A Minimalist Approach to Offline Reinforcement Learning (2106.06860v2)

Published 12 Jun 2021 in cs.LG, cs.AI, and stat.ML

Abstract: Offline reinforcement learning (RL) defines the task of learning from a fixed batch of data. Due to errors in value estimation from out-of-distribution actions, most offline RL algorithms take the approach of constraining or regularizing the policy with the actions contained in the dataset. Built on pre-existing RL algorithms, modifications to make an RL algorithm work offline comes at the cost of additional complexity. Offline RL algorithms introduce new hyperparameters and often leverage secondary components such as generative models, while adjusting the underlying RL algorithm. In this paper we aim to make a deep RL algorithm work while making minimal changes. We find that we can match the performance of state-of-the-art offline RL algorithms by simply adding a behavior cloning term to the policy update of an online RL algorithm and normalizing the data. The resulting algorithm is a simple to implement and tune baseline, while more than halving the overall run time by removing the additional computational overhead of previous methods.

Authors (2)

Scott Fujimoto (17 papers)
Shixiang Shane Gu (34 papers)

Citations (690)

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Summary

A Minimalist Approach to Offline Reinforcement Learning

The paper "A Minimalist Approach to Offline Reinforcement Learning" by Scott Fujimoto and Shixiang Shane Gu presents a streamlined methodology for applying reinforcement learning (RL) in an offline setting. The work focuses on simplifying the integration of offline constraints into existing RL algorithms, minimizing complexity while maintaining competitive performance against state-of-the-art techniques.

Overview

Offline RL, often referred to as batch RL, deals with learning from a static dataset without additional environment interactions. This paradigm poses challenges due to the potential extrapolation error from out-of-distribution actions—actions not encountered during data collection which can lead to inaccurate value estimations. Current solutions typically involve complex modifications to RL algorithms, introducing new hyperparameters and components like generative models.

The authors propose a simplified approach leveraging the TD3 (Twin Delayed Deep Deterministic Policy Gradient) algorithm. Their primary innovation lies in augmenting the policy update step with a behavior cloning (BC) regularization term and normalizing the dataset. By modifying just a few lines of code, they demonstrate that a minimalist algorithm, termed TD3+BC, achieves performance on par with existing complex state-of-the-art offline RL methods.

Methodology

The paper introduces two key changes to the TD3 algorithm:

Behavior Cloning Regularization: A BC term is added to the policy update. This encourages the policy to favor actions present in the dataset:

$\pi = \argmax_\pi E_{(s,a) \sim \mathcal{D}} [\lambda Q(s,\pi(s)) - \|\pi(s) - a\|^2]$

Here, $\lambda$ is a hyperparameter that adjusts the strength of BC.

State Normalization: Features of the dataset states are normalized to improve stability:

$s_i = \frac{s_i - \mu_i}{\sigma_i + \epsilon}$

where $\mu_i$ and $\sigma_i$ are the mean and standard deviation of the dataset's state features.

The adaptation is straightforward and computationally efficient, providing a strong baseline with reduced complexity and runtime.

Empirical Results

The proposed TD3+BC was benchmarked on the D4RL suite—a set of continuous control tasks. It consistently matched or outperformed more sophisticated algorithms like CQL and Fisher-BRC, especially in terms of implementation simplicity and reduced computation time, often halving the required processing overhead of other methods.

The paper also reveals challenges in offline RL, such as variability in policy performance due to the fixed nature of the dataset. These insights underscore the necessity for algorithms that are both effective and easily tunable across various datasets and domains.

Implications and Speculations

The implications of this work are significant for the theoretical and practical development of offline RL. It suggests that simpler methods may offer robust alternatives to complex, resource-intensive approaches. The minimalist strategy could encourage broader accessibility and ease of experimentation in offline RL domains, potentially catalyzing new applications in environments where data collection is constrained.

Further research could focus on refining the balance between RL and imitation terms, or applying similar minimalist principles to other RL frameworks. Investigating the identified challenges, particularly regarding policy variability and robustness, remains a critical avenue for future exploration in offline RL advancements. The paper serves as a compelling reminder of the potential within simplicity and the importance of foundational, rather than purely incremental or complex, advancements in AI research.

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GitHub

GitHub - sfujim/TD3_BC: Author's PyTorch implementation of TD3+BC, a simple variant of TD3 for offline RL (361 stars)