summaries: This paper introduces a novel approach to learning parameterized policies. The way I understand the work: it jointly optimizes a policy objective, a probabilistic dynamic model objective, and a compression objective. A bottleneck encoding distribution is key here, which is subject to both model predictions (given previous action and encoding) and compression (given current observation). In spirit, it is similar to variational information bottleneck (VIB) but can be seen as an extension of that to RL. So, the interpretation of this optimization is that the agent is aiming for encoding distribution that is maximally predictive of the next observation and maximally compressive of the observation while the agent is maximizing the future aggregate reward. The idea is neat. The optimization can also be easily put together. Some theoretical and empirical results are given.  In this paper, the authors propose a new reinforcement learning method, called robust predictable control, that optimizes a policy under limited representational resources. Similar to previous methods, they formalize the problem in a fully observable MDP and learn a compressed representation of the state space using a variational information bottleneck constraint. Crucially, they apply the bottleneck on sequences of states. Through a clever factorization of the variational prior, they obtain a prior dynamics model in the compressed state space. This dynamics model is predictive and the agent only needs additional representational resources if the encoded states are not predicted by the prior dynamics already. Using a set of standard RL benchmarks, the authors show that this set up improves compression and enforces predictable and robust behaviors .  This paper proposes the method "robust predictable control (RPC)". It proposes to use a information bottleneck between inputs and actions. However, in contrast to previous works, it does so not on a per-observation basis, but on a per-sequence basis using a latent model. Similar to some of the prior work, the encoding cost is not added to the loss, but subtracted from the reward.  The paper "Robust Predictable Control" describes a method to learn policies with an information bottleneck between environment observations and a latent representation which is used for decision-making. The bottleneck is posed between *sequences* of observation and latent states, in contrast to prior work, where a bottleneck is posed between individual states and latents independently. A crucial component of the proposed method is an action-conditioned dynamics model in the latent space. The proposed training objective is jointly minimized by training an encoder, latent dynamics model, and policy.  First, this encourages the encoder and dynamics model to yield good predictions of encoded environment observations.  Furthermore, by training the policy, the expected task return is maximized while ensuring that the behaviour of the agent is well-predictable.  The latent dynamics model formulation can be exploited to derive theoretical guarantees between open-loop and information-constrained closed-loop control.  Several experiments show that the information-constrained agent performs well in the nominal case but also on distorted environments and offers smooth transitions between different behaviours depending on the tightness of the information constraint.
abstractText: Many of the challenges facing today’s reinforcement learning (RL) algorithms, such as robustness, generalization, transfer, and computational efficiency, are closely related to compression. Prior work has convincingly argued why minimizing information is useful in the supervised learning setting, but standard RL algorithms lack an explicit mechanism for compression. The RL setting is unique because (1) its sequential nature allows an agent to use past information to avoid looking at future observations and (2) the agent can optimize its behavior to prefer states where decision making requires few bits. We take advantage of these properties to propose a method (RPC) for learning simple policies. This method brings together ideas from information bottlenecks, model-based RL, and bits-back coding into a simple and theoretically-justified algorithm. Our method jointly optimizes a latent-space model and policy to be self-consistent, such that the policy avoids states where the model is inaccurate. We demonstrate that our method achieves much tighter compression than prior methods, yielding up to 5× higher reward than a standard information bottleneck. As a result of this compression, the policies learned by our method are robust and generalize well to new tasks.1
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sections.heading: 1 Introduction
sections.text: Many areas of reinforcement learning (RL) research focus on specialized problems, such as learning invariant representations, improving robustness to adversarial attacks, improving generalization, or building better world models. These problems are often symptoms of a deeper underlying problem: autonomous agents use too many bits from their environment. For the purpose of decision making, most information about the world is irrelevant. For example, a lane keeping feature on a car may take as input high-resolution camera input (millions of bits), but only needs to extract a few bits of information about the relative orientation of the car in the lane. Agents that rely on more bits of information run the risk of overfitting to the training task.
  Agents that use few bits of information enjoy a number of appealing properties. These agents can better cope with high-dimensional sensory inputs (e.g., dozens of cameras on a self-driving car) and will be forced to learn representations that are more broadly applicable. Agents that throw away most information will be agnostic to idiosyncrasies in observations, providing robustness to missing or corrupted observations and allowing for transfer to different scenarios. For example, if an agent ignores 99.9% of bits, then corrupting a random bit is unlikely to change the agent’s behavior. Moreover, an agent that minimizes bits will prefer states where the dynamics are easy to predict, meaning that the agent’s resulting behavior will be easier to model. Thus, compression not only changes an agent’s representation, but also changes its behavior: an agent that can only use a limited number of bits will avoid risky behaviors that require more bits to execute (see Fig. 3a).
  The generalization and robustness of a machine learning model is directly related to the complexity of that model. Indeed, standard techniques for reducing complexity, such as the information bottleneck [1, 43], can be directly applied to the RL setting [14, 21, 31, 42]. While these approaches make
  1Project site with videos and code: https://ben-eysenbach.github.io/rpc
  35th Conference on Neural Information Processing Systems (NeurIPS 2021).
  the policy’s action a simple function of the state, they ignore the temporal dimension of decision making. Instead, we will focus on learning policies whose temporally-extended behavior is simple, where we use compression to measure simplicity. Our key observation is that a policy’s behavior is simple if it is predictable.
  Our method improves upon prior methods that apply an information bottleneck to RL [14, 21, 31] by recognizing two important properties of the decision making setting. First, because agents make a sequence of decisions, they can use salient information at one time step to predict salient information at the next time step. These predictions can decrease the amount of information that the agent needs to sense from the environment. We will show that learning a predictive model is not an ad-hoc heuristic, but rather a direct consequence of minimizing information using bits-back coding [12, 19]. Second, unlike supervised learning, the agent can change the distribution over states, choosing behaviors that visit states that are easier to compress. For example, imagine driving on a crowded road. Aggressively passing and tailgating other cars may result in reaching the destination faster, but requires careful attention to other vehicles and fast reactions. In contrast, a policy optimized for using few bits would not pass other cars and would leave a larger following distance (see Fig. 3b). Combined, these two capabilities result in a method that jointly trains a latent space model and a control policy, with the policy being rewarded for visiting states where that model is accurate. Unlike typical model-based methods, our method explicitly optimizes for the accuracy of open-loop planning and results in a model and policy that are self-consistent.
  The main contribution of this paper is an RL algorithm, robust predictable control (RPC), for learning policies that use few bits of information. We will refer to such policies as compressed policies. RPC brings together ideas from information bottlenecks, model-based RL, and bits-back coding into a simple and theoretically-justified algorithm. RPC attains up to 5× higher return than a standard information bottleneck when compared at the same bitrate. Experiments demonstrate that the compressed policies learned by RPC are more robust than those learned by alternative approaches, generalize well to new tasks, and learn behaviors that can be composed for hierarchical RL.
sections.heading: 2 Related Work
sections.text: Our work builds on a large body of prior work, both in supervised learning and reinforcement learning, that argues that compression is a good objective for learning representations and improving robustness [4, 19, 27, 30, 38, 39, 42, 44]. We measure compression by the mutual information between its inputs and outputs [4, 43], a metric that has been used to study the generalization properties [43] and representations learned by neural networks [1]. Our work extends these results to the RL setting by observing that the agent can change its behavior (i.e., the data distribution) to be more easily compressed. In the RL community, prior work has used the variational information bottleneck (VIB) [2] to minimize communication between agents in a multi-agent setting [45] and to improve exploration in a goal-reaching setting [14]. The most related RL methods are those that use an information bottleneck in RL to improve generalization [21, 31]. Whereas these prior methods compress observations individually, we will compress sequences of observations, similar to a sequential VAE [8, 23]. This difference, which corresponds to learning a latent-space model, improves compression and increases robustness on downstream tasks.
  Prior work in RL has sought robustness, transfer, and generalization in many ways. For example, prior RL methods often use explicit representation learning objectives to accelerate learning of image-based tasks [13, 26, 28, 36]. The presentations learned by our method, like those learned by prior contrastive learning methods [26, 34, 36], do not require reconstructing observations. Prior work has also studied numerous strategies for learning policies that are resilient to perturbations in the environment, a problem known as robust RL [20, 22, 33, 41]. While prior robust RL methods typically involve solving a two-player game, we show that compression is a simpler mechanism for achieving some robustness benefits. Finally, the problem of learning RL policies that generalize has been studied by many prior papers [9, 10].
sections.heading: 3 Reinforcement Learning with Fewer Bits
sections.text: This section introduces the idea that predicting the future allows RL policies to operate with fewer bits. We derive this idea from first principles, develop it into a complete RL method, then discuss connections with model-based RL, bits-back coding, and other related topics.
sections.heading: 3.1 Notation and Preliminaries
sections.text: An agent interacts in an MDP defined by states st and actions at; we will use “states” and “observations” interchangeably. The agent samples actions from a policy πθ(at | st, st−1, st−2, · · · ).2 We will construct this policy by learning an encoder φ(zt | st) (which encodes a state st into a representation zt), and a high-level policy πzθ(at | zt), which decodes a representation zt into an action at. The environment dynamics are defined by an initial state s1 ∼ p1(s1) and a transition function p(st+1 | st,at). The RL objective is to maximize the expected γ-discounted sum of rewards: maxπ Eπ [ ∑∞ t=1 γ
  tr(st,at)]. The discount factor can be viewed as saying that the episode terminates at every time step with probability (1− γ), an interpretation we will use in Sec. 3.2. A model is simpler if it expresses a simpler input-output relationship [4, 43]. We will measure the complexity of a function using an information bottleneck, which is the mutual information I(x;y) between an input x and an output y [1, 43]. The variational information bottleneck provides a tractable upper bound on mutual information [1, 2]:
  I(x;y) ≤ Ep(x,y) [ log ( p(y | x) m(y) )] ,
  where m(y) is an arbitrary prior distribution. Applying the information bottleneck to an intermediate layer z = φ(x) is sufficient for bounding the mutual information between input x and output y.
  Following prior work on compression in RL [21, 31], we aim to maximize rewards while minimizing the number of bits. While there are many ways to apply compression to RL (e.g., compressing actions, goals, or individual observations), we will focus on compressing sequences of states. The input is a sequence of states, s1:∞ , (s1, s2, · · · ); the output is a sequence of actions, a1:∞ , (a1,a2, · · · ). The objective is to learn a representation φθ(zt | st) and policy πθ(at | zt) that maximize reward, subject to the constraint that the policy uses on average C > 0 bits of information per episode:
  max θ
  Eπ,φ [ ∞∑ t=1 γtr(st,at) ] s.t. Eπ[I(s1:∞; z1:∞)] ≤ C, (1)
  While prior work on compression in RL [21, 31] has applied the VIB to states independently, we will aim to compress entire sequences of observations. Applying the VIB to sequences allows us to use more expressive choices of the prior m(z1:∞). Our prior will use previous representations to predict the prior for the next representation, allowing us to obtain a tighter bound on mutual information.
sections.heading: 3.2 Using Fewer Bits by Predicting the Future
sections.text: Predicting the future allows agents to observe fewer bits from future observations. To further decrease the number of bits used, the agent can change its behavior to visit states that are more easily compressed. States where the dynamics are hard to predict will require more bits, so the agent will prefer visiting states where its learned model can accurately predict the next state.
  This intuition corresponds to solving the optimization problem in Eq. 1 with a prior that is factored autoregressively: m1:∞(z1:∞ | a1:∞) = m(z1) ∏ tmθ(zt+1 | zt,at). Note that the prior has learnable parameters θ. We apply the VIB to obtain an upper bound on the constraint in Eq. 1:
  Eπ[I(s1:∞; z1:∞)] ≤ Eπ [ log ( p(z1:∞ | s1:∞) m(z1:∞) )] = Eπ [ ∞∑ t=1 γt(log φθ(zt | st)− logmθ(zt+1 | zt,at)) ] . (2) This objective is different from prior work that applies the VIB to RL [14, 21, 31] because the prior mθ(zt) is predicted, rather than fixed to be a unit Normal distribution. The discount factor γ reflects the assumption from Sec. 3.1 that the episode terminates with probability (1− γ) at each time step; of course, no bits are used after the episode terminates. Our final objective optimizes the policy π, the encoder φ, and the prior m to maximize reward and minimizing information:
  max θ Eπθ,φθ,mθ [ ∞∑ t=1 γtr(st,at) ] s.t. Eπθ,φθ,mθ [∑ t γt (log φθ(zt | st)− logmθ(zt+1 | zt,at)) ] ≤ C. (3) For the encoder, this objective looks like a modification of the information bottleneck where the prior is predicted from the previous representation and action. Of course, we cannot predict the first
  2We condition the policy on sequences of states because, even if the task is Markovian, the agent’s choice to omit certain bits from the representation can turn an otherwise fully-observed task into a partially-observed task.
  representation, so we substitute use a unit Normal distribution for the prior at initial time step. For the policy, this objective looks like the standard RL objective but with a different reward function, one that includes an additional information cost:
  r̃λ(st,at, st+1) , r(st,at) + λ (logmθ(zt+1 | zt,at)− log φθ(zt+1 | st+1))︸ ︷︷ ︸ (negative of the) information cost , (4)
  where λ is the cost per bit. The term log φθ(zt | st) corresponds to the number of bits required to represent the representation zt. The term logmθ(zt+1 | zt,at) reflects how well the agent can predict the next representation. In effect, the agent has to “pay” for bits of the observation, but it gets a “refund” on bits that it predicted from the previous time step, analogous to bits-back coding [12, 19]. Said in other words, we want to minimize the excess bits required to infer zt from the current state, relative to the number of bits required to predict zt from the previous (zt−1,at−1). Note that the standard VIB approaches to RL [14, 21, 31] do not receive this refund because they compress observations independently, rather than compressing sequences of observations.
  Note that the policy, encoder, and model are all jointly optimized with respect to the same objective, so updates to any component increases the objective for all components. Importantly, the agent optimizes not only its representation but also its behavior to minimize this information cost: the agent learns a representation that is easily predictable and learns to visit states where that representation is easily predictable. We therefore call our method robust predictable control (RPC).
  4 A Practical Algorithm
  Our method for optimizing (Eq. 3) is an actor-critic method applied to the informationaugmented reward function in Eq. 4. We introduce a Q function,
  Qψ(st,at) = E [∑
  t
  γtr̃(st,at, st+1) ] ,
  corresponding to the augmented reward function r̃. We optimize this Q function using standard temporal difference learning:
  L(ψ) = 1 2 (Qψ(st,at)− yt)2 (5)
  where yt = br̃t + γQψ(st+1,at+1)csg and b·csg is the stop-gradient operator [6, 11]. Since our overall objective (Eq. 1) only entails compressing the policy, not the Q function, we condition the Q function directly on the state. To optimize the encoder, prior, and policy, we reexpress our objective (Eq. 1) in terms of the immediate reward plus the Q function (see derivation in Appendix A.1):
  L(θ; st) = Ezt−1∼φθ(st−1),st∼p(st|st−1,at−1) zt∼φ(st),at∼πzθ (at|zt) [Qψ(st,at)+λ (logmθ(zt | zt−1,at−1)− log φθ(zt | st))]. (6) Since the encoder is stochastic, we compute gradients using the reparametrization trick [25]. The fact that all three components are jointly optimized with respect to the same objective makes implementation of RPC surprisingly simple. Unlike sequential VAEs and RNNs, training agents with RPC does not require sampling frommθ(zt+1 | zt,at) and does not require backpropagation through time. In our implementation, we instantiate the encoder φθ(st), the prior mθ(zt+1 | zt,at), and the high-level policy πzθ(at | zt) as neural networks with parameters θ. The Q function Qψ(st,at) is likewise represented as a neural network with parameters φ. We update the dual parameter λ ≥ 0 using dual gradient descent. We use standard tricks such as target networks and taking the minimum over two Q values. We refer the reader to Appendix B and the open-sourced code for details.
sections.heading: 5 Connections and Analysis
sections.text: In this section we discuss the connections between robust predictable control and other areas of RL. We then prove that RPC enjoys certain robustness guarantees.
sections.heading: 5.1 Connections
sections.text: This section explains connections between RPC and related ideas in the RL literature, with the aim of building intuition into how RPC works and providing an explanation for why RPC should learn robust policies and useful representations. We include a further discussion of the relationship to MaxEnt RL, the expression for the optimal encoder, and the value of information in Appendix A.3.
  Model-Based RL. The prior mθ(zt | zt−1,at−1) learned by RPC can be viewed as a dynamics model. Rather than predicting what the next state will be, this model predicts what the representation of the next state will be. While the model is trained to make accurate predictions, the policy is also trained to visit states and actions where the model will be more accurate.
  Representation learning. What, precisely, does the representation zt represent? Given a prior m(zt+1 | zt,at) and latent-conditioned policy πz(at | zt), we can use the current representation zt to predict good actions at both the current time step and (by unrolling the prior) at future time steps. Thus, the representation zt can be thought of as a compact representation of open-loop action sequences. Our experiments demonstrate that this compact representation of action sequences, once learned on one task, can be used to quickly learn a range of downstream tasks.
  We can also view RPC as learning a new action space for the MDP, where the reward function is now r̃ (Eq. 4). What we previously called the encoder, φ(zt | st), is now the policy for selecting the actions zt in this new MDP. The − log φ(zt | st) term in from r̃ is the entropy of this encoder, so the encoder is performing MaxEnt RL on this new MDP. This new MDP encodes a strong prior for open-loop policies: sampling actions from the prior yields high reward.
  Open-loop control. RPC learns a model m(zt | zt−1,at−1) that predicts the state representation at the next time step. Thus, we can unroll our policy in an open-loop manner, without observing transitions from the true system dynamics. Because the model and policy are trained to be selfconsistent, we expect that the highly compressed policies learned by RPC will perform well in this open-loop setting, as compared to uncompressed policies (see experiments in Sec. 6).
sections.heading: 5.2 Theoretical Guarantees
sections.text: We now present analysis showing that the compressed policies learned by RPC generalize better. We also discuss a close connection between compression and open-loop control, proving that RPC learns policies and models that are self-consistent in a way that guarantees good performance when even when the agent cannot observe new observations. We do not intend to to analyze all benefits deriving from model compression (see, e.g., [3, 4]). All proofs are in Appendix C.
  In supervised learning, it is well known that compression results in models that generalize better from the training set to the testing set. Our main result applies this same reasoning to RL. We assume that the policy has been trained on an empirical distribution of MDPs, and will be evaluated on a new MDP sampled from that sample distribution. For the following result, we assume that the given stochastic MDP is a mixture of deterministic MDPs, each described by a b-bit random string (see Ng and Jordan [35]). Lemma 5.1. Let stochastic MDPM and policy π be given. DefineRπ(M) to be the expected reward on MDPM. Then the probability that the policy’s expected return on an observed (deterministic) MDP M̂ is much different than the policy’s expected return on the stochastic MDP is bounded by the policy’s bitrate:
  PM̂[|R π(M̂)−Rπ(M)| > ] ≤ C + 1
  2b 2 − 1 .
  The proof is a direct application of Bassily et al. [4, Theorem 8]. This result may be of interest in the offline RL setting, where observed trajectories effectively constitute a deterministic MDP; that policies that use fewer bits will be less likely to overfit to the offline dataset.
  Not only do compressed policies generalize to new MDPs, they are also robust to missing observations. Intuitively, a policy that uses zero bits of information will perform identically in the open-loop setting. The following result shows that our model compression objective corresponds to maximizing a lower bound on the expected return of the open-loop policy. Let πopen be the open-loop policy corresponding to the composition of the prior m(zt+1 | zt,at) and the high-level policy πz(at | zt). Let πreactive
  be the reactive policy corresponding to the composition of the encoder φ(zt | st) and the high-level policy πz(at | zt). Lemma 5.2. Let encoder φ(zt | st), policy πz(at | zt), prior m(zt | zt−1,at−1), and reward function r(st,at) > 0 be given. Then our model compression objective (Eq. 3) with reward function (1− γ) log r(st,at) is a lower bound on the expected return of the open-loop policy.
  Eπopen(τ) [ ∞∑ t=1 γtr(st,at) ] ≥ f ( Eπreactive(τ) [ ∞∑ t=1 γt ( (1− γ) log r(st,at) + logm(zt | zt−1,at−1)− log φ(zt | st) )]) ,
  where f(x) = γ1−γ e x γ is a monotone increasing function of x.
  Moreover, the policies learned by RPC will receive similar returns in settings where they can and cannot observe observations, a result that may be useful for quantifying the regret incurred when using the learned representation zt as an action space of temporally-extended behaviors. Let Rmax = maxτ R(τ) be the maximum return of any trajectory.
  Lemma 5.3. The expected return of the open-loop policy πopen is at most Rmax √ C/2 worse than the expected return of the reactive policy πreactive:
  Eπopen [∑
  t
  γtr(st,at) ] ≥ Eπreactive [∑ t γtr(st,at) ] −Rmax √ C/2.
  In summary, our theoretical results suggest that compressed policies generalize well and, by virtue of their open-loop guarantees, may be useful for planning. We emphasize that the theoretical benefits of model compression have been well studied in the supervised learning literature. As our method is likewise performing model compression, we expect that it will inherit a wide range of additional guarantees, such as guarantees about sample complexity.
sections.heading: 6 Experiments
sections.text: Our experiments have two aims. First, we will demonstrate that RPC achieves better compression than alternative approaches, obtaining a higher reward for the same number of bits. Second, we will study the empirical properties of compressed policies learned by our method, such as their robustness and ability to learn representations suitable for hierarchical RL. We do not intend this section to exhaustively demonstrate every possible benefit from compression; we acknowledge that there are many purported benefits of compression, such as exploration and sample efficiency, which we do not attempt to study here. We include additional experiments in Appendix B.
  6.1 Evaluating Compression
  feedforward architecture predicted prior augmented reward
  RPC (ours) 3 3 3 VIB [21] 3 7 7 VIB+reward [31] 3 7 7 VIB+RNN 7 7 3 Our first experiment studies whether RPC outperforms alternative approaches to compression, which we summarize in the inline table. We compare against a standard VIB [21] and an extension that adds the information cost to the reward, “VIB+reward” [31]. This baseline can be viewed as a special case of RPC where the blue arrows in Fig. 1 are removed. Finally, since the predictive prior in RPC is similar to a recurrent network, we compare against an extension of the VIB [21] that uses an LSTM (“RNN + VIB”). Unlike RPC, this baseline requires training on entire trajectories (rather than individual transitions) and requires backpropagating gradients through time. We evaluate all methods on four tasks from
  OpenAI-Gym [5] and two image-based tasks from dm-control [40]. Because of computational constraints, we omit the RNN+VIB baseline on the image-based tasks.
  We plot results in Fig. 2. To make the rewards comparable across tasks, we normalize the total return by the median return of the best method. On almost all tasks, RPC achieves higher returns than prior methods for the same bitrate. For example, when learning the Walker task using 0.3 bits per observation, RPC achieves a return of 3,562, the VIB+RNN baseline achieves a return of 1,978 (-44%), and the other VIB baselines achieve a return of around 530 (-85%). We include full results on a wider range of bitrates in Appendix Fig. 10. While, in theory, the VIB+RNN baseline could implement RPC internally, in practice it achieved lower returns, perhaps because of optimization challenges associated with training LSTMs [7]. Even if the VIB+RNN baseline could implement the strategy learned by RPC, RPC is simpler (it does not require training on trajectories) and trains about 25% times faster (it does not require backpropagation through time).
sections.heading: 6.2 Visualizing Compressed Policies
sections.text: Behavior of compressed policies. To visualize how compression changes behavior, we applied RPC to two simulated driving tasks shown in Fig. 3a (top left), which are based on prior work [29]. In the first task, the agent can pass cars by driving into the lane for oncoming traffic; the second task uses the same simulated environment but restricts the agent to remain in its own lane to study active cruise control in isolation. The rewards for these tasks corresponds to driving to the right as quickly as possible, without colliding with any other vehicles. In the first task, we observe that compressed policies passed fewer cars than uncompressed policies. Intuitively, a passing maneuver requires many bits of information about the relative positions of other cars, so we expect that compressed policies to engage in fewer passing maneuvers. Fig. 3a (bottom) shows that the bitrate of RPC is directly correlated with the number of cars passed. In the second task, we observe that compressed policies leave a larger following distance from the leading car (Fig. 3b (top)). Fig. 3b (bottom) shows that the number of bits used per observation increases when the car is within 15m of another car. Thus, by maintaining a following distance of more than 30m, the compressed policy can avoid these situations where it would have to use more bits. See the project website for videos.3
  Representations of compressed policies. A policy learned by compression must trade off between maximizing reward and paying to receive bits of information. Using the HalfCheetah-v2 task, we plot the value of information versus cost of information for many sampled states. See Appendices A.4 and B.5 for details. As shown in Fig. 4 (top), the RPC uses more bits from observations that are more valuable for maximizing future returns. Then, to visualize the learned representation zt, we sample the representation from randomly sampled states, and plot the numbers of bits used for each coordinate. Fig. 4 (bottom) shows that RPC learns sparse representations. Whereas the uncompressed policy uses all coordinates, a policy compressed with bitrate 10 uses only 10/50 coordinates and a policy compressed with bitrate 0.3 uses only 2/50 coordinates.
  Intuitively, the ways in which compression changes the behavior and representations of policies should make these policies more robust to disturbances. Our next experiments explicitly test this hypothesis, which would validate the analysis in Sec. 5.2.
  3Project site with videos and code: https://ben-eysenbach.github.io/rpc
sections.heading: 6.3 Robustness
sections.text: The connection between compression and robustness has been well established in the literature (e.g., [16, 47]). Our next set of experiments test the robustness of compressed policies to different types of disturbances: missing observations (i.e., open-loop control), adversarial perturbations to the observations, and perturbations to the dynamics (i.e., robust RL). We emphasize that, because these experiments focus on robustness, the policy is trained and tested in different environments.
  Robustness to missing observations and open-loop control. Since compressed policies rely on fewer bits of input from the observations, we expect that they not only will be less sensitive to missing observations, but also will actively modify their behavior to adopt strategies that require fewer bits from the observation. In this experiment, we drop each observation
  independently with probability p ∈ [0, 1], where p = 1 corresponds to using a fully open-loop policy. For RPC, we handle missing observations by predicting the representation from the previous time step. Our two main baselines take a policy used by standard RL and learn either a latent-space model or a state-space model. When the observation is missing, these baselines make predictions using the learned model. We also compare against RNN+VIB, the strongest baseline from Fig. 2. When observations are missing, the LSTM’s input for that time step is sampled from the prior. Fig. 5 shows that all methods perform similarly when no observations are dropped, but RPC achieves a higher reward than baselines when a larger fraction of observations are dropped. This experiment shows that more effective compression, as done by RPC, yields more robust policies. This robustness may prove useful in real-world environments where sensor measurements are missing or corrupted.
  Adversarial Robustness. Compressed policies extract fewer bits from each observation, so we expect that compressed policies will be more robust to adversarial perturbations to the observation. While prior work has proposed purpose-designed methods for achieving robustness [33, 41], here we investigate whether compression is simple yet effective means for achieving a modicum of robustness. We do not claim that compression is the best method for learning robust policies.
  We first study adversarial perturbations to the dynamics on the Ant-v2 environment. Given a policy π(at | st) and the current state st, the adversary aims to apply a small perturbation to that state to make the policy perform as poorly as possible. We implement the adversary using projected gradient descent [32]; see Appendix B for details. Fig. 6 (left) shows the expected re-
  turn as we increase the magnitude of the attack. The compressed policy is more resilient to larger attacks than the uncompressed policy. Our next experiment looks at perturbations to observations. Unlike the previous experiment, we let the adversary perturb every step in an episode; see Appendix B for full details. Fig. 6 (right) shows that policies that use fewer bits achieve higher returns in this adversarial setting. When the Ant-v2 agent uses too many bits (3 or more), the adversarial perturbations to the dynamics flip the agent over, whereas agents that use fewer bits remain upright.
  Robust RL. Our final set of experiments look at higher-level perturbations to the dynamics, as are typically studied in the robust RL community [33, 41]. Using the same Ant-v2 environment as before, we (1) increase the mass of each body element by a fixed multiplier, or (2) decrease the friction of each body geometry by a fixed multiplier. These experiments test whether the learned policies are robust to more massive
  robots or more “slippery” settings. Fig. 7 shows that compressed policies generalize to larger masses and smaller frictions more effectively than an uncompressed policy. Comparing the policies learned
  by RPC with a bit rate of 0.3 bits versus 3.0 bits, we observe that the bit rate effectively balances performance versus robustness. See Appendix Fig. 11 for a larger version of this plot with error bars.
sections.heading: 6.4 Hierarchical RL using the Learned Representation
sections.text: RPC learns a representation of temporally-extended action sequences (see Sec. 5.1). We hypothesize that these representations can accelerate the learning of new tasks in a hierarchical setting. Our goal is not to propose a complete hierarchical RL system, but rather evaluate whether these action representations are suitable for high-level control. During training, we apply RPC to a goal-conditioned object pushing task, shown in Fig. 8 (top-left). The initial position of the object and the goal position are randomized, so we expect that different representations zt will correspond to high-level behaviors of moving the end effector and object to different positions. At test-time, the agent is presented with a new task. The agent will attempt to solve the task by commanding one or two behaviors zt. See Appendix B for details and pseudocode.
  We compare the action representations learned by RPC to three baselines. The first baseline is “Action Repeat”, which constantly outputs the same action, effectively allowing the agent to command desired poses. To test whether RPC has learned a prior over useful action sequences, we use a variant of RPC (“RPC (randomly initialized)” ) where the policy and model are randomly initialized. Finally, “RL from scratch” applies a state-of-the-art off-policy RL algorithm (SAC [17]) to the task. Taken together, these baselines allow us to study whether RPC not only learns behaviors that do more than move to particular poses, but also learns behaviors for obstacle avoidance and object manipulation. We apply all methods to four tasks and present results in Fig. 8. First, as a sanity check, we apply all methods to the training task, finding that RPC quickly finds a single zt that solves the task. On the remaining three tasks, we likewise observe that the representations learned by RPC allow for much faster learning than the baselines. The final task, “pusher wall” requires chaining together multiple representations zt to solve. While the “RL from scratch” baseline eventually matches and then surpasses the performance of RPC, RPC accelerates learning in the low-data regime.
sections.heading: 7 Conclusion
sections.text: In this paper, we presented a method for learning robust and predictable policies. Our objective differs from prior work by compressing sequences of observations, resulting in a method that jointly trains a policy and a model to be self-consistent. Not only does our approach achieve better compression than prior methods, it also learns policies that are more robust (Fig. 6). We also demonstrate that our method learns representations that are suitable for use in hierarchical RL.
  Our work suggests a few areas for future work. Ideas based on RPC may be useful for model-based RL, as training the policy to be self-consistent with the model may help prevent model exploitation. Similarly, because the skills learned by RPC are optimized to be predictable, we expect that they will perform well when used for open-loop planning. As compression is closely related to the energy required to implement a function on an ideal physical system [37], RPC might be useful to building energy-efficient control policies.
  Limitations. The main limitation of this work is that policies that use few bits will often receive lower reward on the training tasks. Also, for the purpose of exploration, the most informative states may be those that are hardest to compress. While the first limitation is likely irreconcilable, the second might be lifted by maximizing information collected during exploration but minimizing information for policy optimization.
  Acknowledgments. We thank Dibya Ghosh and the members of the Salakhutdinov lab for feedback throughout the project. We thank Oscar Ramirez for help setting up the image-based experiments and thank Rico Jonschkowski for introductions to potential collaborators. We thank Ryan Julian, Vincent Vanhoucke, and anonymous reviewers for feedback on the paper, and thank Abhishek Gupta for discussions during the early stages of this project.
  This material is supported by the Fannie and John Hertz Foundation and the NSF GRFP (DGE1745016).
title: Robust Predictable Control