Bridging the Gap Between Value and Policy Based Reinforcement Learning
Ofir Nachum, Mohammad Norouzi, Kelvin Xu, Dale Schuurmans
We formulate a new notion of softmax temporal consistency that generalizes the standard hard-max Bellman consistency usually considered in value based reinforcement learning (RL). In particular, we show how softmax consistent action values correspond to optimal policies that maximize entropy regularized expected reward. More importantly, we establish that softmax consistent action values and the optimal policy must satisfy a mutual compatibility property that holds across any state-action subsequence. Based on this observation, we develop a new RL algorithm, Path Consistency Learning (PCL), that minimizes the total inconsistency measured along multi-step subsequences extracted from both both on and off policy traces. An experimental evaluation demonstrates that PCL significantly outperforms strong actor-critic and Q-learning baselines across several benchmark tasks.