Monte-Carlo Planning: Theoretically Fast Convergence Meets Practical Efficiency

Popular Monte-Carlo tree search (MCTS) algorithms for online planning, such as e-greedy tree search and UCT, aim at rapidly identifying a reasonably good action, but provide rather poor worst-case guarantees on performance improvement over time. In contrast, a recently introduced MCTS algorithm BRUE guarantees exponential-rate improvement over time, yet it is not geared towards identifying reasonably good choices right at the go. We take a stand on the individual strengths of these two classes of algorithms, and show how they can be effectively connected. We then rationalize a principle of "selective tree expansion", and suggest a concrete implementation of this principle within MCTS. The resulting algorithms favorably compete with other MCTS algorithms under short planning times, while preserving the attractive convergence properties of BRUE.