Paper:
Published as a conference paper at ICLR 2018 NERVENET: LEARNING STRUCTURED POLICY WITH GRAPH NEURAL NETWORKS
We address the problem of learning structured policies for continuous control. In traditional reinforcement learning, policies of agents are learned by multi-layer perceptrons (MLPs) which take the concatenation of all observations from the environment as input for predicting actions. In this work, we propose NerveNet to explicitly model the structure of an agent, which naturally takes the form of a graph. Specifically, serving as the agent's policy network, NerveNet first propagates information over the structure of the agent and then predict actions for different parts of the agent. In the experiments, we first show that our NerveNet is comparable to state-of-the-art methods on standard MuJoCo environments. We further propose our customized reinforcement learning environments for benchmarking two types of structure transfer learning tasks, i.e., size and disability transfer, as well as multi-task learning. We demonstrate that policies learned by NerveNet are significantly more transferable and generalizable than policies learned by other models and are able to transfer even in a zero-shot setting.

Review:
The authors present an interesting application of Graph Neural Networks to learning policies for controlling "centipede" robots of different lengths. They leverage the non-parametric nature of graph neural networks to show that their approach is capable of transferring policies to different robots more quickly than other approaches. The significance of this work is in its application of GNNs to a potentially practical problem in the robotics domain. The paper suffers from some clarity/presentation issues that will need to be improved. Ultimately, the contribution of this paper is rather specific, yet the authors show the clear advantage of their technique for improved performance and transfer learning on some agent types within this domain.
Some comments:
- Significant: A brief statement of the paper's "contributions" is also needed; it is unclear at first glance what portions of the work are the authors' own contributions versus prior work, particularly in the section describing the GNN theory.
- Abstract: I take issue with the phrase "are significantly better than policies learned by other models", since this is not universally true. While there is a clear benefit to their technique for the centipede and snake models, the performance on the other agents is mostly comparable, rather than "significantly better"; this should be reflected in the abstract.
- Figure 1 is instructive, but another figure is needed to better illustrate the algorithm (including how the state of the world is mapped to the graph state h, how these "message" are passed between nodes, and how the final graph states are used to develop a policy). This would greatly help clarity, particularly for those who have not seen GNNs before, and would make the paper more self-contained and easier to follow. The figure could also include some annotated examples of the input spaces of the different joints, etc. Relatedly, Sec. 2.2.2 is rather difficult to follow because of the lack of a figure or concrete example (an example might help the reader understand the procedure without having to develop an intuition for GNNs).
- There is almost certainly a typo in Eq. (4), since it does not contain the aggregated message \bar{m}_u^t.
Smaller issues / typos:
- Abstract: please spell out spell out multi-layer perceptrons (MLP).
- Sec 2.2: "servers" should be "serves"
- "performance By" on page 4 is missing a "."
Pros:
- The paper presents an interesting application of GNNs to the space of reinforcement learning and clearly show the benefits of their approach for the specific task of transfer learning.
- To the best of my knowledge, the paper presents an original result and presents a good-faith effort to compare to existing, alternative systems (showing that they outperform on the tasks of interest).
Cons:
- The contributions of the paper should be more clearly stated (see comment above).
- The section describing their approach is not "self contained" and is difficult for an unlearned reader to follow.
- The problem the authors have chosen to tackle is perhaps a bit "specific", since the performance of their approach is only really shown to exceed the performance on agents, like centipedes or snakes, which have this "modular" quality.
I certainly hope the authors improve the quality of the theory section; the poor presentation here brings down the rest of the paper, which is otherwise an easy read.