PDL Abstract

Runahead A*: Speculative Parallelism for A* with Slow Expansions

ICAPS 2023, Prague, Czech Republic, July 8-13, 2023.

Mohammad Bakhshalipour, Mohamad Qadri, Dominic Guri, Seyed Borna, Ehsani‡, Maxim Likhachev, Phillip B. Gibbons

Carnegie Mellon University
‡ University of Washington

A* suffers from limited parallelism. The maximum level of traditional parallelism in A* is the same as the degree of the search graph nodes, which is too small in many applications. As such, A* cannot fully leverage the multithreading capabilities of modern processors. In this paper, we go beyond traditional parallelism and introduce speculative parallelism for A*. We observe that A*’s node expansions exhibit predictable patterns in applications like path planning. Based on this observation, we propose Runahead A* (RA*). When a node is being expanded, RA⋆ predicts future likely-to-be-expanded nodes, performs their corresponding computation on separate threads, and memoizes the computation results. Later when a predicted node is selected for expansion, rather than performing its computation, the memoized results are used, saving significant time in slow-expansion applications. We study five applications of A*. We show that when its prediction accuracy is high, RA* offers significant speedup over vanilla A* for slow-expansion applications. With 16 threads, RA*’s speedup for such applications ranges from 3.1× to 14.1×. We also study and provide insight into when, why, and to what extent node expansions are predictable. We provide an implementation of RA* at: