Executing Task Graphs Using Work-Stealing

Abstract

NABBIT is a work-stealing library for execution of task graphs with arbitrary dependencies which is implemented as a library for the multithreaded programming language Cilk++. We prove that Nabbit executes static task graphs in parallel in time which is asymptotically optimal for graphs whose nodes have constant in-degree and out-degree. To evaluate the performance of Nabbit, we implemented a dynamic program representing the Smith-Waterman algorithm, an irregular dynamic program on a two-dimensional grid. Our experiments indicate that when task-graph nodes are mapped to reasonably sized blocks, Nabbit exhibits low overhead and scales as well as or better than other scheduling strategies. The Nabbit implementation that solves the dynamic program using a task graph even manages in some cases to outperform a divide-and-conquer implementation for directly solving the same dynamic program. Finally, we extend both the Nabbit implementation and the completion-time bounds to handle dynamic task graphs, that is, graphs whose nodes and edges are created on the fly at runtime.