Paper:
Deep reinforcement learning (RL) has achieved breakthrough results on many tasks, but has been shown to be sensitive to system changes at test time. As a result, building deep RL agents that generalize has become an active research area. Our aim is to catalyze and streamline community-wide progress on this problem by providing the first benchmark and a common experimental protocol for investigating generalization in RL. Our benchmark contains a diverse set of environments and our evaluation methodology covers both in-distribution and outof-distribution generalization. To provide a set of baselines for future research, we conduct a systematic evaluation of deep RL algorithms, including those that specifically tackle the problem of generalization. We find that classic deep RL algorithms often perform as well or better than those that aim to generalize.

Review:
This paper presents a new benchmark for studying generalization in deep RL along with a set of benchmark results. The benchmark consists of several standard RL tasks like Mountain Car along with several Mujoco continuous control tasks. Generalization is measured with respect to changes in environment parameters like force magnitude and pole length. Both interpolation and extrapolation are considered.
The problem considered in this paper is important and I agree with the authors that a good set of benchmarks for studying generalization is needed. However, a paper proposing a new benchmark should have a good argument for why the set of problems considered is interesting. Similarly, the types of generalization considered should be well motivated. This paper doesn’t do a good job of motivating these choices.
For example, why is Mountain Car a good task for studying generalization in deep RL? Mountain Car is a classic problem with a two-dimensional state space. This is hardly the kind of problem where deep RL shines or is even needed at all. Similarly, why should we care whether an agent trained on the Cart Pole task can generalize to a pole length between 2x and 10x shorter than the one it was trained on without being allowed to update its policy? Both the set of tasks and the distributions of parameters over which generalization is measured seem somewhat arbitrary.
Similarly, the restriction to methods that do not update its policy at test time also seems arbitrary since this is somewhat of a gray area. RL^2, which is one of the baselines in the paper, uses memory to adapt its policy to the current environment at test time. How different is this from an agent that updates its weights at test time? Why allow one but not the other?
In addition to these issues with the proposed benchmark, the baseline results don’t provide any new insights. The main conclusion is that extrapolation is more difficult than interpolation, which is in turn more difficult than training and testing on the same task. Beyond that, the results are very confusing. Two methods for improving generalization (EPOpt and RL^2) are evaluated and both of them seem to mostly decrease generalization performance. I find the poor performance of RL^2-A2C especially worrisome. Isn’t it essentially recurrent A2C where the reward and action are fed in as inputs? Why should the performance drop by 20-40%?
Overall, I don’t see the proposed tasks becoming a widely used benchmark for evaluating generalization in deep RL. There are just too many seemingly arbitrary choices in the design of this benchmark and the lack of interesting findings in the baseline experiments highlights these issues.
Other comments:
- “Massively Parallel Methods for Deep Reinforcement Learning” by Nair et al. introduced the human starts evaluation condition for Atari games in order to measure generalization to potentially unseen states. This should probably be discussed in related work.
- It would be good to include the exact architecture details since it’s not clear how rewards and actions are given to the RL^2 agents.