Distributed Tree Search and Its Application to Alpha-Beta Pruning

Chris Ferguson, Richard E. Korf

We propose a parallel tree search algorithm based on the idea of tree-decomposition in which different processors search different parts of the tree. This generic algorithm effectively searches irregular trees using an arbitrary number of processors without shared memory or centralized control. The algorithm is independent of the particular type of tree search, such as single-agent or two-player game, and independent of any particular processor allocation strategy. Uniprocessor depth-first and breadth-first search are special cases of this generic algorithm. The algorithm has been implemented for alpha-beta search in the game of Othello on a 32-node Hypercube multiprocessor. The number of node evaluations grows approximately linearly with the number of processors P, resulting in an overall speedup for alpha-beta with random node ordering of p.75 . Furthermore we present a novel processor allocation strategy, called Bound-and-Branch, for parallel alpha-beta search that achieves linear speedup in the case of perfect node ordering. Using this strategy, an actual speedup of 12 is obtained with 32 processors.

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