PPO-Clip Attains Global Optimality: Towards Deeper Understandings of Clipping

• URL: http://arxiv.org/abs/2312.12065v2
• Date: Mon, 19 Feb 2024 11:27:21 GMT
• Title: PPO-Clip Attains Global Optimality: Towards Deeper Understandings of Clipping
• Authors: Nai-Chieh Huang, Ping-Chun Hsieh, Kuo-Hao Ho, I-Chen Wu
• Abstract summary: We establish the first global convergence results of a PPO-Clip variant in both tabular and neural function approximation settings. Our theoretical findings also mark the first characterization of the influence of the clipping mechanism on PPO-Clip convergence.
• Score: 16.772442831559538
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Proximal Policy Optimization algorithm employing a clipped surrogate objective (PPO-Clip) is a prominent exemplar of the policy optimization methods. However, despite its remarkable empirical success, PPO-Clip lacks theoretical substantiation to date. In this paper, we contribute to the field by establishing the first global convergence results of a PPO-Clip variant in both tabular and neural function approximation settings. Our findings highlight the $O(1/\sqrt{T})$ min-iterate convergence rate specifically in the context of neural function approximation. We tackle the inherent challenges in analyzing PPO-Clip through three central concepts: (i) We introduce a generalized version of the PPO-Clip objective, illuminated by its connection with the hinge loss. (ii) Employing entropic mirror descent, we establish asymptotic convergence for tabular PPO-Clip with direct policy parameterization. (iii) Inspired by the tabular analysis, we streamline convergence analysis by introducing a two-step policy improvement approach. This decouples policy search from complex neural policy parameterization using a regression-based update scheme. Furthermore, we gain deeper insights into the efficacy of PPO-Clip by interpreting these generalized objectives. Our theoretical findings also mark the first characterization of the influence of the clipping mechanism on PPO-Clip convergence. Importantly, the clipping range affects only the pre-constant of the convergence rate.

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