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Title: Towards Simplicity in Deep Reinforcement Learning: Streamlined Off-Policy Learning

Paper content:
The field of Deep Reinforcement Learning (DRL) has recently seen a surge in the popularity of maximum entropy reinforcement learning algorithms.
Their popularity stems from the intuitive interpretation of the maximum entropy objective and their superior sample efficiency on standard benchmarks.
In this paper, we seek to understand the primary contribution  of the entropy term to the performance of maximum entropy algorithms.
For the Mujoco benchmark, we demonstrate that the entropy term in Soft Actor Critic (SAC) principally addresses the bounded nature of the action spaces.
With this insight, we propose a simple normalization scheme which allows a streamlined algorithm without entropy maximization match the performance of SAC.
Our experimental results demonstrate a need to revisit the benefits of entropy regularization in DRL.
We also propose a simple non-uniform sampling method for selecting transitions from the replay buffer during training.
We further show that the streamlined algorithm with the simple non-uniform sampling scheme outperforms SAC and achieves state-of-the-art performance on challenging continuous control tasks.
Off-policy Deep Reinforcement Learning (RL) algorithms aim to improve sample efficiency by reusing past experience.
Recently a number of new off-policy Deep Reinforcement Learning algorithms have been proposed for control tasks with continuous state and action spaces, including Deep Deterministic Policy Gradient (DDPG) and Twin Delayed DDPG (TD3) (Lillicrap et al., 2015; Fujimoto et al., 2018) .
TD3, which introduced clipped double-Q learning, delayed policy updates and target policy smoothing, has been shown to be significantly more sample efficient than popular on-policy methods for a wide range of Mujoco benchmarks.
The field of Deep Reinforcement Learning (DRL) has also recently seen a surge in the popularity of maximum entropy RL algorithms.
Their popularity stems from the intuitive interpretation of the maximum entropy objective and their superior sample efficiency on standard benchmarks.
In particular, Soft Actor Critic (SAC), which combines off-policy learning with maximum-entropy RL, not only has many attractive theoretical properties, but can also give superior performance on a wide-range of Mujoco environments, including on the high-dimensional environment Humanoid for which both DDPG and TD3 perform poorly (Haarnoja et al., 2018a; b; Langlois et al., 2019) .
SAC has a similar structure to TD3, but also employs maximum entropy reinforcement learning.
In this paper, we first seek to understand the primary contribution of the entropy term to the performance of maximum entropy algorithms.
For the Mujoco benchmark, we demonstrate that when using the standard objective without entropy along with standard additive noise exploration, there is often insufficient exploration due to the bounded nature of the action spaces.
Specifically, the outputs of the policy network are often way outside the bounds of the action space, so that they need to be squashed to fit within the action space.
The squashing results in actions persistently taking on their maximal values, so that there is insufficient exploration.
In contrast, the entropy term in the SAC objective forces the outputs to have sensible values, so that even with squashing, exploration is maintained.
We conclude that the entropy term in the objective for Soft Actor Critic principally addresses the bounded nature of the action spaces in the Mujoco environments.
With this insight, we propose Streamlined Off Policy (SOP), a streamlined algorithm using the standard objective without the entropy term.
SOP employs a simple normalization scheme to address the bounded nature of the action spaces, allowing satisfactory exploration throughout training.
We also consider replacing the aforementioned normalization scheme with inverting gradients (IG) The contributions of this paper are thus threefold.
First, we uncover the primary contribution of the entropy term of maximum entropy RL algorithms when the environments have bounded action spaces.
Second, we propose a streamlined algorithm which do not employ entropy maximization but nevertheless matches the sampling efficiency and robustness performance of SAC for the Mujoco benchmarks.
And third, we combine our streamlined algorithms with a simple non-uniform sampling scheme to achieve state-of-the art performance for the Mujoco benchmarks.
We provide anonymized code for reproducibility 1 .
In this paper we first showed that the primary role of maximum entropy RL for the Mujoco benchmark is to maintain satisfactory exploration in the presence of bounded action spaces.
We then developed a new streamlined algorithm which does not employ entropy maximization but nevertheless matches the sampling efficiency and robustness performance of SAC for the Mujoco benchmarks.
Our experimental results demonstrate a need to revisit the benefits of entropy regularization in DRL.
Finally, we combined our streamlined algorithm with a simple non-uniform sampling scheme to achieve state-of-the art performance for the Mujoco benchmark.

assistant: text: We propose a new DRL off-policy algorithm achieving state-of-the-art performance.