Model-free reinforcement learning (RL) methods are succeeding in a growing number of tasks, aided by recent advances in deep learning. However, they tend to suffer from high sample complexity, which hinders their use in real-world domains. Alternatively, model-based reinforcement learning promises to reduce sample complexity, but tends to require careful tuning and to date have succeeded mainly in restrictive domains where simple models are sufficient for learning. In this paper, we analyze the behavior of vanilla model-based reinforcement learning methods when deep neural networks are used to learn both the model and the policy, and show that the learned policy tends to exploit regions where insufficient data is available for the model to be learned, causing instability in training. To overcome this issue, we propose to use an ensemble of models to maintain the model uncertainty and regularize the learning process. We further show that the use of likelihood ratio derivatives yields much more stable learning than backpropagation through time. Altogether, our approach Model-Ensemble Trust-Region Policy Optimization (ME-TRPO) significantly reduces the sample complexity compared to model-free deep RL methods on challenging continuous control benchmark tasks.

通过使用深度神经网络同时学习模型和策略，我们分析了基于模型的增强学习方法的行为，并展示出学习到的策略倾向于利用模型学习不足的区域，导致训练不稳定。为了解决这个问题，我们提出使用模型集合来维护模型的不确定性并规范学习过程，并进一步展示，与“Backpropagation through time”相比，使用“likelihood ratio”导数可以获得更加稳定的学习。通过我们的方法ME-TRPO，在具有挑战性的连续控制基准任务中，显著减少了比基于模型的深度RL方法所需的样本数量。

模型集成信任区域策略优化