Reinforcement Learning Theory

Jun 12, 2019



Provably Efficient Imitation Learning from Observation Alone We study Imitation Learning (IL) from Observations alone (ILFO) in large-scale MDPs. While most IL algorithms rely on an expert to directly provide actions to the learner, in this setting the expert only supplies sequences of observations. We design a new model-free algorithm for ILFO, Forward Adversarial Imitation Learning (FAIL), which learns a sequence of time-dependent policies by minimizing an Integral Probability Metric between the observation distributions of the expert policy and the learner. FAIL provably learns a near-optimal policy with a number of samples that is polynomial in all relevant parameters but independent of the number of unique observations. The resulting theory extends the domain of provably sample efficient learning algorithms beyond existing results that typically only consider tabular RL settings or settings that require access to a near-optimal reset distribution. We also demonstrate the efficacy ofFAIL on multiple OpenAI Gym control tasks. Dead-ends and Secure Exploration in Reinforcement Learning Many interesting applications of reinforcement learning (RL) involve MDPs that include many dead-end" states. Upon reaching a dead-end state, the agent continues to interact with the environment in a dead-end trajectory before reaching a terminal state, but cannot collect any positive reward, regardless of whatever actions are chosen by the agent. The situation is even worse when existence of many dead-end states is coupled with distant positive rewards from any initial state (it is called Bridge Effect). Hence, conventional exploration techniques often incur prohibitively large training steps before convergence. To deal with the bridge effect, we propose a condition for exploration, called security. We next establish formal results that translate the security condition into the learning problem of an auxiliary value function. This new value function is used to capany" given exploration policy and is guaranteed to make it secure. As a special case, we use this theory and introduce secure random-walk. We next extend our results to the deep RL settings by identifying and addressing two main challenges that arise. Finally, we empirically compare secure random-walk with standard benchmarks in two sets of experiments including the Atari game of Montezuma's Revenge. Statistics and Samples in Distributional Reinforcement Learning We present a unifying framework for designing and analysing distributional reinforcement learning (DRL) algorithms in terms of recursively estimating statistics of the return distribution. Our key insight is that DRL algorithms can be decomposed as the combination of some statistical estimator and a method for imputing a return distribution consistent with that set of statistics. With this new understanding, we are able to provide improved analyses of existing DRL algorithms as well as construct a new algorithm (EDRL) based upon estimation of the expectiles of the return distribution. We compare EDRL with existing methods on a variety of MDPs to illustrate concrete aspects of our analysis. Hessian Aided Policy Gradient Reducing the variance of estimators for policy gradient has long been the focus of reinforcement learning research. While classic algorithms like REINFORCE find an $\epsilon$-approximate first-order stationary point in $\OM({1}/{\epsilon^4})$ random trajectory simulations, no provable improvement on the complexity has been made so far. This paper presents a Hessian aided policy gradient method with the first improved sample complexity of $\OM({1}/{\epsilon^3})$. While our method exploits information from the policy Hessian, it can be implemented in linear time with respect to the parameter dimension and is hence applicable to sophisticated DNN parameterization. Simulations on standard tasks validate the efficiency of our method. Provably Efficient Maximum Entropy Exploration Suppose an agent is in a (possibly unknown) Markov Decision Process in the absence of a reward signal, what might we hope that an agent can efficiently learn to do? One natural, intrinsically defined, objective problem is for the agent to learn a policy which induces a distribution over state space that is as uniform as possible, which can be measured in an entropic sense. We provide an efficient algorithm to construct such a maximum-entropy exploratory policy, when given access to a black box planning oracle (which is robust to function approximation). Furthermore, when restricted to the tabular setting where we have sample based access to the MDP, our proposed algorithm is provably efficient method, both in terms of sample size and computational complexity. Key to our algorithmic methodology is utilizing the conditional gradient method (a.k.a. the Frank-Wolfe algorithm) which utilizes an approximate MDP solver. Combining parametric and nonparametric models for off-policy evaluation We consider a model-based approach to perform batch off-policy evaluation in reinforcement learning. Our method takes a mixture-of-experts approach to combine parametric and non-parametric models of the environment such that the final value estimate has the least expected error. We do so by first estimating the local accuracy of each model and then using a planner to select which model to use at every time step as to minimize the return error estimate along entire trajectories. Across a variety of domains, our mixture-based approach outperforms the individual models alone as well as state-of-the-art importance sampling-based estimators. Sample-Optimal Parametric Q-Learning Using Linearly Additive Features Consider a Markov decision process (MDP) that admits a set of state-action features, which can linearly express the process' s probabilistic transition model. We propose a parametric Q-learning algorithm that finds an approximate-optimal policy using a sample size proportional to the feature dimension K and invariant with respect to the size of the state space. To further improve its sample efficiency, we exploit the monotonicity property and intrinsinc noise structure of the Bellman operator, provided the existence of anchor state-actions that imply implicit non-negativity in the feature space. We augment the algorithm using techniques of variance reduction, monotonicity preservation and confidence bounds. It is proved to find a policy which is ϵ-optimal from any initial state with high probability using\wtO(K/ϵ^2(1−γ)^3) sample transitions for arbitrarily large-scale MDP with a discount factor γ∈(0,1). A matching information-theoretical lower bound is proved, confirming the sample optimality of the proposed method with respect to all parameters (up to polylog factors). Transfer of Samples in Policy Search via Multiple Importance Sampling We consider the transfer of experience samples in reinforcement learning. Most of the previous works in this context focused on value-based settings, where transferring instances conveniently reduces to the transfer of (s,a,s',r) tuples. In this paper, we consider the more complex case of reusing samples in policy search methods, in which the agent is required to transfer entire trajectories between environments with different transition models. By leveraging ideas from multiple importance sampling, we propose robust gradient estimators that effectively achieve this goal, along with several techniques to reduce their variance. In the case where the transition models are known, we theoretically establish the robustness to the negative transfer for our estimators. In the case of unknown models, we propose a method to efficiently estimate them when the target task belongs to a finite set of possible tasks and when it belongs to some reproducing kernel Hilbert space. We provide empirical results to show the effectiveness of our estimators. Exploration Conscious Reinforcement Learning Revisited The Exploration-Exploitation tradeoff is one of the main problems of Reinforcement Learning. In practice, this tradeoff is resolved by using some inherent exploration mechanism, such as the ϵ-greedy exploration or adding Gaussian action noise, while still trying to learn an optimal policy. We take a different approach, defining a surrogate optimality objective: an optimal policy with respect to the exploration scheme. As we show throughout the paper, although solving this criterion does not necessarily lead to an optimal policy, the problem becomes easier to solve. We continue by analyzing this notion of optimality, devise algorithms derived from this approach, which reveal connections to existing work, and test them empirically on tabular and deep Reinforcement Learning domains. Kernel-Based Reinforcement Learning in Robust Markov Decision Processes The robust Markov decision processes (MDP) framework aims to address the problem of parameter uncertainty due to model mismatch, approximation errors or even adversarial behaviors. It is especially relevant when deploying the learned policies in real-world applications. Scaling up the robust MDP framework to large or continuous state space remains a challenging problem. The use of function approximation in this case is usually inevitable and this can only amplify the problem of model mismatch and parameter uncertainties. It has been previously shown that, in the case of MDPs with state aggregation, the robust policies enjoy a tighter performance bound compared to standard solutions due to its reduced sensitivity to approximation errors. We extend these results to the much larger class of kernel-based approximators and show, both analytically and empirically that the robust policies can significantly outperform the non-robust counterpart.



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