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Title: CEM-RL: Combining evolutionary and gradient-based methods for policy search

Paper content:
Deep neuroevolution and deep reinforcement learning (deep RL) algorithms are two popular approaches to policy search.
The former is widely applicable and rather stable, but suffers from low sample efficiency.
By contrast, the latter is more sample efficient, but the most sample efficient variants are also rather unstable and highly sensitive to hyper-parameter setting.
So far, these families of methods have mostly been compared as competing tools.
However, an emerging approach consists in combining them so as to get the best of both worlds.
Two previously existing combinations use either an ad hoc evolutionary algorithm or a goal exploration process together with the Deep Deterministic Policy Gradient (DDPG) algorithm, a sample efficient off-policy deep RL algorithm.
In this paper, we propose a different combination scheme using the simple cross-entropy
method (CEM) and Twin Delayed Deep Deterministic policy gradient (TD3), another off-policy deep RL algorithm which improves over DDPG.
We evaluate the resulting method, CEM-RL, on a set of benchmarks classically used in deep RL.
We show that CEM-RL benefits from several advantages over its competitors and offers a satisfactory trade-off between performance and sample efficiency.
Policy search is the problem of finding a policy or controller maximizing some unknown utility function.
Recently, research on policy search methods has witnessed a surge of interest due to the combination with deep neural networks, making it possible to find good enough continuous action policies in large domains.
From one side, this combination gave rise to the emergence of efficient deep reinforcement learning (deep RL) techniques BID17 BID25 .
From the other side, evolutionary methods, and particularly deep neuroevolution methods applying evolution strategies (ESs) to the parameters of a deep network have emerged as a competitive alternative to deep RL due to their higher parallelization capability BID23 .Both
families of techniques have clear distinguishing properties. Evolutionary
methods are significantly less sample efficient than deep RL methods because they learn from complete episodes, whereas deep RL methods use elementary steps of the system as samples, and thus exploit more information BID27 . In particular
, off-policy deep RL algorithms can use a replay buffer to exploit the same samples as many times as useful, greatly improving sample efficiency. Actually, the
sample efficiency of ESs can be improved using the "importance mixing" mechanism, but a recent study has shown that the capacity of importance mixing to improve sample efficiency by a factor of ten is still not enough to compete with off-policy deep RL BID22 . From the other
side, sample efficient off-policy deep RL methods such as the DDPG algorithm BID17 are known to be unstable and highly sensitive to hyper-parameter setting. Rather than opposing
both families as competing solutions to the policy search problem, a richer perspective consists in combining them so as to get the best of both worlds. As covered in Section
2, there are very few attempts in this direction so far.After presenting some background in Section 3, we propose in Section 4 a new combination method that combines the cross-entropy method (CEM) with DDPG or TD3, an off-policy deep RL algorithm which improves over DDPG. In Section 5, we investigate
experimentally the properties of this CEM-RL method, showing its advantages both over the components taken separately and over a competing approach. Beyond the results of CEM-RL
, the conclusion of this work is that there is still a lot of unexplored potential in new combinations of evolutionary and deep RL methods.
We advocated in this paper for combining evolutionary and deep RL methods rather than opposing them.
In particular, we have proposed such a combination, the CEM-RL method, and showed that in most cases it was outperforming not only some evolution strategies and some sample efficient offpolicy deep RL algorithms, but also another combination, the ERL algorithm.
Importantly, despite being mainly an evolutionary method, CEM-RL is competitive to the state-of-the-art even when considering sample efficiency, which is not the case of other deep neuroevolution methods BID24 .Beyond
these positive performance results, our study raises more fundamental questions. First,
why does the simple CEM algorithm perform so well on the SWIMMER-V2 benchmark? Then,
our empirical study of importance mixing did not confirm a clear benefit of using it, neither did the effect of adding noise on actions. We suggest
explanations for these phenomena, but nailing down the fundamental reasons behind them will require further investigations. Such deeper
studies will also help understand which properties are critical in the performance and sample efficiency of policy search algorithms, and define even more efficient policy search algorithms in the future. As suggested
in Section 5.2.3, another avenue for future work will consist in designing an ERL algorithm based on CEM rather than on an ad hoc evolutionary algorithm. Finally, given
the impact of the neural architecture on our results, we believe that a more systemic search of architectures through techniques such as neural architecture search (Zoph & Le, 2016; BID7 may provide important progress in performance of deep policy search algorithms.

assistant: text: We propose a new combination of evolution strategy and deep reinforcement learning which takes the best of both worlds