Optimizing RAM-latency dominated applications

Abstract

Many apparently CPU-limited programs are actually bottlenecked by RAM fetch latency, often because they follow pointer chains in working sets that are much bigger than the CPU's on-chip cache. For example, garbage collectors that identify live objects by tracing inter-object pointers can spend much of their time stalling due to RAM fetches. We observe that for such workloads, programmers should view RAM much as they view disk. The two situations share not just high access latency, but also a common set of approaches to coping with that latency. Relatively general-purpose techniques such as batching, sorting, and "I/O" concurrency work to hide RAM latency much as they do for disk. This paper studies several RAM-latency dominated programs and shows how we apply general-purpose approaches to hide RAM latency. The evaluation shows that these optimizations improve performance by a factor of 1.3x. Counter-intuitively, even though these programs are not limited by CPU cycles, we found that adding more cores can yield better performance.