Deterministic Policy Gradients With General State Transitions

TL;DR

This paper extends deterministic policy gradient methods to a new setting with mixed stochastic and deterministic state transitions, providing theoretical guarantees, a novel algorithm, and empirical evidence of improved performance.

Contribution

It introduces a generalized setting for deterministic policy gradients, proves their existence under certain conditions, and proposes the GDPG algorithm combining model-based and model-free techniques.

Findings

GDPG outperforms DDPG and other baselines in convergence and rewards

Theoretical proof of policy gradient existence in generalized setting

Closed-form expression for the policy gradient

Abstract

We study a reinforcement learning setting, where the state transition function is a convex combination of a stochastic continuous function and a deterministic function. Such a setting generalizes the widely-studied stochastic state transition setting, namely the setting of deterministic policy gradient (DPG). We firstly give a simple example to illustrate that the deterministic policy gradient may be infinite under deterministic state transitions, and introduce a theoretical technique to prove the existence of the policy gradient in this generalized setting. Using this technique, we prove that the deterministic policy gradient indeed exists for a certain set of discount factors, and further prove two conditions that guarantee the existence for all discount factors. We then derive a closed form of the policy gradient whenever exists. Furthermore, to overcome the challenge of high sample complexity of DPG in this setting, we propose the Generalized Deterministic Policy Gradient (GDPG) algorithm. The main innovation of the algorithm is a new method of applying model-based techniques to the model-free algorithm, the deep deterministic policy gradient algorithm (DDPG). GDPG optimize the long-term rewards of the model-based augmented MDP subject to a constraint that the long-rewards of the MDP is less than the original one. We finally conduct extensive experiments comparing GDPG with state-of-the-art methods and the direct model-based extension method of DDPG on several standard continuous control benchmarks. Results demonstrate that GDPG substantially outperforms DDPG, the model-based extension of DDPG and other baselines in terms of both convergence and long-term rewards in most environments.