Model-Free, Regret-Optimal Best Policy Identification in Online CMDPs
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Abstract (arXiv)
This paper considers the best policy identification (BPI) problem in online Constrained Markov Decision Processes (CMDPs). We are interested in algorithms that are model-free, have low regret, and identify an approximately optimal policy with a high probability. Existing model-free algorithms for online CMDPs with sublinear regret and constraint violation do not provide any convergence guarantee to an optimal policy and provide only average performance guarantees when a policy is uniformly sampled at random from all previously used policies. In this paper, we develop a new algorithm, named Pruning-Refinement-Identification (PRI), based on a fundamental structural property of CMDPs proved before, which we call limited stochasticity. The property says for a CMDP with N constraints, there exists an optimal policy with at most N stochastic decisions. The proposed algorithm first identifies at which step and in which state a stochastic decision has to be taken and then fine-tunes the distributions of these stochastic decisions. PRI achieves trio objectives: (i) PRI is a model-free algorithm; and (ii) it outputs an approximately optimal policy with a high probability at the end of learning; and (iii) PRI guarantees tildeO(H√K) regret and constraint violation, which significantly improves the best existing regret bound tildeO(H4 √SAK^frac45) under a model-free algorithm, where H is the length of each episode, S is the number of states, A is the number of actions, and the total number of episodes during learning is 2K+tildecal O(K0.25). We further present a matching lower via an example that shows under any online learning algorithm, there exists a well-separated CMDP instance such that either the regret or violation has to be Omega(H√K), which matches the upper bound by a polylogarithmic factor.
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References · 19
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7 more
- 2023Provably Efficient Model-Free Algorithms for Non-stationary CMDPscited by 21
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- 2023Intrinsic Rewards from Self-Organizing Feature Maps for Exploration in Reinforcement Learningcited by 1
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