as C-NP-complete ones); and 3. show that every computation possesses a short certificate vouching its correctness. FInally, if a special type of computationally sound proof exists, we show that Blum's notion of program checking can be meaningfully broadened so as to prove that NP-complete languages are checkable.

Proving Correctness of a Controller Algorithm for the RAID Level 5 System

Proving Correctness of a Controller Algorithm for the RAID Level 5 System Vazirir, Mandana; Lynch, Nancy A.; Wing, Jeannette Mos RAID controllers implemented in industry are complicated and difficult to reason about. This complexity has led to software and hardware systems that are difficult to debug and hard to modify. To overcome this problem Courtright and Gibson have developed a rapidf prototyping framework for RAID architectures which relies on a generic controller algorithm [1]. The designer of a new architecture needs to specify parts of the generic controller algorithm and must justify the validity of the controller algorithm obtained. However the latter task may be difficult due to the concurrency of operations on the disks. This is the reason why it would be useful to provide designers with an automated verification tool tailored specificially for the RAID prototyping system. As a first step towards building such a tool, our approach consists of studying several controller algorithms manually, to determine the key properties that need to be verified. This paper presents the modeling and verification of a controller algorithm for the RAID Level 5 System [5]. We model the system using I/O automata [6], give an external requirements specification, and prove that the model implements its specification. We use a key invariant to find an error in a controller algorithm for the RAID Level 6 System [5].

Signing with Partially Adversarial Hashing

Sun, 01 Feb 1998 00:00:00 GMT

Signing with Partially Adversarial Hashing Micali, Silvio; Reyzin, Leonid Digital signatures usually utilize one-way hash functions designed by other parties. It is thus possible that such hash functions are adverserially designed so as to enable forging signatures in otherwise secure schemes. We initiate the study of signing

On the Inapproximability of the Shortest Vector in a Lattice Within Some Constant Factor

Thu, 01 Jan 1998 00:00:00 GMT

On the Inapproximability of the Shortest Vector in a Lattice Within Some Constant Factor Micciancio, Danielle We show that computing the approximate length of the shortest vector in a lattice within a factor c is NP-hard for randomized reductions for any constant c < ? (2).

Space - Time Scheduling of Instruction-Level Parallelism on a Raw Machine

Mon, 01 Dec 1997 00:00:00 GMT

Space - Time Scheduling of Instruction-Level Parallelism on a Raw Machine Lee, Walter; Barua, R.; Srikrishna, D.; Babb, Jonathan; Sarkar, V.; Amarasinghe, Saman; Agarwal, Anant Advances in VLSI technology will enable chips with over a billion transistors within the next decade. Unfortunately, the centralized-resource architectures of modern microprocessors are ill-suited to exploit such advances. Achieving a high level of parallelism at a reasonable clock speed requires distributing the processor resources - a trend already visible in the dual-register-file architecture of the Alpha 21264.

Specifying and Using a Partitionable Group Communication Service*

Fri, 01 Aug 1997 00:00:00 GMT

Specifying and Using a Partitionable Group Communication Service* Fekete, Alan; Lynch, Nancy A.; Shvartsman, Alexander A. A new, simple formal specification is presented for a partitionable view-oriented group communication service. The specification consists of a state machine to express safety requirements and a timed trace property to express performance and fault-toleran.

Acquisition of a Large Pose-Mosaic Dataset

Thu, 01 Jan 1998 00:00:00 GMT

Acquisition of a Large Pose-Mosaic Dataset Coorg, Satyan; Master, Neel; Teller, Seth We describe the generation of a large pose-mosaic dataset: a collection of several thousand digital images, grouped by spatial position into spherical mosaics, each annotated with estimates of the acquiring camera's 6 DOF pose (3 DOF position and 3 DOF orientation) in an absolute coordinate system. The pose-mosaic dataset was generated by acquiring images, grouped by spatial position into nodes (essentially, spherical mosaics). A prototype mechanical pan-tilt head was manually deployed to acquire the data. Manual surverying provided initial position estimates for each node. A back-projecting scheme provided initial rotational estimates. Relative rotations within each node, along with internal camera parameters, were refined automatically by an optimization-correlation scheme. Relative translations and rotations among nodes were refined according to point correspondences, generated automatically and by a human operator. The resulting pose-imagery is self-consistent under a variety of evaluation metrics. Pose-mosaics are useful "first-class" data objects, for example in automatic reconstruction of textured 3D CAD models which represent urban exteriors.

Lazy Reference Counting for Transactional Storage Systems

Wed, 01 Oct 1997 00:00:00 GMT

Lazy Reference Counting for Transactional Storage Systems Castro, Miguel; Adya, Atul; Liskov, Barbara HAC is a novel technique for managing the direct the client cache in a distributed, persistent object storage system. In a companion paper, we showed that it outperforms other techniques across a wide range of cache sizes and workloads. This report describes HAC's solution to a specific problem: how to discard indirection table entries in an indirect pointer swizzling scheme.

The Sensitivity of Communication Mechanisms to Bandwidth and Latency

The Sensitivity of Communication Mechanisms to Bandwidth and Latency Chong, Frederic T.; Barua, Rajeev; Dahlgren, Fredrik; Kubiatowicz, John D.; Agarwal, Anant The goal of this paper is to gain insight into the relative performance of communication mechanisms as bisection bandwidth and network latency vary. We compare shared memory with and without prefetching, message passing with interrupts and with polling, and bulk transfer via DMA. We present two sets of experiments involing four irregular applications on the MIT Alewife multiprocessor. First, we introduce I/O cross-traffic to vary bisection bandwidth. Second, we change processor clock speeds to vary relative network latency. We establish a framework from which to understand a range of results. On Alewife, shared memory provides good performance, even on producer-consumer applications with little data-reuse. On machines with lower bisection bandwidth and higher network latency, however, message-passing mechanisms become important. In particular, the high communication volume of shared memory threatens to become difficult to support on future machines without expensive, high-dimensional networks. Furthermore, the round-trip nature of shared memory may not be able to tolerate the latencies of future networks.

Matching and Pose Refinement with Camera Pose Estimates

Mon, 01 Jan 1996 00:00:00 GMT

Matching and Pose Refinement with Camera Pose Estimates Coorg, Satyan; Teller, Seth This paper describes novel algorithms that use absolute camera pose information to identify correspondence among point features in hundreds or thousands of images. Our incidence counting algorithm is a geometric approach to matching: it makes features by extruding them into an absolute 3-D coordinate system, then searching 3-D space for regions into which many features project.

Light Traps

Tue, 01 Oct 1996 00:00:00 GMT

Light Traps Dawson, R.J. Macg.; McDonald, B.E.; Mycielski, J.; Pachter, L. In the February 1992 issue of the American Mathematical Monthly, J. E. Connett [1] asked whether it is possible to construct a 'light trap': a reflective-sided container with the property that a beam of light, shone into it from an appropriate direction, would be reflected inside it over and over again and never escape. Connett suggests that such a trap might be of value as a device to store light rays; however, the market for escape-proof golf holes might be even more lucrative!

Protein Folding in the Generalized Hydrophobic-Polar Model on the Triangular Lattice

Wed, 01 May 1996 00:00:00 GMT

Protein Folding in the Generalized Hydrophobic-Polar Model on the Triangular Lattice Decatur, Scott E. We consider the problem of determining the three-dimensional folding of a protein given its one-dimensional amino sequence. The model we use is based on the Hydrophobic-Polar (HP) model [2] on cubic lattices in which the goal is to find the fold with the maximum number of contacts between non-covalently linked hydrophobic amino acids.

A Survey of Active Network Research

A Survey of Active Network Research Tennenhouse, David L.; Smith, Jonathan M.; Sincoskie, W. David; Wetherall, David J.; Minden, Gary J. Active networks are a novel approach to network architecture in which the switches of the network perform customized computations on the messages flowing through them. This approach is motivated by both lead user applications, which perform user-driven computation at nodes within the network today, and the emergence of mobile code technologies that make dynamic network service innovation attainable. In this paper, we discuss two approaches to the realization of active networks and provide a snapshot of the current research issues and activities.

UDM: User Direct Messaging for General-Purpose Multiprocessing

Fri, 01 Mar 1996 00:00:00 GMT

UDM: User Direct Messaging for General-Purpose Multiprocessing Mackenzie, Kenneth; Kubiatowicz, John; Frank, Matthew; Lee, Walter; Victor, Lee; Agarwal, Anant; Kaashoek, M. Frans User Direct Messaging (UDM) allows user-level, processor-to- processor messaging to coexist with general multiprogramming and virtual memory. Direct messaging, where processors launch and receive messages in tens of cycles directly via network interface FIFO's as opposed to indirectly via memory, offers high message bandwidth and low delivery latency by avoiding memory delay and buffer management overhead.

Verification of the Randomized Consensus Algorithm of Aspnes and Herlihy: a Case Study*

Sun, 01 Jun 1997 00:00:00 GMT

Verification of the Randomized Consensus Algorithm of Aspnes and Herlihy: a Case Study* Pogosyants, Anna; Segala, Roberto; Lynch, Nancy A. The Probabilistic I/O Automaton model of [20] is used as the basis for a formal presentation and proof of the randomized consensus algorithm of Aspnes and Herlihy. The algorithm guarantees termination within expected polynomial time. The Aspnes-Herlihy algorithm is a rather complex algorithm. Processes move through a succesion of asynchronous rounds, attempting to agree at each round. At each round, the agreement attempt involves a distributed random walk.

An Oblivious Data Structure and its Applications to Cryptography

Sat, 01 Jun 1996 00:00:00 GMT

An Oblivious Data Structure and its Applications to Cryptography Micciancio, Danielle

Parameterized Types and Java

Wed, 01 May 1996 00:00:00 GMT

Parameterized Types and Java Bank, Joseph A.; Liskov, Barbara; Myers, Albert C. Java offers the real possibility that most programs can be written in a type-safe language However, for Java to be broadly useful, it needs additional expressive power. This paper extends Java in one area where more power is needed: support for parametric polymorphism, which allows the definition and implementation of generic abstractions.

Conservative Radiance Interpolants for Ray Tracing

Mon, 01 Apr 1996 00:00:00 GMT

Conservative Radiance Interpolants for Ray Tracing Teller, Seth; Bala, Kavita; Dorsey, Julie Classical ray-tracing algorithms compute radiance returning to the eye along one or more sample rays through each pixel of an image. The output of a ray-tracing algorithm, although potentially photorealistic, is a two-dimensional quality an image array of radiance values and is not directly useful from any viewpoint other than the one for which it was computed.

Cilk: An Efficient Multithreaded Runtime System

Mon, 01 Jan 1996 00:00:00 GMT

Cilk: An Efficient Multithreaded Runtime System Blumofe, Robert D.; Joerg, Christopher F.; Kuszmaul, Bradley C.; Leiserson, Charles E.; Randall, Keith H.; Yuli, Zhou Cilk (pronounced "silk") is a C-based runtime system for multithreaded parallel programming. In this paper, we document the efficiency of the Cilk work-stealing scheduler, both empirically and analytically. We show that on real and synthetic applications, the "work" and "critical-path length" of a Cilk computation can be used to model performance accurately.

A Brief Overview of the GSM Radio Interface

Fri, 01 Mar 1996 00:00:00 GMT

A Brief Overview of the GSM Radio Interface Turletti, Thierry This technical memorandum contains a compilation of several papers, reports and books relative to the GSM-900 radio interface. It is not exhaustive and it is restricted to the Traffic Channel/Full-Rate Speech (TCH/FS).

A Spacially and Temporally Coherent Object Space Visibility Algorithm

Thu, 01 Feb 1996 00:00:00 GMT

A Spacially and Temporally Coherent Object Space Visibility Algorithm Coorg, Satyan; Teller, Seth Efficiently identifying polygons that are visible from a changing synthetic viewpoint is an important problem in computer graphics. In many complex geometric models, most parts of the model are invisible from the instantaneous viewpoint. Despite this, hidden-surface algorithms like the z-buffer or BSP tree often expend significant computation processing invisible portions of the model.

Modelling and Verification of Automated Transit Systems, Using Timed Automata, Invariants and Simulations

Fri, 01 Dec 1995 00:00:00 GMT

Modelling and Verification of Automated Transit Systems, Using Timed Automata, Invariants and Simulations Lynch, Nancy A. This paper contains an overview of recent and current work in the M.I.T. Theory of Distributed Systems research group on modelling, verifying and analyzing problems arising in automated transit systems. The problems we consider are inspired by design work in Personal Rapid Transit (PRT) project at Raytheon (as described to us by Toy Johnson, Steve Spielman and Norm Delisle), and in the California PATH project (as described to us by Shankar Sastry, Datta Godbole and John Lygeros) [7, 6,13, 3].

Hybrid I/O Automata

Fri, 01 Dec 1995 00:00:00 GMT

Hybrid I/O Automata Lynch, Nancy A.; Segala, Roberto; Vaandrager, Frits; Weinberg, H. B. We propose a new hybrid I/O automaton model that is capable of describing both continuous and discrete behavior. The model, which extends the timed I/O automaton model of [12, 7] and the phase transition system models of [15, 2], allows communication among components using both shared variables and shared actions.

Enhanced Certificate Revocation System

Enhanced Certificate Revocation System Micali, Silvio We apply off-linne digital signatures to provide a novel approach to certificate revocation. Our approach dismisses with traditional CRLs and yields pubilc-key infrastructures that are several-hundred times cheaper to run than traditional ones. More generally, our technology also yields effective methods to lengthen the validity of a digital signature.

Symmetric Alternation Captures BPP

Wed, 01 Nov 1995 00:00:00 GMT

Symmetric Alternation Captures BPP Russell, Alexander; Sundaram, Ravi We introduce the natural class Sp2 containing those languages which may be expressed in terms of two symmetric quantifiers. This class lies between ? and ? and naturally generates a "symmetric" hierarchy corresponding to the polynomial-time hierarchy. We demonstrate, using the probabilistic method, new containment theorems for BPP.

Temporally Coherent Conservative Visibility

Wed, 01 Nov 1995 00:00:00 GMT

Temporally Coherent Conservative Visibility Coorg, Satvan; Teller, Seth Efficiently identifying polygons that are visible from a changing synthetic viewpoint is an important problem in computer graphics. even with hardware support, simple algorithms like depth-buffering cannot achieve interactive frame rates when applied to geometric models with many polygons. However, a visibility algorithm that exploits the occlusion properties of the scene to identify a superset of visible polygons, without touching most invisible polygons, could achieve fast rates while viewing such models.

Automatic Partitioning of Parallel Loops and Data Arrays for Distributed Shared-memory Multiprocessors

Fri, 01 Sep 1995 00:00:00 GMT

Automatic Partitioning of Parallel Loops and Data Arrays for Distributed Shared-memory Multiprocessors Agarwal, Anant; Kranz, David A.; Natarajan, Venkat This paper presents a theoretical framework for automatically partitioning parallel loops to minimize cache coherency traffic on shared-memory multiprocessors. While several previous papers have looked at hyperplane partitioning of iteration spaces to reduce communication traffic, the problem of deriving the optimal tiling parameters for minimal communication in loops with general affine index expressions had remained open. Our paper solves this open problem by presenting a method for deriving an optimal hyperparallelepiped tiling of iteration spaces for minimal communication in multiprocessors with caches. We show that the same theoretical framework can also be used to determine optimal tiling parameters for both data and loop partitioning in distributed memory multicomputers. Our framework uses matrices to represent iteration and data space mappings and the notion of uniformly intersecting references to capture temporal locality in array references. We introduce the notion of data footprints to estimate the communication traffic between processors and use linear algebraic methods and lattice theory to compute precisely the size of data footprints. We have implemented this framework in a compiler for Alewife, a distributed shared-memory multiprocessor.

Guaranteeds Partial Key-escrow

Tue, 01 Aug 1995 00:00:00 GMT

Guaranteeds Partial Key-escrow Micali, Silvio

On the Redundancy Achieved by Huffman Codes

Fri, 01 Sep 1995 00:00:00 GMT

On the Redundancy Achieved by Huffman Codes De Prisco, Roberto; De Santis, Alfredo It has been recently proved that the redundancy r of any discrete memoryless source satisfies r < 1 -H(pn), where pn is the least likely source letter probability. This bound is achieved only by sources consisting of two letters. We prove a sharper bound if the number of source letters is greater than two. Also provided is a new upper bound on r, as function of the two least likely source letter probabilities which improve on previous results.

The Power of Team Exploration: Two Robots Can Learn Unlabeled Directed Graphs

Fri, 01 Sep 1995 00:00:00 GMT

The Power of Team Exploration: Two Robots Can Learn Unlabeled Directed Graphs Bender, Michael A.; Slonim, Donna K. We show that two cooperating robots can learn exactly any strongly-connected directed graph with n indistinguishable nodes in expected time polynomial in n. We introduce a new type of homing sequence for robots, which helps the robots recognize certain previously-seen nodes. We represent an algorithm in which the robots learn the graph and the homing sequence simultaneously by actively wandering through the graph.

What are principal typings and what are they good for?

Tue, 01 Aug 1995 00:00:00 GMT

Rank 2 Type Systems and Recursive Definitions

Tue, 01 Aug 1995 00:00:00 GMT

Charge-Based Proportional Scheduling

Wed, 01 May 1996 00:00:00 GMT

Charge-Based Proportional Scheduling Maheshwari, Umesh Most priority-based schedulers lack the ability to control the relative execution rates of applications. A recent scheme, called lottery scheduling [WW94], uses randomization to control the execution rates of threads in proportion to the tickets allocated to them.

Stride Scheduling: Deterministic Proportional- Share Resource Management

Thu, 01 Jun 1995 00:00:00 GMT

Stride Scheduling: Deterministic Proportional- Share Resource Management Waldspurger, Carl A.; Weihl, William E. This paper presents stride scheduling, a deterministic scheduling technique that efficiently supports the same flexible resource management abstractions introduced by lottery scheduling. Compared to lottery scheduling, stride scheduling archives significantly improved accuracy over relative throughput rates, with significantly lower response time variability.

Local Rule Switching Mechanism for Viral Shell Geometry

Thu, 01 Jun 1995 00:00:00 GMT

Local Rule Switching Mechanism for Viral Shell Geometry Berger, Bonnie; Shor, Peter W. In a previous paper [Berger et al., PNAS 91 7732,1994] a theory of virus shell formation was proposed in which shell assembly is directed by local interactions of the coat ans scaffolding subunits. This theory requires that the same chemical subunits assume different, stable conformations depending on their position in the shell.

'C: A Language for High-Level, Efficient, and Machine-independant Dynamic Code Generation

'C: A Language for High-Level, Efficient, and Machine-independant Dynamic Code Generation Engler, Dawson R.; Hsieh, Wilson C.; Kaashoek, M. Frans Dynamic code generation allows specialized code sequences to be crafted using runtime information. Since this information is by definition not available statically, the use of dynamic code generation can achieve performance inherently beyond that of static code generation. Previous attempts to support dynamic code generation have been low-leveled, expensive, or machine-dependent.

Algorithms for Modeling and Measuring Proteins

Thu, 01 Jun 1995 00:00:00 GMT

Algorithms for Modeling and Measuring Proteins Slonim, Donna K. In this paper we investigate efficient algorithms for computing the volume and surface area of protein molecules are modeled by sets of overlapping spheres in R 3. We summarize and critique three papers in the field, and we add several new contributions of our own.

A Case Study of Shared Memory and Message Passing: The Triangle Puzzle

Sun, 01 Jan 1995 00:00:00 GMT

A Case Study of Shared Memory and Message Passing: The Triangle Puzzle Lew, Kevin This thesis is the first controlled case study that compares shared-memory and message-passing implementations of an application that solves the triangle puzzle and runs on actual hardware: only the communication interfaces used by the implementations vary; all other system components remained fixed. The implementations run on the MIT Alewife machine, a cache-coherent, distributed-shared-memory multiprocessor that efficiently supports both the shared-memory and message-passing programming models.

Communication-Minimal Partitioning of Parallel Loops and Data Arrays for Cache-Coherent Distributed -Memory Multiprocess

Sun, 01 Jan 1995 00:00:00 GMT

Communication-Minimal Partitioning of Parallel Loops and Data Arrays for Cache-Coherent Distributed -Memory Multiprocess Barua, Rajeev; Kranz, David; Agarwal, Anant Harnessing the full performance potential of cache-coherent distributed shared memory multiprocessors without inordinate user effort requires a compilation technology that can automatically manage multiple levels of memory hierarchy. This paper describes a working compiler for such machines that automatically partitions loops and data arrays to optimize locality of access.

Addressing Partitioned Arrays in Distributed Memory Multiprocessors - the Software Virtual Memory Approach

Thu, 01 Dec 1994 00:00:00 GMT

Addressing Partitioned Arrays in Distributed Memory Multiprocessors - the Software Virtual Memory Approach Barua, Rajeev; Kranz, David; Agarwal, Anant Harnessing the full performance potential of cache-coherent distributed shared memory multiprocessors without inordinate user effort requires a compilation technology that can automatically manage multiple levels of memory hierarchy. This paper describes a working compiler for such machines that automatically partitions loops and data arrays to optimize locality of access.

Symmetric Alteration Captures BPP

Wed, 01 Nov 1995 00:00:00 GMT

Symmetric Alteration Captures BPP Russell, Alexander; Sundaram, Ravi We introduce the natural class Sp2 containing those languages which may be expressed terms of two symmetric quantifiers. This class lies between ?p2 and ? and naturally generates a "symmetric" hierarchy corresponding to the polynomial-time hierarchy. We demonstrate, using the probabilistic method, new containment theorems for BPP.

On the Mathematics of Virus Shell Assembly

Fri, 01 Jul 1994 00:00:00 GMT

On the Mathematics of Virus Shell Assembly Berger, Bonnie; Shor, Peter W. A local rule theory is developed which shows that the self-assembly of icosahedral virus shells may depend on only the lower-level interactions of a protein subunit with its neighbors, i.e. local rules, rather than on larger structural building blocks. The local rule theory provides a framework for understanding the assembly of icosahedral viruses.

Implementing Sequentially Consistant Shared Objects using Broadcast and Point-To-Point Communications

Thu, 01 Jun 1995 00:00:00 GMT

Implementing Sequentially Consistant Shared Objects using Broadcast and Point-To-Point Communications Fekete, Alan; Kaashoek, M. Frans; Lynch, Nancy A. A distributed algorithm that implements a sequentially consistent collection of shared read/update objects using a combination of broadcast and point-to-point communication is presented and proved correct. This algorithm is a generalization of one used in the Orca shared object system. The algorithm caches objects in the local memory of processors according to application needs; each read operation accesses a single copy of the object, while each update accesses all copies. Copies of all the objects are kept consistent using a strategy based on sequence numbers for broadcasts.

CRL: High - Performance All-Software Distributed Shared Memory*

Wed, 01 Mar 1995 00:00:00 GMT

CRL: High - Performance All-Software Distributed Shared Memory* Johnson, Kirk L.; Kaashoek, M. Frans; Wallach, Deborah A. This paper introduces the C Region Library (CRL), a new all-software distributed shared memory (DSM) system. CRL requires no special compiler, hardware , or operating system support beyond the ability to send and receive messages. It provides a simple, portable shared address space programming model that it capable of delivering good performance on a wide range of multiprocessor and distributed system architectures.

Piecemeal Graph Exploration by a Mobile Robot*

Sun, 01 Jan 1995 00:00:00 GMT

Piecemeal Graph Exploration by a Mobile Robot* Awerbuch, Baruch; Betke, Margrit; Rivest, Ronald; Singh, Mona We study how a mobile robot can piecemeal learn an unknown environment. The robot's goal is to learn a complete map of its environment, while satisfying the constraint that it must return every so often to its starting position (for refueling, say). The environment is modelled as an arbitrary, undirected graph, which is initially unknown to the robot.

Using Non-interactive Proofs to Achieve Independence Efficiently and Securely

Tue, 01 Nov 1994 00:00:00 GMT

Using Non-interactive Proofs to Achieve Independence Efficiently and Securely Gennaro, Rosario Independence or simultaneous broadcast is a fundamental tool to achieve security in fault tolerant distributed computing. It allows n players to commit to independently chosen values. In this paper we present a constant round protocol to perform this task. Previous solutions were 0(log n) rounds. In the process we develop a new and stronger formal definition from this problem.

The Use of the Domain Name System for Dynamic References in an Online Library

Sun, 01 May 1994 00:00:00 GMT

The Use of the Domain Name System for Dynamic References in an Online Library Alavi, Ali Persistent, dynamic references (or links) to remove documents are an essential part of an online library. This thesis examines two distributed database systems, X.500 and the Domain Name System(DNS), upon which to build dynamic references. DNS was chosen and was used to design a model and build a sample dynamic reference system. This system seems to exhibit the scalability, robustness, usuability, and efficiency necessary for building global distributed online libraries.

How Can We Compute with Arrays of Nanstructures?

Mon, 01 Aug 1994 00:00:00 GMT

How Can We Compute with Arrays of Nanstructures? Biafore, Michael In part the goal of the Ultra Program is to extract useful computation from nanometer scale effects. To accomplish this goal those of us who are computer scientists must communicate clearly to those of you who are chemists and device physicists precisely what kinds of "computational primitives" you need to obtain from a nanoscale structure before we can contemplate using it as a building block for ultra-dense ultra-fast computation.

The Generalized Railroad Crossing: A Case Study in Formali Verification of Real-time Systems

Tue, 01 Nov 1994 00:00:00 GMT

The Generalized Railroad Crossing: A Case Study in Formali Verification of Real-time Systems Heitmeyer, Constance; Lynch, Nancy A. A new solution to the Generalized Railroad Crossing problem, based on timed automata, invariants and simulation mappings, is presented and evaluated. The solution shows formally the correspondence between four system descriptions: an axiomatic specification, an operational specification, a discrete system implementation, and a system implementation that works with a continuous gate model.

An Efficient Implementation of A Hierarchical Weighted Fair Queue Packet Scheduler

Sun, 01 May 1994 00:00:00 GMT

An Efficient Implementation of A Hierarchical Weighted Fair Queue Packet Scheduler Ndiaye, Oumar The technical developments in computer networks in recent years have spawned the possibility of merging different services into a single Integrated Service Packet Network (ISPN). The types of service quality required by each of the individual services in an ISPN often differ greatly. Thus, the packet scheduling algorithms used in such networks must be flexible enough to allocate the available link shares according to the service quality requirements of the different services.

Application of Minimal Perfect Hashing in Main Memory Indexing

Sun, 01 May 1994 00:00:00 GMT

Application of Minimal Perfect Hashing in Main Memory Indexing Ho, Yuk With the rapid decrease in the cost of random access memory (RAM), it will soon become economically feasible to place full-text indexes of a library in main memory. One essential component of the indexing system is a hashing algorithm, which maps a keyword into the memory address of the index information corresponding to that keyword. This thesis studies the application of the minimal perfect hashing algorithm in main memory indexing. This algorithm is integrated into the index search engine of the Library 2000 system, a digital on-line library system. The performance if this algorithm is compared with that of the open-address hashing scheme. We find that although the minimal perfect hashing algorithm needs fewer keyword comparisons per keyword search on average, its hashing performance is slower than the open-addressing scheme.

Hybrid Caching for Scalable Oject Systems (Think Globally, Act Locally)

Fri, 01 Apr 1994 00:00:00 GMT

Hybrid Caching for Scalable Oject Systems (Think Globally, Act Locally) O'Toole, James; Shrira, Liuba Object-based client caching allows clients to keep more frequently accessed objects while discarding colder objects that reside on the same page. However, when these objects are modified and sent to the server, it may need to read the corresponding page from disk to install the update. These installation reads are not required with a page-based cache because whole pages are sent to the server.

Opportunistic Log: Efficient Installation Reads in a Reliable Object Server

Fri, 01 Apr 1994 00:00:00 GMT

Opportunistic Log: Efficient Installation Reads in a Reliable Object Server O'Toole, James; Shrira, Liuba In a distributed storage system, client caches managed on the basis of small granularity objects can provide better memory utilization then page-based caches. However, object servers, unlike page servers, must perform additional disk reads. These installation reads are required to install modified objects onto their corresponding disk pages.

Coordinated Resource Management in a Replicated Objected Server

Tue, 01 Feb 1994 00:00:00 GMT

Coordinated Resource Management in a Replicated Objected Server Ghemawat, Sanjay; Gruber, Robert; O'Toole, James, Jr.; Shrira, Liuba We propose several new techniques for resource management in a replicated object server. By coordinating cache and disk usage among the replicas, these techniques increase throughput and reduce fetch latency. Cache splitting speeds up fetches by avoiding redundant cache entries, effectively increasing the cache size. Coordinated writing coordinates disk writes to ensure that one replica is always available to service fetches. We investigate the performance of a replicated server using these techniques, and we present simulation results showing that these techniques provide substantial performance improvements across a variety of workloads.

Optimal Clock Synchronization Under Different Delay Assumptions

Fri, 01 Apr 1994 00:00:00 GMT

Optimal Clock Synchronization Under Different Delay Assumptions Attiya, Hagit; Herzberg, Amir; Rajsbaum, Sergio The problem of achieving optimal clock synchronization in a communication network with arbitrary topology and perfect clocks (that do not drift) is studied. Clock synchronization algorithms are presented for a large family of delay assumptions. Our algorithms are modular and consist of three major components. The first component holds for any type of delay assumptions; the second component holds for a large, natural family of local delay assumptions; the third component has to be tailored for each specific delay assumption. Optimal clock synchronization algorithms are derived for several types of delay assumptions by appropriately tuning the third component. The delay assumptions include lower and upper delay bounds, no bounds at all, and bounds on the difference of the delay in opposite directions. In addition, our model handles systems where some processors are connected by broadcast networks in which every message arrives to all processors at approximately the same time. A composition theorem allows combinations of different assumptions for different lins or even for the same link; such mixtures are common in practice. Our results acheive the best possible precision in each execution. This notion of optimality is stronger than the more common notion of worst case optimality. The new notion of optimality applied to systems where the worst case behavior of any clock synchronization algorithm is inherently unbounded.

FUGU: Implementing Translation and Protection in a Multiuser, Multimodel Multiprocessor

Sat, 01 Oct 1994 00:00:00 GMT

FUGU: Implementing Translation and Protection in a Multiuser, Multimodel Multiprocessor Mackenzie, Kenneth; Kubiatowicz, John; Agarwal, Anant; Kaashoek, M. Frans Multimodel multiprocessors provide both shared memory and message passing primitives to the user for efficient communication. In a multiuser machine, translation permits machine resource to be virtualized and protection permits users to be isolated. The challenge in a multiuser multiprocessor is to provide translation and protection sufficient for general-purpose computing without compromising communication performance, particularly the performance of communication between parallel threads belong to the same computation.

Verifiable Secret Sharing as Secure Computation

Tue, 01 Mar 1994 00:00:00 GMT

Verifiable Secret Sharing as Secure Computation Gennaro, Rosario; Micali, Silvio We present a stronger notion of verifiable secret sharing and exhibit a protocol implementing it. We show that our new notion is preferable to the old ones whenever verifiable secret sharing is used as a tool within larger protocols, rather than being a goal in itself.

A Secure and Efficient Digital Signature Algorithm

Tue, 01 Mar 1994 00:00:00 GMT

A Secure and Efficient Digital Signature Algorithm Micali, Silvio

PAC-Learning Prolog Clauses With or Without Errors

Tue, 01 Feb 1994 00:00:00 GMT

PAC-Learning Prolog Clauses With or Without Errors Gennaro, Rosario Recently researchers have been interested in trying to expand the domain of learnability to subsets of first-order logic, in particular Prolog programs. This new research area has been named Inductive Logic Programming (ILP). In a nutshell we can describe a generic ILP problem as following: given a set E of (positive and negative) examples of a target predicate, and some background knowledge B about the world (usually a logic program including facts and auxiliary predicates), the task is to find a logic program H (our hypothesis) such that all positive examples can be deduced from B and H, while no negative example can. In this paper we review some of the results achieved in this area and discuss the techniques used. Moreover we prove the following new results: (1) Predicates described by non-recursive, local clauses of at most k literals are PAC-learnable under any distribution. This generalizes a previous result that was valid only for constrained clauses. (2) Predicates that are described by k non-recursive local clauses are PAC-learnable under any distribution. This generalizes a previous result that was non constructive and valid only under some class of distributions. Finally we introduce what we believe is the first theoretical framework for learning Prolog clauses in the presence of errors. To this purpose we introduce a new noise model, that we call the fixed attribute noise model, for learning propositional concepts over the Boolean domain. This new noise model can be of its own interest.

A Comparison of Simulation Techniques and Algebraic Techniques for Verifying Concurrent Systems

Mon, 01 Nov 1993 00:00:00 GMT

A Comparison of Simulation Techniques and Algebraic Techniques for Verifying Concurrent Systems Lynch, Nancy A.; Segala, Roberto Simulation-based assertional techniques and process algebraic techniques are two of the major methods that have been proposed for the verification of concurrent and distributed systems. It is shown how each of these techniques can be applied to the task of verifying systems described as input/output automata; both of these ways, first using forward simulations, an execution correspondence lemma, and a simple fairness argument, and second using deductions within the process algebra DIOA for I/O automata. An extended evaluation and comparison of the two methods is given.

Anatomy of a Message in the Alewife Multiprocessor

Mon, 01 Feb 1993 00:00:00 GMT

Anatomy of a Message in the Alewife Multiprocessor Kubiatowicz, John; Agarwal, Anant

Analyzing Multiprocessor Cache Behavior Through Data Reference Modeling

Mon, 01 Feb 1993 00:00:00 GMT

Analyzing Multiprocessor Cache Behavior Through Data Reference Modeling Tsai, Jory; Agarwal, Anant This paper develops a data reference modeling technique to estimate with high accuracy the cache miss ratio in cache-coherent multiprocessors. The technique involves analyzing the dynamic data referencing behavior of parallel algorithms. Data reference modeling first identifies of different types of shared data blocks accessed during the execution of a parallel algorithm, then captures in a few parameters the cache behavior of each shared block as a function of the problem size, number of processors, and cache size, and finally constructs an analytical expression for each algorithm to estimate the cache miss ratio.

Simulation Techniques for Proving Properties of Real-time Systems

Mon, 01 Nov 1993 00:00:00 GMT

Simulation Techniques for Proving Properties of Real-time Systems Lynch, Nancy A. The method of simulations is an important technique for reasoning about real-time and other timing-based systems. It is adapted from an analogous method for untimed systems. This paper presents the simulation method in the context of a very general automaton (i.e., labelled transition system) model for timing-based systems. Sketches are presented of several typical examples for which the method has been used successfully. other complementary tools are also described, in particular invariants for safety proofs, progress functions for timing proofs, and execution correspondences for liveness proofs.

Software-Extended Coherent Shared Memory: Performance and Cost

Fri, 01 Oct 1993 00:00:00 GMT

Software-Extended Coherent Shared Memory: Performance and Cost Chaiken, David; Agarwal, Anant This paper evaluates the tradeoffs involved when designing a directory-based protocol that implements coherent shared memory through a combination of hardware and software mechanisms. The fundamental design decisions involve balancing the size and cost of the hardware directory and control, the complexity of the software interface, and the overall performance of the system. In order to study these design problems, we experiment with a spectrum of cache-coherence schemes, raging from a full-map directory that supports all sharing patterns in hardware to an implementation that performs all memory-side actions in software.

The Revitalized Relationship Between Probabilistically Checkable Debate Systems, IP, and PSpace

Wed, 01 Sep 1993 00:00:00 GMT

The Revitalized Relationship Between Probabilistically Checkable Debate Systems, IP, and PSpace Russell, Alexander; Sundaram, Ravi In 1990, PSPACE was shown to be identical to IP, the class of languages with interactive proofs [11, 2]. Recently, PSPACE was again recharacterized, this time in terms of (Random) Probabilistically Checkable Debate Systems [4, 5]. In particular, it was shown that SPACE = PCDS[log n, 1] = RPCDS [log n, 1]. We study the relativized behaviour of the classes defined by these debates systems in comparison with the classes IP and PSPACE.

Virtual Wires: Overcoming Pin Limitations in FPGA-based Logic Emulators

Sun, 01 Nov 1992 00:00:00 GMT

Virtual Wires: Overcoming Pin Limitations in FPGA-based Logic Emulators Babb, Jonathan; Tessier, Russell; Agarwal, Anant Existing FPGA-based logic emulators suffer from limited inter-chip communication bandwidth, resulting in low gate utilization (10 20 percent). This resource imbalance increases the number of chips needed to emulate a particular logic design and thereby decreases emulation speed, since signals must cross more chip boundaries. Current emulators only use a fraction of potential communication bandwidth because they dedicate each FPGA pin (physical wire) to a single emulated signal (logical wire). These logical wires are not active simultaneously are only switched at emulation clock speeds.

Compile-time Loop Splitting for Distributed Memory Multiprocessors

Mon, 01 Nov 1993 00:00:00 GMT

Compile-time Loop Splitting for Distributed Memory Multiprocessors Tanguay, Donald O., Jr. In a distributed memory multiprocessor, a program's task is partitioned among the processors to exploit parallelism, and the data are partitioned to increase referential locality. Though the purpose of partitioning is to shorten the execution time of an algorithm, each data reference can become a complex expression based upon the data partitions. As an attempt to minimize the computation needed for array references, loop splitting can further divide a partitioned loop into segments that allow the code hoisting and strength reduction optimizations. This thesis introduces two methods of loop splitting, rational and interval. While rational splitting divides the loop into equal-length GCD segments, interval splitting specifies segments as an explicit list of intervals. These two methods have been implemented and studied. Under our execution model, the loop in the algorithms analyzed executes an average of 2 to 3 times faster after loop splitting.

Column-associative Caches: A Technique for Reducing the Miss Rate of Direct-mapped Caches

Mon, 01 Nov 1993 00:00:00 GMT

Column-associative Caches: A Technique for Reducing the Miss Rate of Direct-mapped Caches Agarwal, Anant; Pudar, Steven D. Direct-mapped caches are a popular design choice for high-performance processors; unfortunately, direct-mapped caches suffer systematic interference misses when more than one address map into the same cache set. This paper describes the design of column-associative caches, which minimize the conflicts that arise in direct-mapped accesses by allowing conflicting addresses to dynamically choose alternate hashing functions, so that most of the conflicting data can reside in the cache. At the same time, however, the critical hit access path is unchanged. The key to implementing this scheme efficiently is the addition to each cache set of a rehash bit, which indicates whether that set stores data that is referenced by an alternate hashing function. When multiple addresses map into the same location, these rehashed locations are preferentially replaced. We demonstrate using trace-driven simulations and an analytical model that a column-associative cache removed virtually all interference misses for large caches, without altering the critical hit access time.

Modeling Multiprogrammed Caches

Modeling Multiprogrammed Caches Agarwal, Anant This paper presents a simple, yet accurate, model for multiprogrammed caches and validates it against trace-driven simulation. The model takes into account nonstationary behavior of processes and process sharing. By making judicious approximations, the paper shows that a very simple expression of the form u^2(p - 1)/tS accurately models the multiprogramming component of the miss rate of large direct-mapped caches. In the above expression, t is the context-switching interval, S is the cache size in blocks, p is the number of processes, and u is the number of unique blocks accesses by a process during the interval t.

Forward and Backward Simulations Part II: Timing-based Systems

Mon, 01 Mar 1993 00:00:00 GMT

Forward and Backward Simulations Part II: Timing-based Systems Lynch, Nancy A.; Vaandrager, Frits A general automaton model for timing-based systems is presented and is used as the context for developing a variety of simulation proof techniques for such systems. These techniques include (1) refinments, (2) forward and backward simulations, (3) hybrid forward-backward and backward-forward simulations, and (4) history and prophecy relations. Relationships between the different types of simulations, as well as soundness and completeness results, are stated and proved.

Forward and Backward Simulations Part I: Untimed Systems (Replaces TM-486)

Mon, 01 Mar 1993 00:00:00 GMT

Forward and Backward Simulations Part I: Untimed Systems (Replaces TM-486) Lynch, Nancy A.; Vaandrager, Frits A unified, comprehensive presentation of simulation techniques for verification of concurrent systems is given, in terms of a simple untimed automaton model. In particular, (1) refinements, (2) forward and backward simulations, (3) hybrid forward-backward and backward-forward simulations, and (4) history and prophecy relations are defined.

Failsafe Key Escrow Systems (Extended Abstract)

Mon, 01 Aug 1994 00:00:00 GMT

Failsafe Key Escrow Systems (Extended Abstract) Leighton, Tom This paper describes a method for escrowing cryptographic keys, which we call Failsafe Key Escrow (FKE). The method is substantially more secure than alternative such as the Fair Public Key Cryptosystem approach advocated by Micali, and it is particular well suited for use in escrowing DSS keys.

Automatic Partitioning of Parallel Loops for Cache-coherent Multiprocessors

Tue, 01 Dec 1992 00:00:00 GMT

Automatic Partitioning of Parallel Loops for Cache-coherent Multiprocessors Agarwal, Anant; Kranz, David; Natarajan, Venkat This paper presents a theoretical framework for automatically partitioning parallel loops to minimize cache coherency traffic on shared-memory multiprocessors. The framework introduces the notion of uniformly intersecting references to capture temporal locality in array references, and the idea of data footprints to estimate the communication traffic between processors. The framework uses lattice theory to compute the size of data footprints. We demonstrate that algorithms based on our framework discover optimal partitions in many cases, such as non-communication-free parallelogram partitions of affine loop index functions, which were not handled by previous algorithms. We also show that our framework correctly reproduces results from previous loop partitioning algorithms proposed by Abraham and Hudak and by Sadayappan and Ramanujam. Because they deal only with index expressions, the algorithms are computationally efficient as well. We have implemented a subset of this framework for rectangular partitioning in a compiler for the cache-coherent Alewife machine.

Action Transducers and Timed Automata

Sun, 01 Nov 1992 00:00:00 GMT

Action Transducers and Timed Automata Lynch, Nancy A.; Vaandrager, Frits The timed automaton model of [29, 30] is a general model for timing-based systems. A notion of timed action transducer is here defined as an automata-theoretic way of representing operations on timed automata. It is shown that two timed trace inclusion relations are substitutive with respect to operations that can be described by timed action transducers. Examples are given of operations that can be describe in this way, and a preliminary proposal is given for an appropriate language of operators for describing timing-based systems.

Experience with Fine-grain Synchronization in MIMD Machines for Preconditioned Conjugate Gradient

Thu, 01 Oct 1992 00:00:00 GMT

Experience with Fine-grain Synchronization in MIMD Machines for Preconditioned Conjugate Gradient Yeung, Donald; Agarwal, Anant This paper discusses our experience with fine-grain synchronization for the preconditioned conjugate gradient method using the modified incomplete Cholesky factorization of the coefficient matrix as a preconditioner. This algorithm represents a large class of algorithms that have been widely used but traditionally difficult to implement efficiently on vector and parallel machines. Through a series of experiments conducted using a simulator of a distributed shared-memory multiprocessor, this paper addresses two major questions related to fine-grain synchronization in the context of this application. First, what is the overall impact of fine-grain synchronization on performance? Second, what are the individual contributions of the following three mechanisms typically provided to support fine-grain synchronization: language-level support, full-empty bits for compact storage and communication of synchronization state, and efficient processor operations on the state bits? The experiments indicate that fine-grain synchronization improves overall performance by a factor of 3.7 on 16 processors using the largest problem size we could simulate; the paper also projects that a significant performance advantage will be sustained for larger problem sizes. Preliminary experience shows that the bulk of the performance advantage for this application can be attributed to exposing increased parallelism through language-level expression of fine-grain synchronization. A smaller fraction relies on a compact-implementation of synchronization state, while an even smaller fraction results from efficient full-empty bit operations. The paper also shows that the last two components are likely to have a greater impact on performance as mechanisms for latency tolerance are employed.

Integrating Message-passing and Shared-memory: Early Experience

Thu, 01 Oct 1992 00:00:00 GMT

Integrating Message-passing and Shared-memory: Early Experience Kranz, David; Johnson, Kirk; Agarwal, Anant; Kubiatowicz, John; Lim, Beng-Hong This paper discusses some of the issues involved in implementing a shared-address space programming model on large-scale, distributed-memory multiprocessors. Because message-passing mechanisms are much more efficient than shared-memory loads and stores for certain types of interprocessor communication and synchronization operations, we argue for building multiprocessors that efficiently support both shared-memory and message-passing mechanisms. We describe an architecture, Alewife, that integrates support for shared-memory and message-passing through a simple interface. We expect the compiler and runtime system to cooperate in using appropriate hardware mechanisms that are most efficient for specific operations. We report on both integrated and exclusively shared-memory implementations of our runtime system and one complete application; the final paper will contain results for other applications as well. The integrated runtime system drastically cuts down the cost of communication incurred by the scheduling, load balancing, and certain synchronization operations. We also present some preliminary performance results comparing the two systems.

Hybrid Atomicity for Nested Transactions

Thu, 01 Oct 1992 00:00:00 GMT

Hybrid Atomicity for Nested Transactions Fekete, Alan; Lynch, Nancy A.; Weihl, William E. This paper defines the notion of hybrid atomicity for nested transaction systems, and presents and verifies an algorithm providing this property. Hybrid atomicity is a modular property; it allows the correctness of a system to be deduced from the fact each object is implemented to have the property. It allows more concurrency than dynamic atomicity, by assigning timestamps to transaction at commit. The Avalon system provides exactly this facility.

More Choices Allow More Faults: Set Consensus Problems in Totally Asynchronous Systems

Tue, 01 Sep 1992 00:00:00 GMT

More Choices Allow More Faults: Set Consensus Problems in Totally Asynchronous Systems Chaudhuri, Soma We define k-set consensus problem as an extension of the consensus problem, where each processors decides on a single value such that the set of decided values in any run is of size at most k. We require the agreement condition that all values decided upon are initial values of some processor. We show that the problem has a simple ( k - 1 )-resilient protocol in a totally asynchronous system.

Dribble-Back Registers: A Technique for Latency Tolerance in Multiprocessors

Mon, 01 Jun 1992 00:00:00 GMT

Dribble-Back Registers: A Technique for Latency Tolerance in Multiprocessors Soundararajan, Vijayaraghavan As parallel machines grow in scale and complexity, latency tolerance of synchronization faults and remote memory accesses becomes increasingly important. One method for tolerating this by multithreading the processor and rapidly context switching between these threads. Fast context switching is most effective when the latencies being tolerated are short compared to the total run lengths of all the resident threads.

The Integration of the Organization Engine and Library 2000

Fri, 01 May 1992 00:00:00 GMT

The Integration of the Organization Engine and Library 2000 Weiss, Ron In the contemporary research environment, users access and manipulate information gathered from diverse data sources. The organization Engine is a prototype being developed at the Cambridge Research Lab of Digital Equipment Corporation for the incorporation of data from disparate sources into a local homogeneous framework. It relies on information management based on the notion of retrieval and manipulation through the organization of the data in a non strict hierarchical structure.

Approximating the Minimum-cost Maximum Flow is P-Complete

Mon, 01 Jun 1992 00:00:00 GMT

Approximating the Minimum-cost Maximum Flow is P-Complete Stein, Clifford; Wein, Joel We show that it is impossible, in NC, to approximate the value of the minimum-cost maximum flow unless P = NC.

Closing the Window of Vulnerability in Multiphase Memory Transcations

Mon, 01 Jun 1992 00:00:00 GMT

Closing the Window of Vulnerability in Multiphase Memory Transcations Kubiatowicz, John; Chaiken, David; Agarwal, Anant Multiprocessor architects have begun to explore several mechanisms such as prefetching, context-switching and software-assisted dynamic cache-coherence, which transform single-phase memory transactions in conventional memory systems into multiphase operations. Multiphase operations introduce a window of vulnerability in which data can be lost before it is used either through protocol invalidation or cache conflicts. Losing data introduces damaging livelock situations. This paper discusses the origins of the window of vulnerability and proposes an architectural framework that closes it. The framework in implemented in Alwife, a large-scale multiprocessor being built at MIT.

Low-cost Support for Fine-grain Synchronization in Multiprocessors

Mon, 01 Jun 1992 00:00:00 GMT

Low-cost Support for Fine-grain Synchronization in Multiprocessors Kranz, David; Lim, Beng-Hong; Agarwal, Anant As multiprocessors scale beyond the limits of a few tens of processors, they must look beyond traditional methods of synchronization to minimize serialization and achieve the high degrees of parallelism required to utilize large machines. By allowing synchronization at the level of the smallest unit of memory, fine-grain synchronization achieves these goals. Unfortunately, supporting efficient fine-grain synchronization without inordinate amounts of hardware has remained a challenge.

Compile-time Techniques for Processor Allocation in Macro Dataflow Graphs for Multiprocessors

Mon, 01 Jun 1992 00:00:00 GMT

Compile-time Techniques for Processor Allocation in Macro Dataflow Graphs for Multiprocessors Prasanna, G.N. Srinivasa; Agarwal, Anant When compiling a progam consisting of multiple nested loops for execution on a multiprocessor, processor allocation is the problem of determining the number of processors over which to partition each nested loop. This paper presents processor allocation techniques for compiling such programs for multiprocessors with local memory. Programs consisting of multiple loops, where the precedence constraints between the loops is known, can be viewed as macro dataflow graphs. Macro dataflow graphs comprise several macro nodes (or macro operations) that must be executed subject to prespecified precedence constraints. Optimal processor allocation specifies the number of processors computing each macro node and their sequencing to optimize run time. This paper presents computing each macro node and their sequencing to optimize run time. This paper presents computationally efficient techniques for determining the optimal processor allocation using estimated speedup functions of the macro nodes. These ideas have been implemented in a structure-driven compiler, SDC, for expressions of matrix operations. The paper presents the performance of the compiler for several matrix expressions on a simulator of the Alewife multiprocessor.

The Impact of Communication Locality on Large-scale Multiprocessor Performance

Mon, 01 Jun 1992 00:00:00 GMT

The Impact of Communication Locality on Large-scale Multiprocessor Performance Johnson, Kirk L. As multiprocessor sizes scale and computer architects turn to interconnection networks with non-uniform communication latencies, the lure of exploiting communication locality to increase performance becomes inevitable. Models that accurately quantify locality effects provide invaluable insight into the importance of exploiting locality as machine sizes and features change. This paper presents a framework for modeling the impact of communication locality on system performance. The framework provides a means for combining simple models of application, processor, and network behavior to obtain a combined model that accurately reflects feedback effects between processors and networks. We introduce a model that characterizes application behavior with three parameters that capture computation grain, sensitivity to communication latency, and amount of locality present at execution time. The combined model is validated with measurements taken from a detailed simulator for a complete multiprocessor system. Using the combined model, we show that exploiting communication locality provides gains which are at most linear in the factor by which average communication distance is reduced when the number of outstanding communication transactions per processor is bounded. The combined model is also used to obtain rough upper bounds on the performance improvement from exploiting locality to minimize communication distance.

Hierarchical Compilation of Macro Dataflow Graphs for Multiprocessors with Local Memory

Thu, 01 Oct 1992 00:00:00 GMT

Hierarchical Compilation of Macro Dataflow Graphs for Multiprocessors with Local Memory Prasanna, G.N. Srinivasa; Agarwal, Anant; Musicus, Bruce R. This paper presents a hierarchical approach for compiling macro dataflow graphs for multiprocessors with local memory. Macro dataflow graphs comprise several nodes (or macros operations) that must be executed subject to prespecified precedence constraints. Programs consisting of multiple nested loops, where the precedence constraints between the loops are known, can be viewed as macro dataflow graphs. The hierarchical compilation approach comprises a processor allocation phase followed by a partitioning phase. In the processor allocation phase, using estimated speedup functions for the macro nodes, computationally efficient techniques establish the sequencing and parallelism of macro operations for close-to-optimal run times. The second phase partitions the computations in each macro node to maximize communication locality for the level of parallelism determined by the processor allocation phase. The same approach can also be used for programs consisting of multiple loop nests, when each of the nested loops can be characterized by a speedup function. These ideas have been implemented in a prototype structure-driven compiler, SDC, for expressions of matrix operations. The paper presents the performance of the compiler for several matrix expressions on a simulator of the Alewife multiprocessor.

Memory Assignment for Multiprocessor Caches Through Graph Coloring

Sat, 01 Feb 1992 00:00:00 GMT

Memory Assignment for Multiprocessor Caches Through Graph Coloring Agarwal, Anant; Guttag, John; Papaefthymiou, Marios It has become apparent that the achieved performance of multiprocessors is heavily dependent upon the quality of the availabel compilers. In this paper we are concerned with compile-time techniques that can be used to achieve better performance by improving cache utilization. Specifically, we investigate the problem of assigning data chunks to memory in a way that will minimize collisions in direct-mapped multiprocessor caches. We show that while this problem is computationally intractable, there are interesting special cases that can be solved in polynomial time. We also present several techniques that can be used when conflict-free assignment is not possible, or when finding a conflict-free assignment is computationally infeasible. These techniques include uniform decaching, which involves not caching specific data blocks, and data replication, which involves making multiple copies of read-only data. Finally, we present a memory assigment technique, grey coloring, that reduces latency in the presence of collisions by distributing cache misses among processors in a way that minimized the total number of cache misses in any specific cache.

Hierarchical Compilation of Macro Dataflow Graphs for Multiprocessors with Local Memory

Tue, 01 Dec 1992 00:00:00 GMT

The Impact of Communication Locality on Large-scale Multiprocessor Performance

Sat, 01 Feb 1992 00:00:00 GMT

An Analysis of Rabin's Randomized Mutual Exclusion Algorithm: Preliminary Report

Sun, 01 Dec 1991 00:00:00 GMT

An Analysis of Rabin's Randomized Mutual Exclusion Algorithm: Preliminary Report Lynch, Nancy A.; Saias, Isaac In 1982, Michael Rabin published a randomized distributed algorithm implementing mutual exclusion for n processes using a read-modify-write primitive on a shared variable with O(log n) values. He claimed that this algorithm satisfied the following informally-stated strong probabilistic no-lockout property. Define the adversary to be the entity controlling the order in which processes take steps; then, for every adversary, any process competing for entrance to the critical section succeeds with probability Ω(1/m), where m is the number of competing processes. In this paper we consider several different ways in which this property can be expressed formally. We express explicitly the dependency of the probability on the adversary and show that this dependency is so strong that the algorithm does not satisfy any of these conditions. In fact, the algorithm does not even satisfya much weaker Ω(1/n) property.

Fast Deterministic Constructions of Low-Diameter Network Decompositions

Sun, 01 Dec 1991 00:00:00 GMT

Fast Deterministic Constructions of Low-Diameter Network Decompositions Berger, Bonnie; Cowen, Lenore

Linearizable Counting Networks

Fri, 01 Nov 1991 00:00:00 GMT

Linearizable Counting Networks Merlihy, Maurice; Shavit, Nir; Waarts, Orli The counting problem requires n asynchronous processors to assign themselves successive values. A solution is linearizable if the order of the values assigned reflects the real-time order in which they were requested. Linearizable counting lies at the heart of concurrent timestamp generation, as well as concurrent implementations of shared counters, FIFO buffers, and similar data structures. We consider solutions to the linearizable counting problem in a multiprocessor architecture in which processors communicate by applying read-modify-write operations to a shared memory. Linearizable counting algorithms can be judged by three criteria: the memory contention produced, whether processors are required to wait for one another, and how long it takes a processor to choose a value (the latency). A solution is ideal if it has low contention, low latency, and it eschews waiting. The conventional software solution, where processors synchornize at a single variable, avoids waiting and has low latency, but has high contention. In this paper we give two new counting network constructions, one with low latency and low contention, but that requires processors to wait for one another, and one with low contention and no waiting, but that has high latency. Finally, we prove that these trade-offs are inescapable: an ideal linearizable counting algorithm is impossible. Since ideal non-linearizable counting algorithms exist, these results establish a substantial complexity gap between linearizable and non-linearizable counting.

Forward and Backward Simulations for Timing-based Systems

Fri, 01 Nov 1991 00:00:00 GMT

Forward and Backward Simulations for Timing-based Systems Lynch, Nancy A.; Vaandrager, Frits A general automaton model for timing-based systems is presented and is used as the context for developing a variety of simulation proof techniques for such systems. As a first step, a comprehensive overview of simulation techniques for simple untimed automata is given. In particular, soundness and completeness results for (1) refinements, (2) forward and backward simulations, (3) forward-backward and backward-forward simulations, and (4) history and prophecy relations are given.

An Algorithm for the Tramp Steamer Problem Based on Mean-weight Cycles

Fri, 01 Nov 1991 00:00:00 GMT

An Algorithm for the Tramp Steamer Problem Based on Mean-weight Cycles Ishii, Alexander T.; Leiserson, Charles E.; Papaefthymiou, Marios C.

Replication in the Harp File System

Thu, 01 Aug 1991 00:00:00 GMT

Replication in the Harp File System Liskov, B.; Ghemawat, S.; Gruber, R.; Johnson, P.; Shrira, L.; Williams, M. This paper describes the design and implementation of the Harp file system. Harp is a replicated Unix file system accessible via the VFS interface. It provides highly available and reliable storage for files and guarantees that file operations are executed atomically in spite of concurrency and failures. It uses a novel variation of the primary copy replication technique that provides good performance because it allows us to trade disk accesses for network communication. Harp is intended to be used within a file service in a distributed network; in our current implementation, it is accessed via NFS. Preliminary performance results indicate that Harp provides equal or better response time and system capacity than an unreplicated implementation of NFS that uses Unix files directly.

A Fast Multiport Memory Based on Single-port Memory Cells

Mon, 01 Jul 1991 00:00:00 GMT

A Fast Multiport Memory Based on Single-port Memory Cells Rivest, Ronald L.; Glasser, L. We present a new design for dual-port memories that uses single-port memory cells but guarantees fast deterministic read/write access. The basic unit of storage is the word, rather than the bit, and addresses conflicts result in bit errors that are removed by correction circuitry. The addressing scheme uses Galois field arithmetic to guarantee that the maximum number of bit errors in any word accessed is one. These errors can be corrected every time with a simple correction scheme. The scheme can be generalized to an arbitrary number of ports.

The MIT Alewife Machine: A Large-scale Distributed-memory Multiprocessor

Sat, 01 Jun 1991 00:00:00 GMT

The MIT Alewife Machine: A Large-scale Distributed-memory Multiprocessor Agarwal, Anant; Chaiken, David; Johnson, Kirk; Kranz, David; Kubiatowicz, John; Kurihara, Kiyoshi; Lim, Beng-Hong; Maa, Gino; Nussbaum, Dan The Alewife multiprocessor project focuses on the architecture and design of a large-scale parallel machine. The machine uses a low dimension direct interconnection network to provide scalable communication band-width, while allowing the exploitation of locality. Despite its distributed memory architecture, Alewife allows efficient shared memory programming through a multilayered approach to locality management. A new scalable cache coherence scheme called LimitLess directories allows the use of caches for reducing communication latency and network bandwidth requirements.

Cost-sensitive Analysis of Communication Protocols

Sat, 01 Jun 1991 00:00:00 GMT

Cost-sensitive Analysis of Communication Protocols Awerbuch, Baruch; Baratz, Alan; Peleg, David This paper introduces the notion of cost-sensitive communication complexity and exemplifies it on the following basic communication problems: computing a global function, network synchornization, clock synchronization, controlling protocols' worst-case execution, connected components, spanning tree, etc., contructing a minimymn spanning tree, constructing a shortest path tree.

The Complexity of Continuous Optimization

Sat, 01 Jun 1991 00:00:00 GMT

The Complexity of Continuous Optimization Rogaway, Phillip Given a polynomial objective function f(x1,…,xn), we consider the problem of finding the maximum of this polynomial inside some convex set D = {x : Ax <= B}. We show that, under a complexity assumption, this extremum cannot be approximated by any polynomial-time algorithm, even exceedingly poorly. This represents an unusual interplay of discrete and continuous mathematics: using a combinatorial argument to get a hardness result for a continuous optimization problem.

Counting Networks

Sat, 01 Jun 1991 00:00:00 GMT

Counting Networks Aspnes, James; Herlihy, Maurice; Shavit, Nir Many fundamental multi-processor coordination problems can be expressed as counting problems: processes must cooperate to assign successive values from a given range, such as addresses in memory of destinations on an interconnection network. Conventional solutions to these problems perform poorly because of synchronization bottlenecks and high memory contention. Motivated by observations on the behavior of sorting networks, we offer a completely new approach to solving such problems. We introduce a new class of networks called counting networks, i.e., networks that can be used to count. We give two counting network constructions of depth log^2 n, using n log^2 n "gates," avoiding the sequential bottlenecks inherent to former solutions, and substantially lowering the memory contention. Finally, to show that counting networks are not merely mathematical creatueres, we provide experimental evidence that they outperform conventional synchronization techniques under a variety of circumstances.

APRIL: A Processor Architecture for Multiprocessing

Tue, 01 Jan 1991 00:00:00 GMT

APRIL: A Processor Architecture for Multiprocessing Agarwal, Anant; Lim, Beng-Hong; Kranz, David; Kubiatowicz, John Processors in large-scale multiprocessors must be able to tolerate large communication latencies and synchronization delays. This paper describes the architecture of a rapid-context-switching processor called APRIL with support for fine-grain threads and synchronization. APRIL achieves high single-thread performance and supports virtual dynamic threads. A commercial RISC-based implementation of APRIL and a run-time software system that can switch contexts in about 10 cycles is described. Measurements taken for several parallel applications on an APRIL simulator show that the overhead for supporting parallel tasks based on futures is reduced by a factor of two over a corresponding implementation on the Encore Multimax. The scalability of a multiprocessor based on APRIL is explored using a performance model. We show that the SPARC-based implementation of APRIL can achieve close to 80% processor utilization with as few as three resident threads per processor in a large-scale cache-based machine with an average base network latency of 55 cycles.

Lazy Task Creation: A Technique for Increasing the Granularity of Parallel Programs

Sat, 01 Jun 1991 00:00:00 GMT

Lazy Task Creation: A Technique for Increasing the Granularity of Parallel Programs Mohr, Eric; Kranz, David; Halstead; Robert H., Jr. Many parallel algorithms are naturally expressed at a fine level of granularity, often finer than MIMD parallel system can exploit efficiently. Most builders of parallel systems have looked to either the programmer or a parallelizing compiler to increase the granularity of such algorithms. In this paper we explore a third approach to the granularity problem by analyzing two strategies for combining parallel tasks dynamically at run-time.

Limitless Directories: A Scalable Cache Coherence Scheme

Sat, 01 Jun 1991 00:00:00 GMT

Limitless Directories: A Scalable Cache Coherence Scheme Chaiken, David; Kubiatowicz, John; Agarwal, Anant Caches enhance the performance of multiprocessors by reducing network traffic and average memory access latency. However, cache-based systems must address the problem of cache coherence. We propose the LimitLESS directory protocol to solve this problem. The LimitLESS scheme uses a combination of hardware and software techniques to realize the performance of full-map directory with the memory overhead of limited directory. This protocol is supported by Alewife, a large-scale multiprocessor. We describe the architectural interfaces needed to implement the LimitLESS directory, and evaluate its performance though simulations of the Alewife machine.

Reliable Communication Over Unreliable Channels

Thu, 01 Oct 1992 00:00:00 GMT

Reliable Communication Over Unreliable Channels Afek, Yehuda; Attiya, Hagit; Fekete, Alan; Fischer, Michael; Lynch, Nancy A.; Mansour, Yishay; Wang, Da-Wei; Zuck, Lenore Layered communication protocols frequently implement a FIFO message facility on top of an unreliable non-FIFO service such as that provided by a packet-switching network. This paper investigates the possibility of implementing a reliable message layer on top of an underlying layer that can lose packets and deliver them out of order, with the additional restriction that the implementation uses only a fixed finite number of different packets. A new formalism is presented to specify communication layers and their properties, the notion of their implementation by I/O automata, and the properties of such implementations. An I/O automaton that implements a reliable layer over an unreliable layer is presented. In this implementation, the number of packets needed to deliver each succeeding message increases permanently as additional packet-loss and reordering faults occur. A proof is given that no protocol can avoid such performance degradation.

An Autoimmune Mechanism for AIDS' T4 Lymphopenia

Mon, 01 Apr 1991 00:00:00 GMT

The Complexity of Decision Versus Search

Mon, 01 Apr 1991 00:00:00 GMT

The Complexity of Decision Versus Search Bellare, Mihir; Goldwasser, Shafi A basic question about NP is whether or not search (the problem of finding a witness) reduces in polynomial time to decision ( the problem deciding whether there exists a witness). The fact that search does reduce to decision for SAT and other NP-complete problems (self-reducibility) is among the most well known facts in the theory of computation. But the general question of whether search reduces to decision for every language in NP remains open. We indicate that the answer is negative: under a natural complexity assumption (that deterministic and non deterministic double exponential time are unequal) we construct a language in NP for which search does not reduce to decision.

An Autoimmune Mechanism for AIDS' T4 Lymphopenia

Fri, 01 Mar 1991 00:00:00 GMT

Are Wait-free Algorithms Fast?

Fri, 01 Mar 1991 00:00:00 GMT

Are Wait-free Algorithms Fast? Attiya, Hagit; Lynch, Nancy A.; Shavit, Nir The time complexity of wait-free algorithms in "normal" executions, where no failures occure and processes operate at approximately the same speed, is considered. A lower bound of log n on the time complexity of any wait-free algorithm that achieves approximate agreements among n processes is proved. In contrast, there exists a non-wait-free algorithm that solves this problem in constant time. This implies an Ω(log n) time separation between the wait-free and non-wait-free computation models. On the positive side, we present an O(log n) time wait-free approximate agreement algorithm; the complexity of this algorithm is within a small constant of the lower bound.

On-line Algorithms for 2-coloring Hypergraphs via Chip Games

Sat, 01 Dec 1990 00:00:00 GMT

On-line Algorithms for 2-coloring Hypergraphs via Chip Games Aslam, Javed A.; Dhagat, Aditi

On the Massively Parallel Solution of The Assignment Problem

Sat, 01 Dec 1990 00:00:00 GMT

On the Massively Parallel Solution of The Assignment Problem Wein, Joel; Zenios, Stavros In this paper we discuss the design, implementation and effectiveness of massively parallel algorithms for the solution of large-scale assignment problems. In particular, we study the auction algorithm of Bertsekas, an algorithm based on the method of multipliers of Hestenes and Powell, and an algorithm based on the alternating direction method of multipliers of Eckstein. We discuss alternative approaches to the massively parallel implementation of the auction algorithm, including Jacobi, Gauss-Seidel and a hybrid scheme. The hybrid scheme, in particular, exploits two different levels of parallelism and an efficient way of communicating the data between them without the need to perform general router operations across the hypercube network. We then study the performance of massively parallel implementations of the two methods of multipliers. Implementations are carried out on the Connection Machine CM-2, and the algorithms are evaluated empirically with the solution of large scale problems. The hybrid scheme significantly outperforms all of the other methods and gives the best computational results to date for a massively parallel solution to this problem.

On-line Scheduling of Parallel Machines

Thu, 01 Nov 1990 00:00:00 GMT

On-line Scheduling of Parallel Machines Wein, Joel; Williamson, David P. We study the problem of scheduling jobs on parallel machines in an on-line fashion, where the processing requirement of a job is not known until the job is completed. Despite this lack of knowledge of the future, we wish to schedule so as to minimize the completion time of the entire set of jobs. In general, the performance of an on-line algorithm is measured by its competitive ratio: the worst case ratio of its performance of an optimal algorithm with total prior knowledge. We study two fundamental models for this problem, that of identical machines, where all the machines run at the same speed, and uniformaly related machines, where the machines run at different speeds. Our results include: 1) Matching upper and lower bounds on the competitive ratio for the case of identical machines. 2) Upper and lower bounds that differ by a constant factor for uniformly related machines. 3) A lower bound for randomized algorithms for identical machines that nearly matches the deterministic upper bound. 4) Several upper and lower bounds for variations on these models.

Bounds on the Time to Reach Agreement in the Presence of Timing Uncertainty

Thu, 01 Nov 1990 00:00:00 GMT

Bounds on the Time to Reach Agreement in the Presence of Timing Uncertainty Attiya, Hagit; Dwork, Cynthia; Lynch, Nancy A.; Stockmeyer, Larry

The MD4 Message Digest Algorithm

Mon, 01 Oct 1990 00:00:00 GMT

The MD4 Message Digest Algorithm Rivest, Ronald L. The MD4 message digest algorithm takes an input message of arbitrary length and produces an output 128-bit "fingerprint" or "message digest," in such a way that it is (hopefully) computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD4 algorithm is thus ideal for digital signature applications: a large file can be securely "compressed" with MD4 before being signed with (say) the RSA public-key cyrptosystem. The MD4 algorithm is designed to be quite fast on 32-bit machines. For example, on a SUN Sparc station, MD4 runs at 1,450,000 bytes/second (11.6 Mbit/sec). In addition, the MD4 algorithm does not require any large substitution tables; the algorithm can be coded quite compactly. The MD4 algorithm is being place in the public domain for review and possible adoption as a standard.

Randomness-efficient Sampling of Arbitrary Functions

Sun, 01 Jul 1990 00:00:00 GMT

Randomness-efficient Sampling of Arbitrary Functions Bellare, Mihir; Rompel, John

How to Sign Given Any Trapdoor Permutation

Fri, 01 Jun 1990 00:00:00 GMT

How to Sign Given Any Trapdoor Permutation Bellare, Mihir; Micali, Silvio We present a digital signature scheme which is based on the existence of any trapdoor permutation. Our scheme is secure in the strongest possible natural sense: namely, it is secure against existential forgery under adaptive chosen message attack.

Atomic Snapshots of Shared Memory

Tue, 01 May 1990 00:00:00 GMT

Atomic Snapshots of Shared Memory Afek, Yehuda; Attiya, Hagit; Dolev, Danny; Gafni, Eli; Merritt, Michael; Shavit, Nir

Modelling Shared State in a Shared Action Model

Thu, 01 Mar 1990 00:00:00 GMT

Modelling Shared State in a Shared Action Model Goldman, Kenneth; Lynch, Nancy A.

Non-ontrusive Synchonizers

Sun, 01 Apr 1990 00:00:00 GMT

Non-ontrusive Synchonizers Awerbuch, Baruch; Peleg, David

Workstation Services and Kerberos Authentication at Project Athena

Wed, 01 Mar 1989 00:00:00 GMT

Workstation Services and Kerberos Authentication at Project Athena Davis, Don; Swick, Ralph This document proposes solutions for two problems obstructing Project Athena's implementation of workstation services. The principal problem is that workstation services demand a more flexible mutual-authentication protocol than Kerberos currently provides. The egregious X access-control hack, xhost, for example, has lack of authentication as its root cause. The protocol weakness is also the reason that public workstations can't accept authenticated connections from rlogin, rcp, rsh, etc. We propose an extension to the Kerberos Ticket Granting Service protocol, that cleanly supports user-to-user mutual authentication. Our second proposal addresses the problem of ticket propagation. Currently, if a user wants tickets that are valid on a remote host, he has to run kinit an encrypted login session, unless he's willing to send his password in cleartext. As an example of the use of our protocol extension, we describe a Kerberos application that would support a limited facility for secure ticket-propagation.

Sharing Memory Robustly in Message-passing Systems

Thu, 01 Feb 1990 00:00:00 GMT

Sharing Memory Robustly in Message-passing Systems Attiya, Hagit; Bar-Noy, Amotz; Dolev, Danny

Multivalued Possibilities Mappings

Wed, 01 Aug 1990 00:00:00 GMT

Multivalued Possibilities Mappings Lynch, Nancy A. Abrastraction mappings are one of the major tools used to construct correctness proofs for concurrent algorithms. Several examples are given of situations in which it is useful to allow the abstraction mappings to be multivalued. The examples involve algorithm optimization, algorithm distribution, and proofs of time bounds.

Stochastic Analysis of Qualitative Dynamics

Fri, 01 Dec 1989 00:00:00 GMT

Stochastic Analysis of Qualitative Dynamics Doyle, Jon; Sacks, Elisha P.

Synthesis of Efficient Drinking Philosphers Algorithms

Wed, 01 Nov 1989 00:00:00 GMT

Synthesis of Efficient Drinking Philosphers Algorithms Welch, Jennifer Lundelius; Lynch, Nancy A. A variant of the drinking philosphers algorithm of Chandy and Misra is described and proved correct in a module way, using the I/O automaton model of Lynch and Tuttle. The algorithm of Chandy and Misra is based on an particular dining philosophers algorithm, and relies on certain properties of its implementation. The drinking philosophers algorithm presented in this paper is able to use an arbitrary dining philosophers algorithm as a true subroutine; nothing about the implementation needs to be known, only that is solves the dining philosophers problem. An important advantage of this modularity is that by substituting a more time-efficient dining philosophers algorithm with O(1) worst-case waiting time is obtained, whereas the drinking philosophers algorithm of Chandy and Misra has O(n) worst-case waiting time (for n philosophers). Formal definitions are given to distinguish the drinking and dining philosophers problems and to specify precisely varying degrees of concurrency.

Impediments to Universal Preference-based Default Theories

Sun, 01 Oct 1989 00:00:00 GMT

Impediments to Universal Preference-based Default Theories Doyle, Jon; Wellman, Michael Research on nonmonotonic and default reasoning has identified several important criteria for preferring alternative default inferences. The theories of reasoning based on each of these criteria may uniformly be viewed as theories of rational inference, in which the reasoner selects maximally preferred states of belief. Though researchers have noted some cases of apparent conflict between the preferences supported by different theories, it has been hoped that these special theories of reasoning may be combined into a universal logic of nonmonotonic reasoning. We show that the different categories of preferences conflict more than has been realized, and adapt formal results from social choice theory to prove that every universal theory of default reasoning will violate at least one reasonable principle of rational reasoning. Our results can be interpreted as demonstrating that, within the preferential framework, we cannot expect much improvement on the rigid lexicographic priority mechanisms that have been proposed for conflict resolution.

Routing with Polynomial Communication-space Tradeoff

Fri, 01 Sep 1989 00:00:00 GMT

Routing with Polynomial Communication-space Tradeoff Awercuch, Baruch; Peleg, David

Online Tracking of Mobile Users

Tue, 01 Aug 1989 00:00:00 GMT

Online Tracking of Mobile Users Awercuch, Baruch; Peleg, David This paper deals with the problem of maintaining a distributed directory server, that enables us to keep track of mobile users in a distributed network. The paper introduces the graph-theoretic concept of regional matching, and demonstrates how fining a regional matching with certain parameters enables efficient tracking. A polynomial-time algorithm that constructs such a regional matching is presented. The communication overhead of our tracking mechanism is within a polylogarithmic factor of the lower bound.

Nnuclear Fusion Through Dimensional Confinement

Tue, 01 Aug 1989 00:00:00 GMT

Nnuclear Fusion Through Dimensional Confinement Smith, Mark A. A formal mechanism for enhancing nnuclear fusion rates is proposed. The enhancement results whenever the reacting nuclei preferentially migrate in a restricted subspace of phase space - in particular, a fractal subspace. An extended Lawsom criterion is derived, and the prospects for this mechanism in condensed matter are discussed.

Theory of Computation Group Research Summary June 1988 - July 1989

Sat, 01 Jul 1989 00:00:00 GMT

Theory of Computation Group Research Summary June 1988 - July 1989 Theory of Computation Group

Time Bounds for Real-time Process Control in the Presence of Time Uncertainty

Sat, 01 Jul 1989 00:00:00 GMT

Time Bounds for Real-time Process Control in the Presence of Time Uncertainty Attiya, Hagit; Lynch, Nancy A.

A Hundred Impossibility Proofs for Distributed Computing

Tue, 01 Aug 1989 00:00:00 GMT

A Hundred Impossibility Proofs for Distributed Computing Lynch, Nancy A.

Type Abstraction Rules for References: A Comparison of Four Which have Achieved Notoriety

Tue, 01 Aug 1989 00:00:00 GMT

Type Abstraction Rules for References: A Comparison of Four Which have Achieved Notoriety O'Toole Jr., James William I present four type abstraction rules which have been introduced by various authors to permit polymorphic type safety in the presence of mutable data. each of the type abstraction rules is discussed in the context of the language in which is was introduced, and the various abstraction rules are compared.

Three Methods for Range Queries in Computational Geometry

Wed, 01 Mar 1989 00:00:00 GMT

Three Methods for Range Queries in Computational Geometry Kipnis, Shlomo This paper surveys a variety of recent results addressing the problem of range queries in computational geometry. The major contribution of this paper is in identifying three general methods for range queries in computational geometry and in classifying many of the recent results into one or more of these approaches. The three methods discussed in this paper are random sampling, search-tree tables, and space-partition trees. This survey assumes some familiarity with basic computational geometry concepts and techniques.

Communication Effects for Message-based Concurrency

Wed, 01 Feb 1989 00:00:00 GMT

Communication Effects for Message-based Concurrency Jouvelot, Pierre; Gifford, David K. We describe a new framework for explicity concurrency that uses an effect system to describe the communication behavior of expressions in a typed polymorphic programming language. Concurrency occurs between processes connected by channels on which messages are transmitted. Communication operations are characterized by two communication effect constructors, out and in, depending on whether a message has been sent or received. Synchronization is only allowed by message passing along shared channels; communication via mutation of global variables is staticially prohibited by our communication effect system, thus restricting the amount of non-determinancy in user programs. Unobservable communication effects can be masked by the effect system. We show that this system is powerful enough to express many other parallel paradigms, like systolic arrays or pipes, in a typed framework. The programmer can thus express concurrency in a rather flexible way while preserving the correctness of implicit detection of parallelism and optimization by the compiler. This new concurrency framework has been implemented in the FX-87 programming language.

Natural Random Numbers

Thu, 01 Sep 1988 00:00:00 GMT

Natural Random Numbers Gofford, David K. We present a method for generaing random numbers from natural noise sources that is able to produce random numbers to any desired level of perfection. The method works by transducing a physical noise source to generate a stream of biased natural bits, and then applying an unbiasing algorithm. The Wiener-Kinchine relation is used to derive the autocorrelation present in the stream of biased bits and to define safe sampling rate. Experimental results from an implementation of our method support our analysis. One consequence of our analysis is that a broad class of natural random number generators, including ours, can not generate absolutely perfect random numbers.

A Lattice-structured Proof Technique Applied to a Minimum Spanning Tree Algorithm

Wed, 01 Jun 1988 00:00:00 GMT

A Lattice-structured Proof Technique Applied to a Minimum Spanning Tree Algorithm Welch, Jennifer Lundelius; Lamport, Leslie; Lynch, Nancy A. Higly-optimized concurrent algorithms are often hard to prove correct because they have no natural decomposition into separately provable parts. This paper presents a proof technique for the modular verification of such non-modular algorithms. It generalizes existing verification techniques based on a totally-ordered hierarchy of refinements to allow a partially-ordered hierarchy - that is, a lattice of different views of the algorithm. The technique is applied to the well-known distributed minimum spanning tree algorithm of Gallager, Humblet, and Spira, which has until recently lacked a rigorous proof.

Combinatorial Algorithms for the Generalized Circulation Problem

Sun, 01 May 1988 00:00:00 GMT

Combinatorial Algorithms for the Generalized Circulation Problem Goldberg, Andrew V.; Plotkin, Serge A.; Tardos, Eva We consider a generalization of the maximum flow problem in which the amounts of flow entering and leaving an arc are linearly related. More precisely, if x(e) units of flow enter an arc e, x(e) ?(e) units arrive at the other end. For instance, nodes of the graph can correspond to different currencies, with the multipliers being the exchange rates. We require conservation of flow at every node except a given source node. The goal is to maximize the amount of flow excess at the source. This problem is a special case of linear programming, and therefore can be solved in polynomial time. In this paper we present the first polynomial time combinatorial algorithms for this problem. The algorithms are simple and intuitive.

Sublinear-time Parallel Algorithms for Matching and Related Problems

Wed, 01 Jun 1988 00:00:00 GMT

Sublinear-time Parallel Algorithms for Matching and Related Problems Goldberg, Andrew V.; Plotkin, Serge A.; Vaidya, Pravin This paper presents the first sublinear-time deterministic parallel algorithms for bipartite matching and several related problems, including maximal node-disjoint paths, depth-first search, and flows in zero-one networks. Our results are based on a better understanding of the combinatorial structure of the above problems, which leads to new algorithmic techniques. In particular, we show how to use maximal matching to extend, in parallel, a current set of node-disjoint paths and how to take advantage of the parallelism that aries when a large number of nodes are "active" during an execution of a push/relabel network flow algorithm. We also show how to apply our techniques to design parallel algorithms for the weighted versions of the above problems. In particular, we present sublinear-time deterministic parallel algorithms for finding a minimum-weight bipartite matching and for finding a minimum-cost flow in a network with zero-one capacities, if the weights are polynomially bounded integers.

Semantical Paradigms: Notes for an Invited Lecture

Fri, 01 Jul 1988 00:00:00 GMT

Semantical Paradigms: Notes for an Invited Lecture Meyer, Albert R.; Cosmadakis, Stavros S. It tooke me quite a few years to understand the point of the continuity in denotational semantics. I'm happy to report below on some recent results which justify my muddle-headedness and help to explain the point too. What follows are some global comments on denotational semantics of teh kinds invited lecturers sometimes indulge themselves in, highlighting "goodness of fit" criteria between semantic domains and symbolic evaluators. For readers impatient with sketchy overviews, two appendices mostly by Cosmadakis provide the key parts of a long proof that Scott domains give a computationally adequate and fully abstract semantics for lambda calculus with simple recursive types.

I/O Automata: A Model for Discrete Event Systems

Tue, 01 Mar 1988 00:00:00 GMT

I/O Automata: A Model for Discrete Event Systems Lynch, Nancy A.

A Modular Proof of Correctness for a Network Synchronizer

Tue, 01 Sep 1987 00:00:00 GMT

A Modular Proof of Correctness for a Network Synchronizer Fekete, A.; Lynch, N.; Shrira, L. In this paper we offer a formal, rigorous proof of the correctness of Awerbuch's algorithm for network synchronization. We specify both the algorithm and the correctness condition using the I/O automaton model, which has previously been used to describe and verify algorithms for concurrency control and resource allocation. We show that the model is also a powerful tool for reasoning about distributed graph algorithmss. Our prood of correctness follows closely the intuitive arguments made by the designer of the algorithm by exploiting the model's natural support for such important design techniques as stepwise refinement and modularity. In particular, since the algorithm uses simpler algorithms for synchronization within and between "clusters" of nodes, our prood can import as lemmas the correctness of these simpler algorithms.

Inferring Decision Trees Using the Minimum Description Length Principle

Tue, 01 Sep 1987 00:00:00 GMT

Inferring Decision Trees Using the Minimum Description Length Principle Quinlan, L. Ross; Rivest, Ronald L. We explore the use of Rissanen's Minimum Description Length Principle for the construction of decision trees. Empirical results comparing this approach to other methods are given.

Lower Bounds for Recognizing Small Cliques on CRCW PRAM's

Sat, 01 Aug 1987 00:00:00 GMT

Lower Bounds for Recognizing Small Cliques on CRCW PRAM's Beame, Paul

The Semantics of Miranda's Algebraic Types

Sat, 01 Aug 1987 00:00:00 GMT

The Semantics of Miranda's Algebraic Types Bruce, Kim B.; Riecker, Jon G. Miranda has two interesting features in its typing system: implicit polymorphism (also known as ML-style polymorphism) and algebraic types. Algebraic types create new types from old and can operate on arbitrary types. This paper argues that functions of types, or type constructors, best represent the meaning of algebraic types. Building upon this idea, we develop a denotational semantics for algebraic types. We first define a typed lambda calculus that specifies type constructors. A semantic model of type constructors is them built, using the ideal model as a basis. (The ideal model gives the most natural semantics for Miranda's implicit polymorphism.) The model is shown to be sound with respect to this lambda calculus. FInally, we demonstrate how to use the model to interpret algebraic types, and prove that the translation produces elements in the model.

Finding Minimum-cost Circulations by Canceling Negative Cycles

Wed, 01 Jul 1987 00:00:00 GMT

Finding Minimum-cost Circulations by Canceling Negative Cycles Goldberg, Andrew V.; Tarjan, Robert E. A classical algorithm for finding a minimum-cost circultaion consists of repeatedly finding a residual cycle of negative cost and canceling it by pushing enough flow around the cycle to saturate an arc. We show that a judicious choice of cycles for canceling leads to a polynomial bound on the number of iterations in this algorithm. This gives a very simple strongly polynomial algorithm that uses no scaling. A variant of the algorithm that uses dynamic trees runs in O(nm(log n)min{log(nC),mlogn}) time on a network of n verticies, m arcs, and arc costs of maximum absolute value C. This bound is comparable to those of the fastest previously known algorithms.

Finding Minimum-cost Circulations by Successive Approximation

Wed, 01 Jul 1987 00:00:00 GMT

Finding Minimum-cost Circulations by Successive Approximation Goldberg, Andrew V.; Tarjan, Robert E.

Formulation of Tradeoffs in Planning Under Uncertainty

Mon, 01 Jun 1987 00:00:00 GMT

Formulation of Tradeoffs in Planning Under Uncertainty Wellman, Michael P. Planning under uncertainty with multiple, competing objectives is impossible when goals are represented as predicates and the effects of actions are modeled as deterministic functions of situations. Decision-theoretic models, on the other hand, do not address the problem of constructing strategies from more primitive representations of actions. In this proposal, I describe a method for formulating plans from large knowledge bases that can accomodate uncertain and partial satisifaction of goals. At the core of the planner is a dominance prover that derives admissibility properties of plan classes. The representation for the effects of actions is based on a qualitative formalism for asserting influences among variables. The planner makes decisions "up to tradeoffs," an intuitive description that seems to characterize the power of a dominance prover based on the qualitative influence formalism.

Controlling Worst-case Performance of a Communication Protocol and Dynamic Resource Management

Fri, 01 May 1987 00:00:00 GMT

Controlling Worst-case Performance of a Communication Protocol and Dynamic Resource Management Awerbuch, Baruch This paper raises a fundamental questions, neglected so far in the literature: how to make a distributed algorithm robust against input errors and wrong probabilistic assumptions about the distribution of the inputs or of the link delays. We introduce a notion of complexity-preserving protocol controller: this is an automatic procedure that controls worst-case execution of any distributed algorithm. We then suggest a controlled with poly-logarithmic overhead. We show that the problem of designing controllers is a special case of another problem, referred to as dynamic resource management. We generalize our solution to solve the latter problem. We believe that the techniques used are basic ones, and will be used to solve a variety of unrelated network problems. Our solution seems to be very practical, since the formal code of the protocol is very simple and thus easy to implement. The technique used in the solution appears to be interesting because a global resource is manipulated locally. This somewhat resembles the "parallel prefix" technique used extensively in parallel computing.

A Space-efficient Algorithm for Finding the Connected Components of Rectangles in the Plane

Sun, 01 Feb 1987 00:00:00 GMT

A Space-efficient Algorithm for Finding the Connected Components of Rectangles in the Plane Leiserson, Charles E.; Phillips, Cynthia A. We present an algorithm for determining the connectivity of a set of N rectangles in the plane, a problem central to avoiding aliasing in VLSI design rule checkers. Previous algorithms for this problem either worked slowly with a small amount of primary memory space, or worked quickly but used more space. Our algorithm uses O(W) primary memory space, where W, the scan width, is the maximum number of rectangles to cross any vertical cut. The algorithm runs in O(N lg N) time and requires no more than O(N) transfers between primary and secondary memory.

Efficient Multichip Partial Concentrator Switches

Sun, 01 Feb 1987 00:00:00 GMT

Efficient Multichip Partial Concentrator Switches Cormen, Thomas H.

Efficient Parallel Algorithms for (_+1)-coloring and Maximal Indepdendent Set Problems

Thu, 01 Jan 1987 00:00:00 GMT

Efficient Parallel Algorithms for (_+1)-coloring and Maximal Indepdendent Set Problems Goldberg, Andrew V.; Plotkin, Serge A. We describe an efficient technique for breaking symmetry in paralle. The technique works especially well on rooted trees and on graphs with a small maximum degree. In particular, we can find a maximal independent set on a constant-degree graph in O(lg*n) time on an EREW PRAM using a linear number of processors. We show how to apply this technique to construct more efficient paralle algorithms for several problems, including coloring of planar graphs and (Δ+1)-coloring of constant-degree graphs. We also prove lower bounds for two related problems.

Murmur Clinic: An Auscultation Expert System

Thu, 01 Jan 1987 00:00:00 GMT

Murmur Clinic: An Auscultation Expert System Leong, Tze-Yun Auscultation is a technique used in cardiac physical examination to detect irregularities by analyzing heart sounds. This paper reports on the development of Murmur Clinic, a cardiac auscultation expert system which is able to interpret and analyze auscultatory findings, and performs a tentative diagnosis based on a formalized diagnostic reasoning process. Descriptions of the scope addressed, the design, the diagnostic algorithm used and implementation of the system, as well as a sample session, and a discussion of limitations and possible improvements are presented.

Communication-efficient Parallel Graph Algorithms

Mon, 01 Dec 1986 00:00:00 GMT

Communication-efficient Parallel Graph Algorithms Leiserson, Charles E.; Maggs, Bruce M. Communication bandwidth is a resource ignored by most parallel random-access machine (PRAM) models. This paper shows that many graph problems can be solved in parallel, not only with polylogarithmic performance, but with efficient communication at each step of the computation. We measure the communication requirements of an algorithm in a model called the distributed random-access machine (DRAM), in whcih communication cost is measured in terms of the congestion of memory access across cuts of an underlying network. The algorithms are based on a communication-efficient variant of the tree contraction technique due to Miller and Reif.

Cellular Automata '86 Conference

Mon, 01 Dec 1986 00:00:00 GMT

Cellular Automata '86 Conference Bennett, Charles H.; Toffoli, Tommaso; Wolfram, Stephen

Data Sharing in Group Work

Wed, 01 Oct 1986 00:00:00 GMT

Data Sharing in Group Work Greif Irene; Sarin, Sunil Data sharing is fundamental to computer-supported cooperative work: people share information through explicit communication channels and through their coordinated use of shared databases. Database support tools are therefore critical to the effective implementation of software for group work. This paper survey data sharing requirements for grouop work, highlight new database technologies that are especially likely to affect our ability to build computer systems supporting group work.

Atomic Shared Register Access by Asynchronous Harward

Wed, 01 Oct 1986 00:00:00 GMT

Atomic Shared Register Access by Asynchronous Harward Vitányi, Paul M.B.; Awerbuch, Baruch The contribution of this paper is two-fold. First, we describe two ways to construct multivalued atomic n-writer n-reader registers. The first solution uses atomic 1-write 1-reader registers and unbounded tags. The other solution uses atomic 1-write n-reader registers and bounded tags. The second part of the paper develops a general methodology to porve atomicity, by identifying a set of criteria which guaranty an effective construction for the required atomic mapping. We apply the method to prove atomicity of the two implementations for atomic multiwriter multireader registers.

Theory of Computation Group Research Summary June 1985 - July 1986

Fri, 01 Aug 1986 00:00:00 GMT

Theory of Computation Group Research Summary June 1985 - July 1986 Theory of Computation Group

Hierarchical Inequality Reasoning

Sun, 01 Feb 1987 00:00:00 GMT

Hierarchical Inequality Reasoning Sacks, Elisha P. This paper describes a program called BOUNDER that proves inequalities between elementary functions over finite sets of constraints. Previous inequality algorithms perform well on some subset of the elementary functions, but poorly elsewhere. Although complex algorithms perform better than simple ones for most functions, exceptions exist. To overcome these problems, BOUNDER maintains a hierarchy of increasingly complex algorithms. When on fails to resolve an inequality, it tries the next. This strategy resolves more inequalities than any single algorithm. It also performs well on hard problems without wasting time on easier ones. The current hierarchy consists of four algorithms: bounds propogation, substitution, derivative inspection, and iterative approximation. Propogation is an extension of interval arithmetic that takes linear time, but ignores constraints between variables and multiple occurences of variables. The remaining algorithms consider these factors, but require exponential time. Substitution is a new, provably correct, algorithm for utilizing constraints between variables. An earlier attempt by Brooks does not terminate on all inputs and exploits fewer constraints. The final two algorithms analyze constraints between variables. Inspection examines the signs of partial derivatives. Iteration is based on several earlier algorithms from interval arithmetic.

Game Tree Searching by Min/Max Approximation

Mon, 01 Sep 1986 00:00:00 GMT

Game Tree Searching by Min/Max Approximation Rivest, Ronald L.

An Artificial Intelligence Approach to Clinical Decision Making

Mon, 01 Sep 1986 00:00:00 GMT

An Artificial Intelligence Approach to Clinical Decision Making Szolovits, Peter; Kassirer, Jerome P.; Long, William J.; Moskowitz, Alan J.; Pauker, Stephen G.; Patil, Ramesh S.; Wellman, Michael P. This memo is the text of a proposal from the MIT Laboratory for Computer Science Clinical Decision Making group to the National Library of Medicine, requesting support for a five-year program of research.

Retiming Synchronous Circuitry

Thu, 01 May 1986 00:00:00 GMT

Retiming Synchronous Circuitry Leiserson, Charles E.; Saxe, James B. This paper shows how the technique of retiming can be used to transform a given sycnhronous circuit into a more efficient circuit under a variety of different cost criteria. We model a circuit as a graph, and we give an O(|V||E|log|V|) algorithm for determining an equivalent circuit with the smallest possible clock period. We show that the problem of determining an equivalent retimed circuit with minimum state (total number of registers) is polynomial-time solvable. This result yields a polynomimal-time optimal solution to the problem of pipelining combinatorial circuitry with minimum register cost. We also give a characterization of optimal retiming based on an efficiently solvable mixed-integer linear programming problem.

Floyd-Hoare Logic Defines Semantics

Thu, 01 May 1986 00:00:00 GMT

Floyd-Hoare Logic Defines Semantics Meyer, Albert R. The first-order patrial correctness assertions provable in Floyd-Hoare logic about an uninterpreted while-program scheme determine the scheme up to equivalence. This settles an open problem of Meyer and Halpern. The simple proof of this fact carries over to other partial correctness axiomatizations given in the literature for wider classes of ALGOL-like program schemes.

Randomized Routing on Fat-trees

Thu, 01 May 1986 00:00:00 GMT

Randomized Routing on Fat-trees Greenberg, Ronald I.; Leiserson, Charles E. Fat-trees are a class of routing networks for hardware-efficient paralle computation. This paper presents a randomized algorithm for routing messages on a fat-tree. The quality of the algorithm is measured in terms of the load factor of a set of messages to be routed, which is a lower bound on the time required to deliver the messages. We show that if a set of messages has load factor lambda on a fat-tree with n processors, the number of delivery cyles (routing attempts) that the algorithm requires is O(lambda + lg n lg lg n) with probability 1-O(1/n). The best previous bound was )(lambda lg n) for the off-line problem where switch settings can be determined in advance. In a VLSI-like model where hardware cost is equated with physical volume, the routing algorithm demonstrates that fat-trees are universal routing networks in the sense that any routing network can be efficiently simulated by a fat-tree of comparable hardward cost.

Nonsequential Computation and Laws of Nature

Thu, 01 May 1986 00:00:00 GMT

Nonsequential Computation and Laws of Nature Vitányi, Paul M.B. Traditionally, computational complexity theory deals with sequential computations. In the computational models the underlying physics is hardly accounted for. This attitude has persisted in common models for parallel computations. Wrongly, we shall argue, since the laws of physic intrude forcefully when we want to obtain realistic estimates of the performance of paralle or distributed algorithms. First, we shall explain why it is reasonable to abstract away from the physical details in sequential computations. Second, we show why certain common approaches in the theory of paralle complexity do not give useful information about the actual complexity of the parallel computation. Third, we give some examples of the interplay between physical considerations and actual complexity of distributed computations.

Representing Change

Thu, 01 May 1986 00:00:00 GMT

Representing Change Sacks, Elisha This paper evaluates knowledge representations for time-dependent information. It compares recent work by Moore, McDermott, and Allen with an ealier proposal by McCarthy and Hayes. Moore's formalism is faulted for its needless and unmotivated complexity and a simpler alternative is outlined. McDermott's formalism is proved inconsistent and unintuitive. Allen achieves the most by attempting the least. He proposes a simple plausible formalism, which makes few ontological or computational commitments. The paper concludes with a high-level discussion of the merits formal logic as a representation for empirical knowledge.

Distributed Control in Computer Networks and Cross-sections of Colored Multidimensional Bodies

Tue, 01 Apr 1986 00:00:00 GMT

Distributed Control in Computer Networks and Cross-sections of Colored Multidimensional Bodies Kranakis, Evangelos; Vitanti, Paul M.B. The number of messages to match a pair of processes in a multiprocessor network with mobile processes is a measure for the cost of setting up temporary communication between processes. We establish lower bounds on the average number of point-to-point transmissions between any pair of nodes in this context. The present analysis allows for the possibility of multiple transmissions (as opposed to a single one) between any two nodes, and also for the possibility of multiple queries (as opposed to the two, i.e. post and a single query considered before). Applications of the results include lower bounds on the number of messages for distributed s-matching, that is, matching a group of s processes, and distributed s-mutual exclusion, that is, s-1 processes may enter a critical section simultaneously, but s process may not, for >=2. The idea of the proof of the combinatorial result needed for this analysis is further extended to obtain a lower bound on the average number of colors occuring in random cross-sections of colored, multidimensional bodies in terms of the total (multidimensional) volume of each color in the whole body.

The Power of the Queue

Tue, 01 Apr 1986 00:00:00 GMT

The Power of the Queue Li, Ming; Longpre, Luc; Vitányi, Paul M.B. Queues, stacks (pushdown stores), and tapes are storage models which have direct applications in compiler design and the general desig of algorithms. Whereas stacks (pushdown store or last-in-first-out storage) have been thoroughly investigated and are well understood, this is much less the case for queues (first-in-first-out storage). This paper contains a comprehensive study comparing queues to stacks and tapes. We address off-line machines with a one-way input, both deterministic and nondeterministic. The techniques relly on algorithmic information theory (Kolmogorov Complexity).

A Survey of Algorithms for Integrating Wafer-scale Systolic Arrays

Thu, 01 May 1986 00:00:00 GMT

A Survey of Algorithms for Integrating Wafer-scale Systolic Arrays Leighton, Tom; Leiserson, Charles VLSI technologists are fast developing wafer-scale integration. Rather than partitioning a silicon wafer into chips as is usually done, the idea behind wafer-scale integration is to assemble an entire system (or network of chips) on a single wafer, thus avoiding the costs and performance loss associated with individual packaging of chips. A major problem with assembling a large system of microprocessors on a single wafer, however, is that some of the processor, or cells, on the wafer are likely to be defective. In the paper, we describe practical procedures for integrating wafer-scale systems "around" such faults. The procedures are designed to minimize the length of the longest wire in the system, thus minimizing the communication time between cells. Although the underlying network problems are NP-complete, we prove that the procedures are reliable by assuming a probabilistic model of cell failure.

Interval and Recency-rank Source Coding: Two On-line Adaptive Variable-length Schemes

Tue, 01 Apr 1986 00:00:00 GMT

Interval and Recency-rank Source Coding: Two On-line Adaptive Variable-length Schemes Elias, Peter In these schemes the encoder maps each message into a codeword in a prefix-free codeword set. In interval encoding the codeword is indexed by the interval since the last previous occurrence of that message, and the codeword set must be countably infinite. In recency rank encoding the codeword is indexed by the number of distinct messages in that interval, and there must be no fewer codewords than messages. The decoder decodes each codewords on receipt. Users need not know message probabilities but must agree on indexings, of the codeword set in an order of increasing length and of the message set in some arbitrary order. The average codeword length over a communications bout is never much larger than the value for an off-line scheme which maps the jth most frequent message in the bout into the jth shortest codeword in the give set, and is never too much larger than the value for off-line Huffman encoding of messages into the codeword set best for the bout message frequencies. Both schemes can do much better than Huffman coding when successive selections of each message type cluster much more than in the independent case.

Knowledge and Common Knowledge in a Byzantine Environment: Crash failures

Tue, 01 Jul 1986 00:00:00 GMT

Knowledge and Common Knowledge in a Byzantine Environment: Crash failures Dwork, Cynthia; Moses, Yoram By analyzing the states of knowledge that the processors attain in an unreliable system of a simple type, we capture some of the basic underlying structure of such systems. In particular, we study what facts become common knowledge at various points in the execution of protocols in an unreliable system. This characterizes the simultaneous actions that can be carried out in such systems. For example, we obtain a complete characterization of the number of rounds required to reach Simultaneous Byzantine Agreement, given the pattern in which failures occur. From this we derive a new protocol for this problem that is optimal in all runs, rather than just always matching the worst-case lower bound. In some cases this protocol attains Simultaneous Byzantine Agreement in as few as 2 rounds. We also present a non-trivial simultaneous agreement problem called bivalent agreement for which there is a protocol that always halts in two rounds. Our analysis applies to simultaneous actions in general, and not just to Byzantine agreement. The lower bound proofs presented here generalize and simplify the previously known proofs.

An Application of Digital Broadcast Communication to Large Scale Information Systems

Tue, 01 Apr 1986 00:00:00 GMT

An Application of Digital Broadcast Communication to Large Scale Information Systems Gifford, David K.; Lucassen, John M.; Berline, Stephen T. A new type of information system is described that combines personal computers, broadcast data communication, and bidirectional communication. The system is designed to use broadcast communciation whenever possible to deliever information to personal computers, which are used for data storage, indexing, and retrieval. This paper starts with an overview of the system, and then discuss the problem of reliable digital broadcast communication in some detail. A parameterized broadcast protocol is described, and we show how to choose protocol parameters based on observed channel error characteristics. A flexible encryption-based protection system is included in the protocol. We discuss the implementation of the system on contemporary personal computers. A broadcast system based on these ideas is now operating in Boston area homes.

Tight Bounds for Minimax Grid Matching, with Applications to the Average Case Analysis of Algorithms

Thu, 01 May 1986 00:00:00 GMT

Tight Bounds for Minimax Grid Matching, with Applications to the Average Case Analysis of Algorithms Leighton, Tom; Shor, Peter The minimax grid matching problem is a fundamental combinatorial problem associated with the average case analysis of algorithms. The problem has arisen in a number of interesting and seemingly unrelated areas, including wafer-scale integration of systolic arrays, two-dimentsional discrepancy problems, and testing pseudorandom number generators. However, the minimax grid matching problem is best known for its application to the maximum up-right matching problem. The maximum up-right matching problem was originally defined by Karp, Luby and Marchetti-Spaccamela in association with algorithms for 2-dimensional bin packing. More recently, the up-right matching problem has arisen in the average case analysis of on-line algorithms for 1-dimensional bin packing and dynamic allocation. In this paper, we solve both the minimax grid matching problem and the maximum up-right matching problem. As a direct result, we obtain tight upper bounds on the average case behavior of the best algorithms known for 2-dimensional bin packing, 1-dimensional on-line packing and on-line dynamic allocation. The results also solve a long-open question in mathematical statistics.

A Randomized Data Structure for Ordered Sets

Thu, 01 May 1986 00:00:00 GMT

A Randomized Data Structure for Ordered Sets Bentley, Jon L.; Leighton Frank Thomson; Lepley, Margaret; Stanat, Donald F.; Steele, J. Michael In this paper, we consider a simple randomized data structure for representing ordered sets, and give a precise combinatorial analysis of the time required to perform various operations. In addition to a practical data structure, this work provides new and nontrivial proabilistic lower bounds and an instance of a practical problem whose randomized complexity is provably less than its deterministic complexity.

Cellular Automata Supercomputers for Fluid Dynamics Modeling

Sun, 01 Dec 1985 00:00:00 GMT

Cellular Automata Supercomputers for Fluid Dynamics Modeling Margolis, Norman; Toffoli, Tommaso; Vichniac, Gerard We report recent developments in the modeling of fluid dynamics, and give experimental results (including dynamical exponents) obtained using cellular automata machines. Because of their locality and uniformity, cellular automata lend themselves to an extremely efficient physical realization; with a suitable architecture, an amount of hardware resources comparable to that of a home computer can achieve (in the simulation of cellular automata) the performance of a conventional supercomputer.

Atomic Data Abstractions in a Distributed Collaborative Editing System (Extended Abstract)

Fri, 01 Nov 1985 00:00:00 GMT

Atomic Data Abstractions in a Distributed Collaborative Editing System (Extended Abstract) Greif, Irene; Selinger, Robert; Weihl, William This paper describes our experience implementing CES, a distributed Collaborative Editing System written in Argus, a language that includes facilities for managing long-lived distributed data. Argus provides atomic actions, which simplify the handling of concurrency and failures, and t the mechanisms for implementing atomic data types, which ensure serializability and recoverability of actions that use them. This paper focuses on the support for atomicity in Argus, especially the support for building new atomic types. Overall the mechanisms in Argus made it relatively easy to build CES; however, we encountered interesting problems in several areas. For example, much of the processing of an atomic action in Argus is handled automatically by the run-time system; several examples are presented that illustrate areas where more explicit control in the implementations of atomic types would be useful.

Dataflow Architectures

Sun, 01 Jan 0002 00:00:00 GMT

Dataflow Architectures Arvind; Culler, David E. Dataflow graphs are described as a machine language for parallel machines. Static and dynamic dataflow architectures are presented as two implementations of the abstract dataflow model. Static dataflow allows at most one token per arc in dataflow graphs and thus only approximates the abstract model where unbounded token storage per arc is assumed. Dynamic architectures tag each token and keep then in a common pool storage, thus permitting a better approximation of the abstract model. The relative merits of the two approaches are discussed. Functional data structures and I-structures are presented as two views of data structures which are both compatible with the dataflow model. These views are contrasted and compared in regard to efficiency and exploitation of potential parallelism in programs. A discussion of major dataflow projects and a prognosis for dataflow architectures are also presented.

Width-3 Permutation Branching Programs

Sun, 01 Dec 1985 00:00:00 GMT

Width-3 Permutation Branching Programs Barrington, David A. We consider a restricted class of width-3 branching programs where each column of nodes depends on a single variable, and the 0-edges and the 1-edges out of each column form a permutation. In this model, parity and the mod-3 function are easy to calculate, but the and-function is hard. We show that any function of n inputs can be calculated in length O(2^n), and that the and-function in particular requires length O(2^n) if the branching program has one accept node and one reject node.

Packet Trains: Measurements and a New Model for Computer Network Traffic

Fri, 01 Nov 1985 00:00:00 GMT

Packet Trains: Measurements and a New Model for Computer Network Traffic Jain, Raj; Routhier, Shawn Traffic measurements on a ring local area computer network at Massachusetts Institute of Technology are presented. The analysis of the arrival pattern shows that the arrival processes are neither Poisson nor Compound Poisson. An alternative model called "packet train" is proposed. In the train model, the traffic on the network consists of a number of packet streams between various pairs of nodes on the network. Each node-pair stream (or node-pair process, as we call them) consists of a number of trains. Each train consists of a number of packets (or cars) going in either direction (from node A to B or from node B to A). The inter-car gap is large (compared to packet transmission time) and random. The inter-train time is even larger. The Poisson and the Compound Poisson arrivals are shown to be special cases of the train arrival model. Another important observation is that the packet arrivals exhibit a "source locality." If a packet is seen on the network going from A to B, the probability of the next packet going from A to B or from B to A is very high. Implications of the train arrivals, and source locality on the design of bridges, gateways and reservation protocols are discussed. A number of open problems requiring development of analysis techniques for systems with train arrival processes are also described.

A New Max-flow Algorithm

Fri, 01 Nov 1985 00:00:00 GMT

A New Max-flow Algorithm Goldberg, Andrew V. All previously known max-flow algorithms worked by finding augmenting paths, either one path at a time (Ford and Fulkerson algorithm), or all shortest augmenting paths at once (by using the level network technique of Dinic). We introduce an alternative way of dealing with the problem. Our method is to push flow through the original network. The algorithm and its analysis are simple and intuitive, yet the algorithm does as well as any other network flow algorithm on dense graphs, achieving O(n^3) running time. The algorithm admits distributed and parallel implementations as well as a sequential implementation. The algorithm requires less storage then the only other parallel max-flow algorithm known (due to Shiloach and Vishkin), and its parallel running time is the same, O(n^2 logn). In fact, our algorithm uses constant amount of storage for every edge or vertex of the network, allowing an implementation under and more realistic distributed model.

Distributed FIFO Allocation of Identical Resources Using Small Shared Space

Tue, 01 Oct 1985 00:00:00 GMT

Distributed FIFO Allocation of Identical Resources Using Small Shared Space Fischer, Michael J.; Lynch, Nancy A.; Burns, James; Borodin, Allan We present a simple and efficient algorithm for the FIFO allocation of k identical resources among asynchronous processes which communicate via shared memory. The algorithm simulates a shared queue but uses exponentially fewer shared memory values, resulting in practical savings of time and space as well as program complexity. The algorithm is robust against processes failure through unannounced stopping, making it attractive also for use in an environment of processes of widely differing speeds. In addition to its practical advantages, we show the algorithm is optimal (to within a constant factor) with respect to shared space complexity.

The CAM-7 Multiprocessor: A Cellular Automata Machine

Sun, 01 Dec 1985 00:00:00 GMT

The CAM-7 Multiprocessor: A Cellular Automata Machine Toffoli, Tommaso; Margolis, Norman

Dscribe: A Scribe Server

Tue, 01 Oct 1985 00:00:00 GMT

Dscribe: A Scribe Server Chung, Janice C. This document gives a complete description of the design and implementation of Dscribe, the Scribe server. Dscribe is a program which allows users on a variety of hosts to have files processed remotely by the Scribe document preparation system. The first part of the document describes the functionality of Dscribe and the motivation for writing the program. It also gives an overview of how the program works. Later sections discuss important design issues and describe the implementation in detail.

Network Control by Bayesian Broadcast

Sun, 01 Sep 1985 00:00:00 GMT

Network Control by Bayesian Broadcast Rivest, Ronald L.

Improvements of Yao's Results on Parity Circuits

Sun, 01 Sep 1985 00:00:00 GMT

Improvements of Yao's Results on Parity Circuits Hastad, Johan

Two Undecidability Results in Probabilistic Automata Theory

Sat, 01 Jun 1985 00:00:00 GMT

Two Undecidability Results in Probabilistic Automata Theory Kilian, Joseph J. The language accepted by a probabilistic finite state acceptor with an isolated cutpoint is known to be regular. We show that determining if a cutpoint is isolated is undecideable.

A Mixed Integer Linear Programming Problem Which is Efficiently Solvable

Mon, 01 Jul 1985 00:00:00 GMT

A Mixed Integer Linear Programming Problem Which is Efficiently Solvable Leiserson, Charles E.; Saxe, James B. Efficient algorithms are known for the simple linear programming problem where each inequality is of the form xj-xi<=aij. Furthermore, these techniques extend to the integer linear programming variant of the problem. This paper gives an efficient solution to the mixed-integer linear programming variant where some, but not necessarily all, of the unknowns are required to be integers. The algorithm we develop is based on a graph representation of the constraint system and runs in O(|V||E|+|V|62lh|V|) time. It has several applications including optimal retiming of synchronous circuitry, VLSI layout compaction in the presence of power and ground buses, and PERT scheduling with periodic constraints.

Unbiased Bits from Sources of Weak Randomness and Probabilistic Communication Complexity

Mon, 01 Sep 1986 00:00:00 GMT

Unbiased Bits from Sources of Weak Randomness and Probabilistic Communication Complexity Chor, Benny; Goldreich, Oded A new model for weak random physical sources is presented. The new model strictly generalizes previous models (e.g. the Santha and Vazirani model [26]). The sources considered output strings according to probability distributions in which no single string is too probable. The new model provides a fruitful viewpoint on problems studied previously as: 1) Extracting almost perfect bits from sources of weak randomness: the question of possibility as well as the question of efficiency of such extraction schemes are addressed. 2) Probabilistic Communication Complexity: it is shown that most functions have linear communication complexity in a very strong probabilistic sense. 3) Robustness of BPP with respect to sources of weak randomness (generalizing a result of Vazirani and Vazirani [29]).

Computer-based Real-time Conferences

Mon, 01 Jul 1985 00:00:00 GMT

Computer-based Real-time Conferences Sarin, Sunil K.; Greif, Irene A real-time conferencing system allows a group of users to conduct a problem-solving meeting from their workstations. Participants in such a conference use the computer to jointly view, edit, and process relevant information, and use voice communication to discuss the information they are sharing. General principles are presented in this paper for selecting a set of user functions in a real-time conferencing system. The available implementation strategies are reviewed and compared, with emphasis on the tradeoffs between reusing existing single-user interactive programs and writing new distributed multi-user programs. Network communication requirements for real-time conferences, and their potential impact on communication protocol standards, are discussed. Real-time conferencing is contrasted with aynchronous communication support such as electronic message systems and shared databases, and the need for the two to work together within the total system environment is emphasized.

What Price for Eliminating Expression Side-effects?

Sat, 01 Jun 1985 00:00:00 GMT

What Price for Eliminating Expression Side-effects? Hailperin, Max Separating a programming language into side-effect-free expressions and effect-only statements should make the language more amenable to axiomatization, as well as providing benefits for style, pedagogy, and implementation efficiency (particularly in parallel-computing environments). This paper shows that such a division does not come at an unreasonable cost in programming convenience. First a dialect of Lisp is defined, in which a distinction is made between statements, which may have side-effects, and expressions, which may not. Next, a representative collection of examples from Abelson and Sussman's Structure and Interpretation of Computer Programs is coded in this dialect of Lisp. Most of the examples divide neatly into functional and imperative portions, and a few relatively clean transformations prove sufficient for the more stubborn cases.

Qualitative Simulation in Medical Physiology: A Progress Report

Sat, 01 Jun 1985 00:00:00 GMT

Qualitative Simulation in Medical Physiology: A Progress Report Kuipers, Benjamin This progress report describes the current status of the application of the QSIM qualitative simulation representation and algorithm to mechanisms drawn from medical physiology. QSIM takes a qualitative description of the structure of a mechanism and produces and qualitative description of its behavior. Here we apply it to a set of different, medically realistic examples, to represent the following kinds of knowledge: 1) Physiology: qualitative simulation handles the response of normally-functioning mechanisms for salt and water balance to a variety of different environmental perturbations. 2) Pathophysiology: local changes to the structure describing a normal mechanism produces a structure that accurately describes the pathophysiology of a set of diseases. 3) Abstraction: the knowledge of the complexity of human physiology can only be handled by organizing it hierarchically. A hierarchy according to the temporal scale of equilibrium processes appears to be promising. 4) Cardiology: a complex structure describing maintenance of heart rate and blood pressure was adequately constructed during a short meeting with a set of computationally sophisticated physicians. 5) Future Directions: we can outline some of the representation barriers in the way of capturing a broader range of medical knowledge.

Probabilistic Analysis of a Network Resource Allocation Algorithm

Sat, 01 Jun 1985 00:00:00 GMT

Probabilistic Analysis of a Network Resource Allocation Algorithm Fischer, Michael J.; Griffeth, Nancy; Guibas, Leonidas J.; Lynch, Nancy A. A distributed algorithm is presented, for allocating a large number of identical resources (such as airline tickets) to requests which can arrive anywhere in a distributed network. Resources, one allocated, are never returned. The algorithm searches sequentially, exhausting certain neighborhoods of the request origin before proceeding to search at great distances. Choice of search direction is made nondeterministically. Analysis of expected response time is simplified by assuming that the search direction is chosen probabilistically, that messages require constant time, that the network is a tree with all leaves at the same distance from the root, and that requests and resources occur only at leaves. It is shown that the response time is approximated by the number of messages of one that are sent during the execution of the algorithm, and that this number of messages is a nondecreasing function of the interarrival time for requests. Therefor, the worst case occurs when requests come in so far apart that they are processed sequentially. The expected time for the sequential case of the algorithm is analyzed by standard techniques. This time is shown to be bounded by a constant, independent of the size of the network. It follows that the expected response time for the algorithm is bounded in the same way.

Electing a Leader in a Synchronous Ring

Mon, 01 Jul 1985 00:00:00 GMT

Electing a Leader in a Synchronous Ring Frederickson, Greg N.; Lynch, Nancy A. We consider the problem of electing a leader in a synchronous ring of n processors. We obtain both positive and negative results. One the one hand, we show that if processor ID's are chosen from some countable set, then there is an alorithm which uses only O(n) messages in the worst case. On the other hand, we obtain two lower bound results. If the algorithm is restructed to use only comparisons of ID's, then we obtain an Ω(n log n) lower bound for the number of messages required in the worst case. Alternatively, there is a (very fast-growing) function f with the following property. If the number of rounds is required to be bounded by some t in the worst case, and ID's are chosen from any set having at leas f(n,t) elements, then any algorithm requires Ω(n log n) messages in the worst case.

Reaching Approximate Agreement in the Presence of Faults

Wed, 01 May 1985 00:00:00 GMT

Reaching Approximate Agreement in the Presence of Faults Dolev, Danny; Lynch, Nancy A.; Pinter, Shlomit S.; Stark, Eugene W.; Weihl, William E. This paper considers a variant of the Byzantine Generals problem, in which processes start with arbitrary real values rather than Booleann values or values from some bounded range, and in which approximate, rather than exact, agreement is the desired goal. Algorithms are presented to reach approximate agreement in aynchronous, as well as synchornous systems. The asynchronous agreement algorithm is an interesting contrast to a result of Fischer, Lynch, and Paterson, who show that exact agreement is not attainable in an asychronous system with as few as one fault process. The algorithms work by successive approximation, with a provable convergence rate that depends on the ratio between the number of faulty processes and the total number of processes. Lower bounds on the convergence rate for algorithms of this form are proven, and the algorithms presented are shown to be optimal.

The Byzantine Firing Squad Problem

Mon, 01 Apr 1985 00:00:00 GMT

The Byzantine Firing Squad Problem Burns, James E.; Lynch, Nancy A. A new problem, the Byzanntine Firing Squad problem, is defined and solved in two versions, Permissive and Strict. Both problems provide for synchronization of initially unsychronized processors in a synchronous network, in the absense of a common clock and in the presence of a limited number of faulty processors. Solutions are given which take the same number of rounds as Byzantine Agreement but might transmit r times as many bits, where r is the number of rounds used. Additional solutions are provided which use at most one (Permissive) or two (Strict) additional rounds and send at most n^2 bits plus four times the number of bits sent by a chosen Byzantine Agreement algorithm.

Qualitative Simulation of Mechanisms

Mon, 01 Apr 1985 00:00:00 GMT

Qualitative Simulation of Mechanisms Kuipers, Benjamin Qualitative simulation is a key inference process in qualitative causal reasoning. However, the precise meaning of the different proposals and their relation with differential equations is often unclear. In this paper, we present a precise definition of qualitative structure and behavior descriptions as abstractions of differential equations and continuously differentiable functions. We present a new algorithm for qualitative simulation that generalizes the best features of existing algorithms, and allows direct comparisons among alternate approaches. Starting with a structural description abstracted from a differential equation, we prove that the QSIM algorithm is guaranteed to produce a qualitative behavior corresponding to any solution to the original equation. We also show that any qualitative simulation algorithm, because of its local point of view, will sometimes produce spurious qualitative behaviors: ones which do not correspond to any mechanism satisfying the structureal description. These observations suggest specific types of care that must be taken in designing applications of qualitative causal reasoning systems, and in constructing and validating a knowledge base of mechanism descriptions.

Generalized Planar Matching

Mon, 01 Apr 1985 00:00:00 GMT

Generalized Planar Matching Berman, Fran; Leighton, Tom; Shor, Peter; Snyder, Larry In this paper, we prove that maximum planar H-matching (the problem of determining the maximum number of node-disjointed copies of the fixed graph H contained in a variable planar graph G) is NP-complete for any connected planar graph H with three or more nodes. We also show that perfect planar H-matching is NP-complete for any connected outerplanar graph H with three or more nodes, and is, somewhat surprisingly, solvable in linear time for triangulated H with four or more nodes. The results generalize and unify several special-case results proved in the literature. The techniques can also be applied to solve a variety of problems, including the optimal tile salvage problem from wafer-scale integration. Although we prove that the optimal tile salvage problem and other like it are NP-complete, we also describe provably good approximation algorithms that are suitable for practical applications.

Tight Bounds on the Complexity of Parallel Sorting

Mon, 01 Apr 1985 00:00:00 GMT

Tight Bounds on the Complexity of Parallel Sorting Leighton, Tom

Patterns in Trees

Tue, 01 Jan 1985 00:00:00 GMT

Patterns in Trees Dershowitz, Nachum; Zaks, Shmuel A very general enumeration formula for occurences of a pattern, or set of patterns, in the class of ordered trees with a given number of edges is presented, and its wide usefulness is demonstrated.

Consensus in the Presence of Partial Synchrony

Tue, 01 Oct 1985 00:00:00 GMT

Consensus in the Presence of Partial Synchrony Dwork, Cynthia; Lynch, Nancy A.; Stockmeyer, Larry The concept of partial synchrony in a distributed system is introduced. Partial synchrony lies between the cases of a synchronous system and an asynchronous system. In a synchronous system, there is a known fixed upper bound Δ on the time required for a message to be sent from one processor to another and a known fixed upper bound Φ on the relative speeds of different processors. In an asynchronous system, no fixed uppper bounds Δ and Φ exist. In one version of partial synchrony, fixed bounds Δ and Φ exist but they are not know a priori. The problem is to design protocols which work correctly in the partially synchronous system regardless of the actual values of the bounds Δ and Φ. In another version of partial synchrony, the bounds are known but they are only guaranteed to hold starting at some unknown time T, and protocols must be designed to work correctly regardless of when the time T occurs. Fault tolerant consensus protocols are given for various cases of parial synchrony and various fault models. Lower bounds are also given which show in many cases that out protocols are optimal with respect to the number of faults tolerated. Our consensus protocols for partially synchronous processors use new protocols for fault-tolerant "distributed clocks" which allow partially synchronous processors to reach some approximately common notion of time.

The Colored Ticket Algorithm

Mon, 01 Aug 1983 00:00:00 GMT

The Colored Ticket Algorithm Fischer, Michael J.; Lynch, Nancy A.; Burns, James; Borodin, Allan Upper and lower bounds are proved for shared space requirements for solution of a problem involving resource allocation among asynchronous processes. The problem is to allocate some number, k≥1, of resources, in an environment in which processes can fail by stopping without warning. Allocation is to be as FIFO as possible, subject to variations imposed by the possibility of failures.

Complexity of Network Synchronization

Tue, 01 Jan 1985 00:00:00 GMT

Complexity of Network Synchronization Awerbuch, Baruch In this paper we investigate the problem of simulation of the synchronous network by the asynchronous one. We propose a new simulation technique, referred to as "Synchronizer" which is a new, simple methodology for desiging efficient distributed algorithms in asynchronous networks. Our Synchronizer exhibits a trade-off between its communication and time complexities, which is proved to be within a constant factor of the lower bound.

Proposal for a Small Scheme Implementation

Mon, 01 Oct 1984 00:00:00 GMT

Proposal for a Small Scheme Implementation Schooler, Richard; Stamos, James W. Scheme is a lexically scoped dialect of LISP developed at MIT. In this report we determine the feasibility of implementing a Scheme-based programming/application environment on a contemporary personal computer such as the Apple Macintosh. The absense of virtual memmory, coupled with a limitation on the maximum amount of physical memory, means that space is at a premium. We suggest the use of bytecodes and sketch a possible instruction set. Because of space constraints, tail-recursion optimization and an efficient mechanism for the reclamation of inaccessible contexts are also examined. Using the built-in operating system and user interface of the Macintosh realizes speed, functionality, and friendliness but raises a number of interesting issues. For example, the Pascal and assembler routines make many assumptions about data representation, type checking, and parameter passing. Since an implementation of Scheme is likely to have radically different conventions, the two environments must be interfaced smoothly and efficiently. In addition to the bytecode instruction set, we specify the virtual machine informally, discuss the implementation of basic and advanced features, and estimate the performance of such an implementation, and finally evaluate the proposed design.

A Simple and Efficient Randomized Byzantine Agreement Algorithm

Wed, 01 Aug 1984 00:00:00 GMT

A Simple and Efficient Randomized Byzantine Agreement Algorithm Chor, Benny; Coan, Brian A. A new randomized Byantine agreement algorithm is presented. This algorithm operates in a synchronous systems of n processors, at most t of which can fail. The algorithm reaches agreement in O(t/log n) expected rounds and O(n^2 t/log n) expected message bits independent of the distribution of processor failures. This performance is further improved to a constant expected number of rounds and O(n^2) message bits if the distribution of processor failures is assumed to be uniform. In either event, the algorithm improves on the known lower bound on rounds for deterministic algorithms. Some other advantages of the algorithm are that it requires no cryptographic techniques, that the amount of local computation is small, and that the expected number of random bits used per processor is only one. It is argued that in many practical applications of Byzantine agreement, the randomized algorithm of this paper achieves superior performance.

A New Fault-tolerant Algorithm for Clock Sychronization

Sun, 01 Jul 1984 00:00:00 GMT

A New Fault-tolerant Algorithm for Clock Sychronization Lundelius, Jennifer; Lynch, Nancy A. We describe a new fault-tolerant algorithm for solving a variant of Lamport's clock synchronization problem. The algorithm is designed for a system of distributed processes that communicate by sending messages. Each process has its own read only physical clock whose drift rate from real time is very small. By adding a value to its physical clock time, the process obtains its local time. The algorithm solves the problem of maintaining closely synchornized local times, assuming that processes' local times are closely synchronized initially. The algorithm is able to tolerate the failure of just under a third of the participating processes. It maintains synchornization to within a small constant, whose magnitude depends upon the rate of clock drift, the message delivery time, and the initial closeness of synchronization. We also give a characterization of how far the clocks drift from real time. Reintegration of a repaired process can be accomlished using a slight modification of the basic algorithm. A similiar style algorithm can also be used to achieve synchronization initially.

Software for Interactive On-line Conferences

Sun, 01 Jul 1984 00:00:00 GMT

Software for Interactive On-line Conferences Sarin, Sunil K.; Greif, Irene A layered architecture for the implementation of real-time conferences is presented. In a real-time conference a group of users, each at his or her own workstation, share identical views of on-line application information. The users cooperate in a problem solving task by interactively modifying or editing the shared view or the underlying information, and can use a voice communication channel for discussion and negotiation. The lower layer in this architecture, named Ensemble, supports the sharing of arbitrary application-defined objects among the participants of a conference, and the manipulation of these objects via one or more application-defined groups of commands called activities. Ensemble provides generic facilities for sharing objects and activities, and for dynamically adding and removing participants in a conference; these can be used in constructing real-time conferencing systems for many different applications. An example is presented of how the Ensemble functions can be used to implement a shared bitmap with independent participant cursors. The relation between this layered architecture and the ISO Open Systems Interconnection reference model is discussed. In particular, it is argued that Ensemble represents a plausible first step toward a Session-layer protocol for "multi-endpoint connections," a neglected area of communication protocol development.

Naming and Directory Issues in Message Transfer Systems

Sun, 01 Jul 1984 00:00:00 GMT

Naming and Directory Issues in Message Transfer Systems Sirbu, Marvin A., Jr.; Sutherland, Juliet B. A message transfer system requires some means for users to determine the addresses of their correspondents. A Directory Service aids users in identifying a particular correspondent and the correspondent's address. In this paper we discuss the technical, economic, organizational and political requirements which must be satisified by a directory service. We develop a language for describing alternative architectures for directory service borrowed from notions of hierarchical computer file system design. We propose a system of naming and directory services which meets the stated requirements based on names which specify a path through a sequence of directories. Finally, we compare our proposal to several alternative designs for directory service which have appeared in the literature.

Three-dimensional Circuit Layouts

Fri, 01 Jun 1984 00:00:00 GMT

Three-dimensional Circuit Layouts Leighton, Tom; Rosenberg, Arnold

Optimal Distributed Algorithms for Sorting and Ranking

Tue, 01 May 1984 00:00:00 GMT

Optimal Distributed Algorithms for Sorting and Ranking Zaks, Shmuel We study the problems of sorting and ranking n processors that have initial values - not necessarily distinct - in a distrubuted system. Sorting means that the initial values have to move around in the network and be assigned to the processors according to their distinct identities, while ranking means that the numbers 1,2,...,n have to be assigned to the processors according to their initial values; ties between initial values can be broken in any chosen way. Assuming a tree network, and assuming that a message can contain an initial value, an identity or a rank, we present an algorithm for the ranking problem that uses, in the worst case, at most 1/2n^2 + O(n) such messages. The algorithm is them extended to perform sorting, using in the worst case at most 3/4n^2 + O(n) messages. Both algorithms are using a total of O(n) space. The algorithms are extended to general networks. The expected behavior of these algorithms for three classes of trees are discussed. Assuming that the initial values, identities and ranks can only be compared within themselves, lower bounds of 1/2n^2 and 3/4n^2 messages are proved for a worst case execution of any algorithm to solve the ranking and sorting problems, correspondingly.

RSA/RABIN Least Significent Bits Are 1/2 + 1/poly(logN) Secure

Tue, 01 May 1984 00:00:00 GMT

RSA/RABIN Least Significent Bits Are 1/2 + 1/poly(logN) Secure Chor, Benny; Goldreich, Oded

The Impact of Synchronous Communication on. The Problem of Electing a Leader in a Ring

Sun, 01 Apr 1984 00:00:00 GMT

The Impact of Synchronous Communication on. The Problem of Electing a Leader in a Ring Lynch, Nancy A.; Frederickson, Greg N. We consider the problem of electing a leader in a synchronous ring of n processors. We obtain both positive and negative results. One the one hand, we show that if processor ID's are chosen from some countable set, then there is an alorithm which uses only O(n) messages in the worst case. On the other hand, we obtain two lower bound results. If the algorithm is restructed to use only comparisons of ID's, then we obtain an Ω(n log n) lower bound for the number of messages required in the worst case. Alternatively, there is a (very fast-growing) function f with the following property. If the number of rounds is required to be bounded by some t in the worst case, and ID's are chosen from any set having at leas f(n,t) elements, then any algorithm requires Ω(n log n) messages in the worst case.

The Semantics of Local Storage of What Makes The Free-list Free?

Sun, 01 Apr 1984 00:00:00 GMT

The Semantics of Local Storage of What Makes The Free-list Free? Halpern, Joseph Y.; Meyer, Albert R.; Trakhtenbrot, B.A. Denotational semantics for an ALGOL-like language with finite-mode procedures, blocks with local storage, and sharing (aliasing) is given by translating programs into an appropriately typed lambda-calculus. Procedures are entirely explained at a purely functional level - independent of the interpretation of program constructs - by continuous models for lambda-calculus. However, the usual (cpo) models are not adequate to model local storage allocation for blocks because storage overflow presents an apparent discontinuity. New domains of store models are offered to solve this problem.

On the Sequential Nature of Unification

Thu, 01 Mar 1984 00:00:00 GMT

On the Sequential Nature of Unification Dwork, Cynthia; Kanellakis, Paris C.; Mitchell, John C. The problem of unification of terms is log-space complete for P. In deriving this lower bound no use is made of the potentially concise representation of terms by directed acyclic graphs. In addition, the problem remains complete even if infinite substitutions are allowed. A consequence of this result is that parallelism cannot significantly improve on the best sequential solutions for unification. The "dual" problem of computing the congruence closure of an equivalence relation is also log-space complete for P. However, we show that for the problem of term matching, an important subcase of unification, there is a good parallel algorithm using O(log^2 n) time and n^O(1) processors on a PRAM. For the O(log^2 n) parallel time upper bound we assume that the terms are presented by directed acyclic graphs; if the longer string representation is used we obtain an O(log n) parallel time bound.

On the Numbers of Close-and-equal Pairs of Bits in a String (with Implications on the Security of RSA'S L.S.B.)

Thu, 01 Mar 1984 00:00:00 GMT

On the Numbers of Close-and-equal Pairs of Bits in a String (with Implications on the Security of RSA'S L.S.B.) Goldreich, Oded We consider the following problem: Let s be a n-bit string with m ones and n-m zeros. Denote by CEt(s) the number of pairs, of equal bits which are within distance t apart, in the string s. What is the minimum value of Cet(*), when the minimum is taken over all n-bit strings which consists of m ones and n-m zeros? We prove a (reasonably) tight lower bound for this combinatorial problem. Implications, on the cryptographic secruity of the least significant bit of a message encrypted by the RSA scheme, follow. E.g. under the assumption that the RSA is unbreakable; there exist no probabilistic polynomial-time algorithm which guesses the least significant bit of a message (correctly) with probability at least 0.725, when given the encryption of the message using the RSA. This is the best result known concerning the security of RSA's least significant bit.

How to Assemble Tree Machines

Thu, 01 Mar 1984 00:00:00 GMT

How to Assemble Tree Machines Bhatt, Sandeep Nautam; Leiserson, Charles E. Many researchers have proposed that ensembles of processing elements be organized as trees. This paper explores how large tree machines can be assembled efficiently from smaller components. A principal constraint considered is the limited number of external connections from an integrated circuit chip. We also explore the emerging capabilities of restructurable VLSI which allows a chip to be customized after fabrication. We give a linear-area chip of m processors and only four off-chip connections which can be used as the sole building block to construct an arbirtarily large complete binary tree. We also present a restructurable linear-areas layout of m processors with O(lg m) pins that can realize an arbitrary binary tree of any size. This layout is based on a solution to the graph-theoretic problem: Given a tree in which each vertex is either black or white, determine how many edges need to be cut in order to bisect the tree into equal-size components, each containing exactly half the black and half the white vertices. These ideas extend to more general graphs using separator theoerems and bifurcators.

Empirical Analysis of a Token Ring Network

Sun, 01 Jan 1984 00:00:00 GMT

Empirical Analysis of a Token Ring Network Feldmeier, David C. The MIT Laboratory for Computer Science 10 Megabit token ring local area network was monitored. Over a one-week period 7 million packets and 1.3 billion bytes passes by the monitor. This thesis compares the MIT ring traffic with that observed on the Xerox Palo Alto Research Center experimental Ethernet by Shoch and Hupp.

An Application of Number Theory to the Organization of Raster Graphics Memory

Sun, 01 Apr 1984 00:00:00 GMT

An Application of Number Theory to the Organization of Raster Graphics Memory Chor, Benny; Leiserson, Charles E.; Rivest, Ronald L.; Shearer, James B. A high-resolution raster-graphics display is usually combined with processing power and a memory organization that facilitates basic graphics operations. For many applications, including interactive text processing, the ability to quickly move or copy small rectangles of pixels is essential. This paper proposes a novel organization of raster-graphics memory that permits all small rectangles to be moved efficiently. The memory organization is based on a doubly periodic assignment of pixels to M memory chips according to a "Fibonacci" lattice. The memory organization guarantees that if a rectilinearly oriented rectangle contains fewer than M/√5 pixels, then all pixels will reside in different memory chips, and thus can be accesses simultaneously. We also define a continuous analogue of the problem which can be posed as, "What is the maximum density of a set of points in the plane such that no two points are contained in the interior of a rectilinearly oriented rectangle of unit area." We show the existence of such a set with density 1/√5, and prove this is optimal by giving a matching upper bound.

On BPP

Thu, 01 Dec 1983 00:00:00 GMT

On BPP Zachos, Stathis K.; Heller, Hans

Reaching Approximate Agreement in the Presence of Faults

Tue, 01 Oct 1985 00:00:00 GMT

On Concurrent Identification Protocols

Thu, 01 Dec 1983 00:00:00 GMT

On Concurrent Identification Protocols Goldreich, Oded We consider communication networks in which it is not possible to identify the source of a message which is broadcastes through the network. A natural question is whether it is possible for two users to identify each other concurrently, through a secure two-party protocol. We show that more than the existence of a secure Public Key Cryptosystem should be assumed in order to present a secure protocol for concurrent identification. We present two concurrent identification protocols: The first one relies on the existence of a center who has distributed "identification tags" two the users; while the second protocol relies on the distribution of "experimental sequences" by instances of a pre-protocol which have taken place between every two users.

The Markov Chain Tree Theorem

Tue, 01 Nov 1983 00:00:00 GMT

The Markov Chain Tree Theorem Leighton, Frank Thomson; Rivest, Ronald L. Let M be a finite first-order stationary Markov chain. We define an arborescence to be a set of edges in the directed graph for M having at most one edge out of every vertex, no cyles, and maximum cardinality. The weight of an arborescence is defined to be the product over each edge in the arborescence of the probability of the transition associated with the edge. We prove that if M starts in state i, its limiting average probability of being in state j is proportional to the sum of the weights of all arborescences having a path from i to j and no edge out of j. We present two proofs. The first is derived from simple graph theoretic identities. The second is derived from the closely-related Matrix Tree Theorem.

Estimateing a Probability using Finite Memory

Tue, 01 Nov 1983 00:00:00 GMT

Estimateing a Probability using Finite Memory Leighton, Frank Thomson; Rivest, Ronald L.

Probabilistic Searching in Sorted Linked Lists

Tue, 01 Nov 1983 00:00:00 GMT

Probabilistic Searching in Sorted Linked Lists Leighton, Frank Thomson; Lepley, Margaret Janko [2] and Bentley, Stanat, and Steele [1] have described probabilistic procedures for data manipulation in sorted linnked lists. Their procedures are based on an algorithm which performs a Member search operation using 2N^1/2 + O(1) expected steps where N is the number of elements in the list. In addition, Bentley, Stanat and Steele have shown that every Member search algorithm requires (2N)^1/2 + Ω(!) expected steps. In this paper, we improve the lower bound result in order to prove that the known algorithm for Member search is optimal.

From Denotational to Operational and Axiomatic Semantics for ALGOL-like Languages: An Overview

Sat, 01 Oct 1983 00:00:00 GMT

From Denotational to Operational and Axiomatic Semantics for ALGOL-like Languages: An Overview Trakhtenbrot, B.A.; Halpern, Joseph Y.; Meyer, Albert R. The advantages of denotational over operational semantics are argued. A denotational semantics is provided for an ALGOL-like language with finite-model procedures, blocks with local storage, and sharing (aliasing). Procedure declarations are completely explained in the ususal framework of complete partial orders, but cpo's are inadequate for the semantics of blocks, and a new class of store models is developed. Partial correctness theory over store models is developed for commands which may contain calls to global procedures, but do not contain function procedures returning storable values.

Understanding ALGOL: A View of a Recent Convert to Denotational Semantics

Sat, 01 Oct 1983 00:00:00 GMT

Understanding ALGOL: A View of a Recent Convert to Denotational Semantics Meyer, Albert R. The advantages of denotational over copy-rule semantics are argued. A denotational semantics is indicated for an ALGOL-like language with finite-mode procedures, blocks with local storage, and sharing (aliasing). Procedure declarations are completely explained in the usual framework of complete partial orders, but cpo's are inadequate for the semantics of blocks, and a new class of store models is described. The semantics justifies a proof system for partial correctness of commands containing global procedures.

How to Construct Random Functions

Mon, 01 Nov 1982 00:00:00 GMT

How to Construct Random Functions Goldreich, Oded; Goldwasser, Shafi; Micali, Silvio We assume that functions that are one-way in a very weak sense exist. We prove that in probabilitic polynomial time it is possible to construct deterministic polynomial time computable functions g:{1,…,2^k} -> {1,…,2^k} that cannot be distinguished by an probabilistic polynomial time algorithm from a random function. Loosely speaking, g provides random access to a K2^k -bit long pad whose entries record the outcome of independent coin flips. This complexity theoretic result has many important applications in Cryptography, Protocols, and Hashing.

Efficient Demand-Driven Evaluation (II)

Thu, 01 Sep 1983 00:00:00 GMT

Efficient Demand-Driven Evaluation (II) Pingali, Keshav; Arvind In Part I of this paper, we presented a scheme whereby a compiler could propogate demands through programs in a powerful stream language L. A data-driven evaluation of the transformed program performed exactly the same computation as a demand-driven evaluation of the original program. In this paper, we explore a different transformation which trades the complexity of demand propogation for a bounded amount of extra computation on some data lines.

Efficient Demand-Driven Evaluation (I)

Thu, 01 Sep 1983 00:00:00 GMT

Efficient Demand-Driven Evaluation (I) Pingali, Keshav; Arvind We describe a program transformation technique for programs in a general stream language L whereby a data-driven evaluation of the transformed program performs exactly the same computation as a demand-driven evaluation of the original program. The transformational technique suggests a simple denotational characterization of demand-driven evaluation.

Two Fundamental Issues in Multiprocessing: The Dataflow Solution

Thu, 01 Aug 1985 00:00:00 GMT

Two Fundamental Issues in Multiprocessing: The Dataflow Solution Arvind; Iannucci, Robert A. To exploit the parallelism inherent in algorithms, any multiprocessor system must address two very basic issues - long memory latencies and waits for synchronization events. It is argued on the basis of the evolution of high performance computers that the processor idle time induced by memory latency and synchronization waits cannot be reduced simultaneously in von Neumann style multiprocessors. Dataflow architectures are offered as an alternative because, given enough parallelism in the program, they can reduce both latency and sychronization costs.

A Program for Therapy of Acid-base and Electrolyte Disorders

Wed, 01 Jun 1983 00:00:00 GMT

A Program for Therapy of Acid-base and Electrolyte Disorders Bromley, Hank This thesis describes work done on the therapy component of an on-going project for the diagnosis and management of acid-base and electrolyte disoders. Therapeutic interventions can be classified as symptomatic or etiologic, and as acute or chronic. We have focused on the problem of acute symptomatic therapy. Based on observation of clinical practice, we have developed a formalization of the domain-independent aspects of the task of acute symptomatic therapy, then applied the formalization to the particular field of acid-base and electrolyte disorders. A rule-based program named ABET (the Acide-Base ad Electrolye Therapy Advisor) has been designed and written to test this formalization. The thesis presents the methods used by ABET, the program's implementation, a sample session, and a discussion of limitations and possible improvements.

Evaluation of an Office Analysis Methodology

Tue, 01 Mar 1983 00:00:00 GMT

Evaluation of an Office Analysis Methodology Sutherland, Juliet; Sirbu, Marvin We have developed a model of the office that describes semi-structured office work. This model underlies an office analysis methodology and an office-specification language. An evaluation of the usefulness and practicality of the model, the specification language, and the methodology has dhown that the model is clearly a useful approach to understanding offices, the specification language is interesting but not as useful in practice as we had hoped, and the methodology is useful but could be improved. We have developed a new methodology that addresses the issue of diagnosis as well as description. This new methodology is still being evaluated, but early results show that it is as useful for training new analysts as the old methodology.

An Approximation Algorithm for Manhattan Routing

Tue, 01 Feb 1983 00:00:00 GMT

An Approximation Algorithm for Manhattan Routing Baker, Brenda S.; Bhatt, Sandeep N.; Leighton, Frank Thomson Density has long been known to be an important measure of difficulty for Manhattan routing. In this paper, we identify a second important measure of difficulty, which we call flux. We show that flux, like density, is a lower bound on channel width. In addition, we present a linear-time algorithm which routes any multipoint net Manhattan routing problem with density d and flux f in a channel of width 2d+O(f). (For 2-point net, the bound is d+O(f).) Thus we show that Manhattan routing is one of the NP-complete problems for which there is a provably good approximation algorithm.

Planar Embedding of Planar Graphs

Tue, 01 Feb 1983 00:00:00 GMT

Planar Embedding of Planar Graphs Dolev, Danny; Leighton, Frank Thomson; Trickey, Howard Planar embedding with minimal area of graphs on an integer grid is an interesting problem in VLSI theory. Valiant [V] gave an algorithm to construct a planar embedding for trees in linear area; he also proved that there are planar graphs that require quadratic area. We fill in a spectrum between Valiant's results by showing that a N-node planar graph has a planar embedding with area O(NF), where F is a bound on the path length from any node to the exterior face. In particular, an outerplanar graph can be embedded without crossovers in linear area. This bound is tight, up to constant factors: for any N and F, there exist graphs requiring Ω(NF) area for planar embedding. Also, finding a minimal embedding area is shown to be NP-complete for forests, and hence for more general types of graphs.

Wafer-scale Integration of Systolic Arrays

Tue, 01 Feb 1983 00:00:00 GMT

Wafer-scale Integration of Systolic Arrays Leighton, Frank Thomson; Leiserson, Charles E. VLSI technologists are fast developing wafer-scale integration. Rather than partitioning a silicon wafer into chips as is usually done, the idea behind wafer-scale integration is to assemble an entire system (or network of chips) on a single wafer, thus avoiding the costs and performance loss associated with individual packaging of chips. A major problem with assembling a large system of microprocessors on a single wafer, however, is that some of the processor, or cells, on the wafer are likely to be defective. In the paper, we describe practical procedures for integrating wafer-scale systems "around" such faults. The procedures are designed to minimize the length of the longest wire in the system, thus minimizing the communication time between cells. Although the underlying network problems are NP-complete, we prove that the procedures are reliable by assuming a probabilistic model of cell failure. We also discuss applications of this work to problems in VLSI layout theory, graphy theory, fault-tolerant systems and planar geometry.

The Implication Problem for Functional and Inclusion Dependencies

Tue, 01 Feb 1983 00:00:00 GMT

The Implication Problem for Functional and Inclusion Dependencies Mitchell, John C. There are two implication problems for functional dependencies and inclusion dependencies: general implication and finite implication. Given a set of dependencies ∑∪{σ}, the problems are to determine whether σ holds in all databases satisfying ∑ or all finite databases satisfying ∑. Contrary to the possibility suggested in [5], there is a natural, complete axiom system for general implication. However, a simple observation shows that both implication problems are recursively unsolvable. It follows that there is no recursively enumerable set of axioms for finite implication.

Randomized Encryption Techniques

Sat, 01 Jan 1983 00:00:00 GMT

Randomized Encryption Techniques Rivest, Ronald L.; Sherman, Alan T. A randomized encryption procedure enciphers a message by randomly choosing a ciphertext from a set of ciphertexts corresponding to the message under the current encryption key. At the cost of increasing the required bandwidth, such procedures may achieve greater cryptographic security than their deterministic counterparts by increasing the apparent size of the message space, eliminating the threat of chosen plaintext attacks, and improving the a priori statistics for the inputs to the encryption algorithms. In this paper we explore various ways of using randomization in encryption.

Implementing Internet Remost Logic on a Personal Computer

Wed, 01 Dec 1982 00:00:00 GMT

Implementing Internet Remost Logic on a Personal Computer Konopelski, Louis J. This thesis demonstrates that a desktop personal computer can support an efficient internet remote login implementation with the same protocols used by large mainframes. It describes a project in which the Telnet remote login protocol, along with the supporting Transmission Control Protocol and Internet Protocol were implemented on an IBM Personal Computer. The utility of the implementation depended heavily on the software speed. Strategies discusses to insure quick performance included tailoring protocols to their clients needs, sharing the overhead of asynchronous actions, and sharing data. A natural order in which to process the protocol data was identified, and two control structures were presented that allowed the protocol modules to run in this order. One of the control structures used procedures and processes, while the other used procedures alone. A full scale protocol was successfully placed in the personal computer. With some foreign hosts, the implementation echoed characters in less than a quarter of a second, and processed a screenful of data in less than three seconds. The protocol software overhead was never the dominating performance bottleneck. The serial line interface limited the character echoing performance while the speed with which the processor could operate its display limited the processing speed of large amounts of data. Memory size was not a significant constraint.

PLY: A System of Plausibility Inference with a Probabilistic Basis

Wed, 01 Dec 1982 00:00:00 GMT

PLY: A System of Plausibility Inference with a Probabilistic Basis Yeh, Alexander An overview is given of a system of plausibility inference that will be developed for use in planning. This system, to be called PLY, will be specifically designed to work with propositions of the form "when A is true (occurs), B is likely to be true (to occur)." Previous systems performing similiar functions have been designed as aids for such tasks as medical diagnosis (MYCIN and others) and mineral prospecting (PROSPECTOR). PLY will have a probabilistics basis. Intuitive assumptions to deal with knowledge not explicitly given to the system will be made with the aid of an information-theoretic measure on the amount of information in a probability distribution. Unlike many other systems, PLY will not use these assumptions when the given knowledge indicates they are not tenable. In addition to standard probabilities, PLY will be able to make use of knowledge (information) in the form of correlations and increased/decreased likelihoods, which most people find easier to estimate than probabilities. PLY's knowledge will be in an organized and structures form, which will help in knowledge acquisition and revisino, faciliate system explanations, and lower the storage requirements of the system.

An Asymptotically Optimal Layout for the Shuffle-exchange Graph

Fri, 01 Oct 1982 00:00:00 GMT

An Asymptotically Optimal Layout for the Shuffle-exchange Graph Kleitman, Daniel; Leighton, Frank Thomson; Lepley, Margaret; Miller, Gary L. The shuffle-exchange graph is one of the best structures known for parallel computation. Among other things, a shuffle-exchange computer can be used to compute discrete Fourier transforms, multiply matrices, evaluate polynomials, perform permutations and sort lists. The algorithms needed for these operations are quite simple and many require no more than logarithmic time and constant space per processor. In this paper, we describe an O(n^2/log^2N)-area layout for the shuffle-exchange graph on a two-dimentional grid. The layout is the first which is known to achieve Thompson's asymptotic lower bound.

Embedding Cryptographic Trapdoors in Arbirtrary Knapsack Systems

Wed, 01 Sep 1982 00:00:00 GMT

Embedding Cryptographic Trapdoors in Arbirtrary Knapsack Systems Shamir, Adi In this paper we show that after sufficiently many modular multiplications, any knapsack system becomes a trapdoor system that can be used in pubic-key cryptography.

The Complexity of Evaluation Relational Queries

Sun, 01 Aug 1982 00:00:00 GMT

The Complexity of Evaluation Relational Queries Cosmadakis, Stavros S. We show that, given a relation R, a relational query ? Involving only projection and join, and a conjectured result r, resting with ?(R)=r is D^p complete. Bounding the size of ?(R) from below (above) is NP-hard (co-NP-hard), and bounding it both ways is D^p hard. Computing the size of ?(R) is #P-hard. We also show that, given two relations R1 and R2 and two queries ?1 and ?2 as above, testing whether ?1)R1)⊆?2(R2) and testing whether ?1(R1)=?2(R2) and both ∏p2-complete, even when R1-R2 or when ?1=?2.

Two Remarks on the Power of Counting

Sun, 01 Aug 1982 00:00:00 GMT

Two Remarks on the Power of Counting Papadimitriou, Christos H.; Zachos, Stathis K. The relationship between the polynomial hierarchy and Valiant's class #P is at present unknown. We show that some low portions of the polynomial hierarchy, namely deterministic polynomial algorithms using an NP oracle at most a logarithmic number of times, can be simulated by one #P computation. We also show that the class of problems solvable by polynomial-time nondeterministic Turing machines which accept whenever there is an odd number of accepting computations is idempotent, that is closed under usage of oracles from the same class.

New Lower Bound Techniques For VLSI

Sun, 01 Aug 1982 00:00:00 GMT

New Lower Bound Techniques For VLSI Leighton, Frank Thomson In this paper, we use crossing number and wire area arguments to find lower bounds on the layout area and maximum edge length of a variety of new and computationally useful networks. In particular, we describe 1) an N-node planar graph which has layout area ⊖ (NlogN) and maximum edge length ⊖(N^1/2/log^1/2N), 2) an N-node graph with an O(x^1/2)-separator which has layout area ⊖ (Nlog^2N) and maximum edge length ⊖ (N^1/2logN/loglogN), and 3) an N-node graph with an O(x^1-1/r)-separator which has maximum edge length ⊖(N1-1/4) for an r≥3.

Hoare's Logic Is Not Complete When It Could Be

Sun, 01 Aug 1982 00:00:00 GMT

Hoare's Logic Is Not Complete When It Could Be Bergstra, J.; Chielinksa, A.; Tiuryn, J. It is known (cf.[2]) that is the Hoare rules are complete for a first-order structure A, then the set of partial correctness assertions true over A is recursive in the first-order theory of A. We show that the converse is not true. Namely, there is a first-order structure C such that the set of partial correctness assertions true over C is recursive in the theory of C, but the Hoare rules are not complete for C.

Foundations for Office Semantics

Thu, 01 Jul 1982 00:00:00 GMT

Foundations for Office Semantics Barber, Gerald R.; Hewitt, Carl In this paper we develop the semantics of work in the office in terms of the concepts of application structure and organizational structure of the office. Application structure is concerned with the rules and constraints of the domain of the office work such as accounting, law, or social security regulations. Organizational structure is concerned with the informal and formal social relationships within the organization. Detailed knowledge of office application structures and organizational structures is necessary in order to understand how they interact and evolve. Problem solving is a pervasive activity within offices which is performed when office workers apply general knowledge about office procedures to the specific cases encountered in their daily work. We discuss how a description system (named OMEGA) can aid in the construction of interactive systems whose intent is to describe the application and organization structures. Using the knowledge embedded within itself about the office OMEGA can help support office workers in their problems solving processes.

Supporting Organizational Problem Solving with a Workstation

Thu, 01 Jul 1982 00:00:00 GMT

Supporting Organizational Problem Solving with a Workstation Barber, Gerald R. This paper describes an approach to supporting work in the office. Using and extending ideas from the field of Artificial Intelligence (AI) we describe office work as a problem solving activity. A knowledge embedding language called Omega is used to embed knowledge of the organization into an office worker's workstation in order to support the office worker in his or her problem solving. A particular approach to reasoning about change and contradiction is discussed. This approach uses Omega's viewpoint mechanism. Omega's viewpoint mechanism is a general contradiction handling facility. Unlike other Knowledge Representation systems, when a contradiction is reached the reasons for the contradiction can be analyzed by the deduction mechanisms without having to resort to a backtracking mechanism. The Viewpoint mechanism is the heart of the Problem Solving Support Paradigm. This paradigm supplements the classical AI view of problem solving. Office workers are supported using the Problem Solving Support Paradigm. An example is presented where Omega's facilities are used to support an office worker's problem solving activities. The example illustrates the use of viewpoints and of Omega's capabilities to reason about it's own reasoning process.

A Principled Design for an Integrate Computational Environment

Thu, 01 Jul 1982 00:00:00 GMT

A Principled Design for an Integrate Computational Environment diSessa, Andrea A. Boxer is a computer language designed to be the base of an integrated computational environment providing a broad array of functionality -- from text editing to programming -- for naïve and novice users. It stands in the line of Lisp inspired languages (Lisp, Logo, Scheme), but differs from these in achieveing much of its understandability from pervasive use of a spatial metaphor reinforced through suitable graphics. This paper describes a set of learnability and understandability issues first and then uses them to motivate design decisions made concerning Boxer and the environment in which it is embedded.

A Telex Gateway for the Internet

Sat, 01 May 1982 00:00:00 GMT

A Telex Gateway for the Internet Meier zu Sieker, Friedrich The design of a gateway connecting one of the networks of the MIT Laboratory for Computer Science to the telex network is discussed. A description of the telex network is given. The relationship of the gateway to other resources of the network environment is considered to obtain directions for the implementation of new resources. The implementation of the gateway on the UNIX operating system outlined.

Layouts for the Suffle-Exchange Graph Based on the Complex Plane Diagram

Tue, 01 Jun 1982 00:00:00 GMT

Layouts for the Suffle-Exchange Graph Based on the Complex Plane Diagram Leighton, Frank Thomson; Lepley, Margaret; Miller, Gary L. The shuffule-exchange graph is one of the best structures known for parallel computation. Among other things, a shuffle-exchange computer can be used to compute discrete. Fourier transforms, multiply matrices, evaluate polynomials, performa permutations and sort lists. The algorithms needed for these operations are extremely simple and many require no more than logarithmic time and constant space per processor. In this paper, we analyze the algebraic structure of the shuffle-exchange graph in order to find area-efficient embeddings of the graph in a two-dimensinoal grid. The results are applicable to the design of Very Large Scale Integration (VLSI) circuit layouts for a shuffle-exchange computer.

Circuit Analysis of Self-timed Elements for NMOS VLSI Systems

Sat, 01 May 1982 00:00:00 GMT

Circuit Analysis of Self-timed Elements for NMOS VLSI Systems Chu, Tam-Anh Scalingof VLSI digital systems introduces new problems to the design of synchronous systems, due to the disproportional increase in wire delays with the decrease in transistor sizes. One the other hand, the asynchronous self-timed design approach, which has been traditionally less attractive, offer a number of advantages for VLSI. Also, this approach can be directly incorporated into a structured design methodology for Packet Communication Architectures. This paper considers a practical self-timed design methodology and studies its implementation in nMOS. The C-element and the arbiter circuit, two main circuit components of self-timed systems, are analyzed to allow the evaluation of the design approach.

Recursive Decomposition Ordering and Multiset Orderings

Tue, 01 Jun 1982 00:00:00 GMT

Recursive Decomposition Ordering and Multiset Orderings Jouannaud, Jean-Pierre; Lescanne, Pierre; Reinig, Fernand The Recursive Decomposition Ordering, a simplification ordering on terms, is useful to prove termination of term rewriting systems. In this paper we give the definition of the decomposition ordering and prove that it is a well-founded simplication ordering containing Dershowitz's Recursive Path Ordering. We also show that the Recursive Decomposition Ordering has a very interesting incremental property. In the second paper, we propose two well-founded orderings on multisets that extend the Dershowitz-Manna ordering. Unlike the Dershowitz-Manna ordering, ours do not have a natural monotonicity property. This lack of monotonicity suggests using monotonicity to provide a new characterization of the Dershowitz -Manna ordering. Section 5 proposes an efficient and correct implementation of that ordering.

Cooperative Office Work, Teleconferencing and Calendar Management: A Collection of Papers

Sat, 01 May 1982 00:00:00 GMT

Cooperative Office Work, Teleconferencing and Calendar Management: A Collection of Papers Greif, Irene This technical memo consists of a collection of papers that have been presented at conferences. They all present results of research in the "Multi-person Informational Work" project in the Office Automation Group.

A File Transfer Program for a Personal Computer

Thu, 01 Apr 1982 00:00:00 GMT

A File Transfer Program for a Personal Computer Wright, Karl D. This thesis explores engineering decisions involved in implementing a network file transfer program on a personal computer in response to criteria of low cost and reasonable efficiency. The issues include choice of hardware, design of the network, choice of implementation language, choice of communication protocols, and choice of software structure. A machine level protocol is designed. A project incorporating these and other ideas is undertaken and the ideas thus evaluated. Insight is gleaned into the performance expected under varying operating system and interrupt enviroments. A notion of an "ideal" operating system interface for applications similiar to file transfer (which can exploit concurrency) is developed. Finally, possible improvements on the actual project are suggested based in part on the efficiency data obtained.

Coping with Syntactic Ambiguity or How to Put the Block in the Box on the Table

Thu, 01 Apr 1982 00:00:00 GMT

Coping with Syntactic Ambiguity or How to Put the Block in the Box on the Table Church, Kenneth; Patil, Ramesh Sentences are far more ambiguous than one might have thought. There may be hundreds, perhaps thousands of syntatic parse trees for certain very natural sentences of English. This fact has been a major problem confronting natural language processing because it indicates that it may require a long time to construct a list of all the parse trees, and furthermore, it isn't clear what to do with the list once it has ben constructed. This list may be so numerous that it is probably not the most convenient representation for communication with the semantic and pragmatic processing modules. In this paper we propose some methods for dealing with syntactic ambiguity in ways that take advantage of certain regularities among the alternative parse trees. These regularities will be expressed as linear combinations of ATN networks, and also as sums and products of formal power series. We will suggest some ways that practical processor can take advantage of this modularity in order to deal more efficiently with combinatoric ambiguity. In particular, we will show how a processor can efficiently compute the ambiguity of an input sentence (or any portion thereof). Furthermore, we will show how to compile certain grammars into a form that can be processed more efficiently. In some cases, including the "every way ambiguous" grammar (e.g., conjunction, prepositional phrases, noun-noun modification), processing time will be reduced from O9n^3) to O(n). Finally, we will show how to uncompile certain highly optimized grammars into a form suitable for linguistic analysis.

Optimizing Synchronous Systems

Mon, 01 Mar 1982 00:00:00 GMT

Optimizing Synchronous Systems Leiserson, Charles E.; Saxe, James B. The complexity of integrated-circuit chips produced today makes it feasible to build inexpensive, special-purpose subsystems that rapidly solve sophisticated problems on behalf of a general-purpose host computer. This paper contributes to the design methodology of efficient VLSI algorithms. We present a transformation that converts synchronous systems into more time-efficient, systolic implementations by removing combinatorial rippling. The problem of determining the optimized system can be reduced to the graph-theoretic single-destination-shortest-paths problem. More importantly from an engineering standpoint, however, the kinds of rippling that can be removed from a circuit at essentially no cost can be easily characterized. For example, if the only global communication in a system is broadcasting from the host computer, the broadcast can always be replaced by local communication.

Termination Assertions for Recursive Programs: Completeness and Axiomatic Definability

Mon, 01 Mar 1982 00:00:00 GMT

Termination Assertions for Recursive Programs: Completeness and Axiomatic Definability Meyer, Albert R.; Mitchell, John C. The termination assertion pq means that whenever the formular p is true, there is an execution of the possibly nondeterministic program S which terminates in a state in qhich q is true. A recursive program S may declare and use local variables and nondeterministic recursive procedures with call-by-address and call-by--value parameters, in addition to accessing undeclared variables and global procedures. Assertions p and q about calls to global procedures are first order formulas extended to express hypotheses about the termination of calls to undeclared global procedures. A complete, effective axiom system with axioms corresponding to the syntax of the programming language is given for the termination assertinos valid over all interpretations. Termination assertions define the semantics of recursive programs in the following sense: if two programs have different input-output semantics, then there is a termination assertion that is valid for one program but not for the other. Thus the complete axiomatization of termination assertions constitutes an axiomatic definition of the semantics of recursive programs.