PRA*: Massively Parallel Heuristic Search

In this paper we describe a variant of A* search designed to run on the massively parallel, SIMD Connection Machine (CM-2). The algorithm is designed to run in a limited memory by use of a retraction technique which allows nodes with poor heuristic values to be removed from the open list, until such time as they may need reexpansion, more promising paths having failed. Our algorithm, called PRA* (for Parallel Retraction A*), is designed to maximize use of the Connection Machine's memory and processors. In addition, the algorithm is guaranteed to return an optimal path when an admissable heuristic is used. Results comparing PRA* to Korf's IDA* for the fteen-puzzle show signi cantly fewer node expansions for PRA*. In addition, empirical results show signi cant parallel speedups, indicative of the algorithm's design for high processor utilization. Please address correspondence to: James Hendler Computer Science Dept. University of Maryland College Park, MD 20742 Institute for Advanced Computer Studies and Computer Science Department. E-mail: [email protected]. y Computer Science Department, Systems Research Center, and Institute for Advanced Computer Studies. E-mail: [email protected] z Systems Research Center, Computer Science Department, and Institute for Advanced Computer Studies. E-mail: [email protected]. x Computer Science Department, Systems Research Center, and Institute for Advanced Computer Studies. E-mail: [email protected].