Paper:
In reinforcement learning, robotic control tasks are often useful for understanding how agents perform in environments with deceptive rewards where the agent can easily become trapped into suboptimal solutions. One way to avoid these local optima is to use a population of agents to ensure coverage of the policy space (a form of exploration), yet learning a population with the "best" coverage is still an open problem. In this work, we present a novel approach to population-based RL in continuous control that leverages properties of normalizing flows to perform attractive and repulsive operations between current members of the population and previously observed policies. Empirical results on the MuJoCo suite demonstrate a high performance gain for our algorithm compared to prior work, including Soft-Actor Critic (SAC).

Review:
RL in environments with deceptive rewards can produce sub-optimal policies. To remedy this, the paper proposes a method for population-based exploration. Multiple actors, each parameterized with policies based on Normalizing Flows (radial contractions), are optimized over iterations using the off-policy SAC algorithm. To encourage diverse-exploration as well as high-performance, the SAC-policy-gradient is supplemented with gradient of an “attraction” or “repulsion” term, as defined using the KL-divergence of current policy to another policy from an online archive. When applying the KL-gradient, the authors find it crucial to only update the flow layers, and not the base Gaussian policy.
I have 2 issues with this paper:
1.	Lack of novelty – Most of the components of the proposed algorithm have been researched in other works. SAC with NF was contributed by Mazoure et. al., and this paper seems to use the exact same architecture.  The diversity-enforcing objective (Equation 1.) was proposed by Hong et. al., albeit not with a separate archive of policies, but using the experience replay (for off-policy) and recent policies (for on-policy). The objective is Section 3.2 is also the same as that in Hong et. al. 
2.	I have major concerns about how the results have been reported:
     a.	The authors have omitted SAC and SAC-NF from Figure 4, and therefore the figure gives the impression that ARAC is significantly better than the presented baselines in some of the environments. For example, take Humanoid-v2. The original SAC paper reports reaching close to 5k score in about 0.5 million timesteps. It therefore seems that most of the benefit of ARAC (over the presented baselines) comes just from using SAC. Another example is Humanoid-sparse, where SAC-NF achieves ~550 (from Table 1), but is not shown in the figure, making ARAC (score ~600) look awesome. The authors also incorrectly mention in text that “ARAC is the only method that can achieve non-zero performance” on this.
     b.	Table 1 reports “maximum” average return. This is very misleading and non-standard in RL. Take Humanoid-sparse for instance. The number reported for ARAC is 816 which is the peak performance during training. As we see in Figure 4, ARAC is unstable and the perf. drops to ~600 at end of training. The peak performance during training is an irrelevant metric.
     c.	Humanoid-rllab range on y-axis looks incorrect
Minor points:
For creating a diverse archive, I’m not sure if k-means on the episodic-returns is the most effective mechanism. As explored in Conti et al., and also mentioned in the introduction of this paper, behavioral-diversity is a more useful characterization, and episodic-returns may not be aligned to that (especially in sparse reward environments).
Some of the missing related work (on population-based diversity): DIAYN, Learning Self-Imitating Diverse Policies, Divide and Conquer RL.