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Title: Model-Ensemble Trust-Region Policy Optimization

Paper content:
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.
Deep reinforcement learning has achieved many impressive results in recent years, including learning to play Atari games from raw-pixel inputs BID0 , mastering the game of Go BID1 , as well as learning advanced locomotion and manipulation skills from raw sensory inputs BID3 BID4 .
Many of these results were achieved using model-free reinforcement learning algorithms, which do not attempt to build a model of the environment.
These algorithms are generally applicable, require relatively little tuning, and can easily incorporate powerful function approximators such as deep neural networks.
However, they tend to suffer from high sample complexity, especially when such powerful function approximators are used, and hence their applications have been mostly limited to simulated environments.
In comparison, model-based reinforcement learning algorithms utilize a learned model of the environment to assist learning.
These methods can potentially be much more sample efficient than model-free algorithms, and hence can be applied to real-world tasks where low sample complexity is crucial BID7 BID3 BID8 .
However, so far such methods have required very restrictive forms of the learned models, as well as careful tuning for them to be applicable.
Although it is a straightforward idea to extend model-based algorithms to deep neural network models, so far there has been comparatively fewer successful applications.The standard approach for model-based reinforcement learning alternates between model learning and policy optimization.
In the model learning stage, samples are collected from interaction with the environment, and supervised learning is used to fit a dynamics model to the observations.
In the policy optimization stage, the learned model is used to search for an improved policy.
The underlying assumption in this approach, henceforth termed vanilla model-based RL, is that with enough data, the learned model will be accurate enough, such that a policy optimized on it will also perform well in the real environment.Although vanilla model-based RL can work well on low-dimensional tasks with relatively simple dynamics, we find that on more challenging continuous control tasks, performance was highly unstable.
The reason is that the policy optimization tends to exploit regions where insufficient data is available to train the model, leading to catastrophic failures.
Previous work has pointed out this issue as model bias, i.e. BID7 BID9 BID10 .
While this issue can be regarded as a form of overfitting, we emphasize that standard countermeasures from the supervised learning literature, such as regularization or cross validation, are not sufficient here -supervised learning can guarantee generalization to states from the same distribution as the data, but the policy optimization stage steers the optimization exactly towards areas where data is scarce and the model is inaccurate.
This problem is severely aggravated when expressive models such as deep neural networks are employed.To resolve this issue, we propose to use an ensemble of deep neural networks to maintain model uncertainty given the data collected from the environment.
During model learning, we differentiate the neural networks by varying their weight initialization and training input sequences.
Then, during policy learning, we regularize the policy updates by combining the gradients from the imagined stochastic roll-outs.
Each imagined step is uniformly sampled from the ensemble predictions.
Using this technique, the policy learns to become robust against various possible scenarios it may encounter in the real environment.
To avoid overfitting to this regularized objective, we use the model ensemble for early stopping policy training.Standard model-based techniques require differentiating through the model over many time steps, a procedure known as backpropagation through time (BPTT).
It is well-known in the literature that BPTT can lead to exploding and vanishing gradients BID11 .
Even when gradient clipping is applied, BPTT can still get stuck in bad local optima.
We propose to use likelihood ratio methods instead of BPTT to estimate the gradient, which only make use of the model as a simulator rather than for direct gradient computation.
In particular, we use Trust Region Policy Optimization (TRPO) BID4 , which imposes a trust region constraint on the policy to further stabilize learning.In this work, we propose Model-Ensemble Trust-Region Policy Optimization (ME-TRPO), a modelbased algorithm that achieves the same level of performance as state-of-the-art model-free algorithms with 100× reduction in sample complexity.
We show that the model ensemble technique is an effective approach to overcome the challenge of model bias in model-based reinforcement learning.
We demonstrate that replacing BPTT by TRPO yields significantly more stable learning and much better final performance.
Finally, we provide an empirical analysis of vanilla model-based RL using neural networks as function approximators, and identify its flaws when applied to challenging continuous control tasks.
In this work, we present a simple and robust model-based reinforcement learning algorithm that is able to learn neural network policies across different challenging domains.
We show that our approach significantly reduces the sample complexity compared to state-of-the-art methods while reaching the same level of performance.
In comparison, our analyses suggests that vanilla model-based RL tends to suffer from model bias and numerical instability, and fails to learn a good policy.
We further evaluate the effect of each key component of our algorithm, showing that both using TRPO and model ensemble are essential for successful applications of deep model-based RL.
We also confirm the results of previous work BID7 BID32 BID22 that using model uncertainty is a principled way to reduce model bias.One question that merits future investigation is how to use the model ensemble to encourage the policy to explore the state space where the different models disagree, so that more data can be collected to resolve their disagreement.
Another enticing direction for future work would be the application of ME-TRPO to real-world robotics systems.

assistant: text: Deep Model-Based RL that works well.