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Title: Anticipatory Asynchronous Advantage Actor-Critic (A4C): The power of Anticipation in Deep Reinforcement Learning

Paper content:
We propose to extend existing deep reinforcement learning  (Deep RL) algorithms by allowing them to additionally choose sequences of actions as a part of their policy.
This modification forces the network to anticipate the reward of action sequences, which, as we show, improves the exploration leading to better convergence.
Our proposal is simple, flexible, and can be easily incorporated into any Deep RL framework.
We show the power of our scheme by consistently outperforming the state-of-the-art GA3C algorithm on several popular Atari Games.
Basic reinforcement learning has an environment and an agent.
The agent interacts with the environment by taking some actions and observing some states and rewards.
At each time step t, the agent observes a state s t and performs an action a t based on a policy π(a t |s t ; θ).
In return to the action, the environment provides a reward r t and the next state s t+1 .
This process goes on until the agent reaches a terminal state.
The learning goal is to find a policy that gives the best overall reward.
The main challenges here are that the agent does not have information about the reward and the next state until the action is performed.
Also, a certain action may yield low instant reward, but it may pave the way for a good reward in the future.Deep Reinforcement Learning BID6 has taken the success of deep supervised learning a step further.
Prior work on reinforcement learning suffered from myopic handcrafted designs.
The introduction of Deep Q-Learning Networks (DQN) was the major advancement in showing that Deep Neural Networks (DNNs) can approximate value and policy functions.
By storing the agent's data in an experience replay memory, the data can be batched BID8 BID9 or randomly sampled BID4 BID12 from different time-steps and learning the deep network becomes a standard supervised learning task with several input-output pairs to train the parameters.
As a consequence, several video games could be played by directly observing raw image pixels BID1 and demonstrating super-human performance on the ancient board game Go .In
order to solve the problem of heavy computational requirements in training DQN, several followups have emerged leading to useful changes in training formulations and DNN architectures. Methods
that increase parallelism while decreasing the computational cost and memory footprint were also proposed BID7 BID6 , which showed impressive performance.A breakthrough was shown in BID6 , where the authors propose a novel lightweight and parallel method called Asynchronous Advantage Actor-Critic (A3C). A3C achieves
the stateof-the-art results on many gaming tasks. When the proper
learning rate is used, A3C learns to play an Atari game from raw screen inputs more quickly and efficiently than previous methods. In a remarkable
followup to A3C, BID0 proposed a careful implementation of A3C on GPUs(called GA3C) and showed the A3C can accelerated significantly over GPUs, leading to the best publicly available Deep RL implementation, known till date.Slow Progress with Deep RL: However, even for very simple Atari games, existing methods take several hours to reach good performance. There is still
a major fundamental barrier in the current Deep RL algorithms, which is slow progress due to poor exploration. During the early
phases, when the network is just initialized, the policy is nearly random. Thus, the initial
experience are primarily several random sequences of actions with very low rewards. Once, we observe
sequences which gives high rewards, the network starts to observe actions and associate them with positive rewards and starts learning. Unfortunately, finding
a good sequence via network exploration can take a significantly long time, especially when the network is far from convergence and the taken actions are near random. The problem becomes more
severe if there are only very rare sequence of actions which gives high rewards, while most others give on low or zero rewards. The exploration can take
a significantly long time to hit on those rare combinations of good moves.In this work, we show that there is an unusual, and surprising, opportunity of improving the convergence of deep reinforcement learning. In particular, we show that
instead of learning to map the reward over a basic action space A for each state, we should force the network to anticipate the rewards over an enlarged action space A + = K k=1 A k which contains sequential actions like (a 1 , a 2 , ..., a k ). Our proposal is a strict generalization
of existing Deep RL framework where we allow to take a premeditated sequence of action at a given state s t , rather than only taking a single action and re-deciding the next action based on the outcome of the first action and so on. Thus the algorithm can pre-decide on a
sequence of actions, instead of just the next best action, if the anticipated reward of the sequence is good enough.Our experiments shows that by simply making the network anticipate the reward for a sequence of action, instead of just the next best actions, the network shows significantly better convergence behavior consistently. We even outperform the fastest known implementation
, the GPU accelerated version of A3C (GA3C). The most exciting part is that that anticipation can
be naturally incorporated in any existing implementation, including Deep Q Network and A3C. We simply have to extend the action set to also include
extra sequences of actions and calculate rewards with them for training, which is quite straightforward.
We propose a simple yet effective technique of adding anticipatory actions to the state-of-the-art GA3C method for reinforcement learning and achieve significant improvements in convergence and overall scores on several popular Atari-2600 games.
We also identify issues that challenge the sustainability of our approach and propose simple workarounds to leverage most of the information from higher-order action space.There is scope for even higher order actions.
However, the action space grows exponentially with the order of anticipation.
Addressing large action space, therefore, remains a pressing concern for future work.
We believe human behavior information will help us select the best higher order actions.

assistant: text: Anticipation improves convergence of deep reinforcement learning.