Generalization in Reinforcement Learning with Selective Noise Injection and Information Bottleneck -
Coming from the perspective of the rest of machine learning, a somewhat odd thing about reinforcement learning that often goes unnoticed is the fact that, in basically all reinforcement learning, performance of an algorithm is judged by its performance on the same environment it was trained on. In the parlance of ML writ large: training on the test set. In RL, most of the focus has historically been on whether automatic systems would be able to learn a policy from the state distribution of a single environment, already a fairly hard task. But, now that RL has had more success in the single-environment case, there comes the question: how can we train reinforcement algorithms that don't just perform well on a single environment, but over a range of environments. One lens onto this question is that of meta-learning, but this paper takes a different approach, and looks at how straightforward regularization techniques pulled from the land of supervised learning can (or can't straightforwardly) be applied to reinforcement learning. In general, the regularization techniques discussed here are all ways of reducing the capacity of the model, and preventing it from overfitting. Some ways to reduce capacity are: - Apply L2 weight penalization - Apply dropout, which handicaps the model by randomly zeroing out neurons - Use Batch Norm, which uses noisy batch statistics, and increases randomness in a way that, similar to above, deteriorates performance - Use an information bottleneck: similar to a VAE, this approach works by learning some compressed representation of your input, p(z|x), and then predicting your output off of that z, in a way that incentivizes your z to be informative (because you want to be able to predict y well) but also penalizes too much information being put in it (because you penalize differences between your learned p(z|x) distribution and an unconditional prior p(z) ). This pushes your model to use its conditional-on-x capacity wisely, and only learn features if they're quite valuable in predicting y However, the paper points out that there are some complications in straightforwardly applying these techniques to RL. The central one is the fact that in (most) RL, the distribution of transitions you train on comes from prior iterations of your policy. This means that a noisier and less competent policy will also leave you with less data to train on. Additionally, using a noisy policy can increase variance, both by making your trained policy more different than your rollout policy (in an off-policy setting) and by making your estimate of the value function higher-variance, which is problematic because that's what you're using as a target training signal in a temporal difference framework. The paper is a bit disconnected in its connection between justification and theory, and makes two broad, mostly distinct proposals: 1. The most successful (though also the one least directly justified by the earlier-discussed theoretical difficulties of applying regularization in RL) is an information bottleneck ported into a RL setting. It works almost the same as the classification-model one, except that you're trying to increase the value of your actions given compressed-from-state representation z, rather than trying to increase your ability to correctly predict y. The justification given here is that it's good to incentivize RL algorithms in particular to learn simpler, more compressible features, because they often have such poor data and also training signal earlier in training 2. SNI (Selective Noise Injection) works by only applying stochastic aspects of regularization (sampling from z in an information bottleneck, applying different dropout masks, etc) to certain parts of the training procedure. In particular, the rollout used to collect data is non-stochastic, removing the issue of noisiness impacting the data that's collected. They then do an interesting thing where they calculate a weighted mixture of the policy update with a deterministic model, and the update with a stochastic one. The best performing of these that they tested seems to have been a 50/50 split. This is essentially just a knob you can turn on stochasticity, to trade off between the regularizing effect of noise and the variance-increasing-negative effect of it. Based on my read of the experiments in the paper, the most impressive thing here is how well their information bottleneck mechanism works as a way to improve generalization, compared to both the baseline and other regularization approaches. It does look like there's some additional benefit to SNI, particularly in the CoinRun setting, but very little in the MultiRoom setting, and in general the difference is less dramatic than the difference from using the information bottleneck.