Paper:
We introduce SLM Lab, a software framework for reproducible reinforcement learning (RL) research. SLM Lab implements a number of popular RL algorithms, provides synchronous and asynchronous parallel experiment execution, hyperparameter search, and result analysis. RL algorithms in SLM Lab are implemented in a modular way such that differences in algorithm performance can be confidently ascribed to differences between algorithms, not between implementations. In this work we present the design choices behind SLM Lab and use it to produce a comprehensive single-codebase RL algorithm benchmark. In addition, as a consequence of SLM Lab's modular design, we introduce and evaluate a discrete-action variant of the Soft Actor-Critic algorithm (Haarnoja et al., 2018)   and a hybrid synchronous/asynchronous training method for RL agents.

Review:
This paper presents a new RL library called « SLM Lab ». Its most relevant features for RL research are: (1) modularity to help re-use existing components (thus reducing the risk of subtle implementation differences when comparing algorithms), (2) implementations of most popular algorithms like DQN & variants, A3C, PPO, SAC, (3) ability to parallelize both actors (through vectorized environments) and the learner (through distributed gradient descent), and (4) utilities for hyper-parameter optimization, reproducible experiments and reporting. The paper also reports performance over Atari games, Roboschool environments as well as some Unity ML-Agents tasks. Finally, it provides a high-level overview of SLM Lab’s capabilities compared to 23 other RL open source libraries, showing that it is the only one that combines: reporting of the performance of the implemented algorithms, ability to specify hyper-parameters in the config file, parallelization, hyper-parameter optimization, and visualization of the results.
Overall this looks like a solid RL library, but I am not convinced that it brings enough novelty to the RL software landscape for a published ICLR paper — it would better fit in a workhop dedicated to ML libraries for instance, thus the weak reject.
Table 3 shows that SLM Lab is the only RL library with such a broad offering of features, and this is definitely impressive, but I would argue that many of these features can typically be added to other libraries by plugging in other open source software. For instance there are several tools for experiment management and hyper-parameter optimization (and for RLLib in particular, hyper-parameter optimization is not checked but is straightforward with Ray Tune). TensorBoard can also often be easily used for visualization.
The parallelization capabilities of SLM Lab also seem limited: if I understand correctly, actor parallelization can only occur on a single machine, and thus an algorithm like Ape-X or R2D2 could not be implemented. If this is correct then it is a major limitation of the framework, since such parallelization across multiple computers can be extremely useful when environments are slow and costly to run (in CPU / RAM).
It is not clear to me to which extent multi-agent is supported. It seems like it is possible to have multiple agents in one environment, but is that enough for general multi-agent RL? (ex: how to specify individual / team rewards? share information between agents? deal with agents not acting all at the same timestep? centralize part of the training / execution?…)
I appreciate that the paper presents a discrete version of SAC, mentioning how easy it was to implement thanks to the modular design of SLM Lab, but results from Table 1 do not look very good (especially since it did not work on some of the environments). Relying on the Gumbel-softmax might not be the most robust & stable way to train a discrete SAC — see e.g. the recent « Soft Actor-Critic for Discrete Action Settings » for a different approach.
Finally, it is also great to have some benchmarks of the algorithms being implemented, but at least for Atari, I am not aware of previous work using the exact same evaluation setting, so it is hard to tell how they compare to other implementations.
In spite of the above, I do not mean to criticize SLM Lab too heavily as from what I can tell it seems to be a a solid library with many useful features, and I am sure many researchers will find it useful in their day-to-day work.
Minor points:
- Anonymity was clearly violated with the two github links
- Having some synthetic result on Atari (like the typically reported median human-normalized score) would be good
- I am personally not a fan of large config JSON files due to the lack of comments in JSON
- A.5 (« Key Implementation Lessons ») is great!
Review update after author feedback: I am on the fence for this paper, but still leaning towards rejection due to the fact I am still not convinced that this library brings that much novelty compared to existing other libraries (although it seems like a nice RL library, I am not sure ICLR is the right venue for talking about it). The authors argue that their benchmark results are a key contribution of the paper, but I do not find these results particularly insightful, especially the Atari ones that are not comparable to previous results (due to using a different evaluation method) and the lack of state-of-the-art algorithms like Rainbow or IQN.