Abstract In recent years, attention has been focused on the relationship between black-box optimization problem and reinforcement learning problem. In this research, we propose the Mirror Descent Search (MDS) algorithm which is applicable both for black box optimization problems and reinforcement learning problems. Our method is based on the mirror descent method, which is a general optimization algorithm. The contribution of this research is roughly twofold. We propose two essential algorithms, called MDS and Accelerated Mirror Descent Search (AMDS), and two more approximate algorithms: Gaussian Mirror Descent Search (G-MDS) and Gaussian Accelerated Mirror Descent Search (G-AMDS). This research shows that the advanced methods developed in the context of the mirror descent research can be applied to reinforcement learning problem. We also clarify the relationship between an existing reinforcement learning algorithm and our method. With two evaluation experiments, we show our proposed algorithms converge faster than some state-of-the-art methods.
Previous studies also proposed reinforcement learning algorithms on the basis of MD method @cite_15 @cite_23 . Mirror Descent TD( @math ) (MDTD) @cite_15 is a value based RL algorithm. The paper @cite_15 employs Minkowski distance with Euclidean space rather than KL divergence. By contrast, we basically employ the Bregman divergences on the simplex space, i.e. non-Euclidean space. Mirror Descent Guided Policy Search (MDGPS) @cite_18 is also associated with our proposed method. They showed mirror descent formulation improved the Guided Policy Search (GPS) @cite_36 . MDGPS has a distinctive feature that it depends both on KL divergence and RKL divergence. However, as is shown in @cite_42 , there are the variety of Bregman divergences on simplex space other than KL divergence and RKL divergence. Moreover, it plays an important role in accelerating the mirror descent @cite_11 . So we explicitly use Bregman divergence in this research.
@cite_18 @cite_36 @cite_42 @cite_23 @cite_15 @cite_11
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This paper explores a new framework for reinforcement learning based on online convex optimization, in particular mirror descent and related algorithms. Mirror descent can be viewed as an enhanced gradient method, particularly suited to minimization of convex functions in highdimensional spaces. Unlike traditional gradient methods, mirror descent undertakes gradient updates of weights in both the dual space and primal space, which are linked together using a Legendre transform. Mirror descent can be viewed as a proximal algorithm where the distance generating function used is a Bregman divergence. A new class of proximal-gradient based temporal-difference (TD) methods are presented based on different Bregman divergences, which are more powerful than regular TD learning. Examples of Bregman divergences that are studied include p-norm functions, and Mahalanobis distance based on the covariance of sample gradients. A new family of sparse mirror-descent reinforcement learning methods are proposed, which are able to find sparse fixed points of an l1-regularized Bellman equation at significantly less computational cost than previous methods based on second-order matrix methods. An experimental study of mirror-descent reinforcement learning is presented using discrete and continuous Markov decision processes. The mirror descent algorithm (MDA) generalizes gradient descent by using a Bregman divergence to replace squared Euclidean distance. In this paper, we similarly generalize the alternating direction method of multipliers (ADMM) to Bregman ADMM (BADMM), which allows the choice of different Bregman divergences to exploit the structure of problems. BADMM provides a unified framework for ADMM and its variants, including generalized ADMM, inexact ADMM and Bethe ADMM. We establish the global convergence and the O(1 T) iteration complexity for BADMM. In some cases, BADMM can be faster than ADMM by a factor of O(n ln n) where n is the dimensionality. In solving the linear program of mass transportation problem, BADMM leads to massive parallelism and can easily run on GPU. BADMM is several times faster than highly optimized commercial software Gurobi. We prove the equivalence of two online learning algorithms, mirror descent and natural gradient descent. Both mirror descent and natural gradient descent are generalizations of online gradient descent when the parameter of interest lies on a non-Euclidean manifold. Natural gradient descent selects the steepest descent direction along a Riemannian manifold by multiplying the standard gradient by the inverse of the metric tensor. Mirror descent induces non-Euclidean structure by solving iterative optimization problems using different proximity functions. In this paper, we prove that mirror descent induced by a Bregman divergence proximity functions is equivalent to the natural gradient descent algorithm on the Riemannian manifold in the dual co-ordinate system. We use techniques from convex analysis and connections between Riemannian manifolds, Bregman divergences and convexity to prove this result. This equivalence between natural gradient descent and mirror descent, implies that (1) mirror descent is the steepest descent direction along the Riemannian manifold corresponding to the choice of Bregman divergence and (2) mirror descent with log-likelihood loss applied to parameter estimation in exponential families asymptotically achieves the classical Cramer-Rao lower bound. We consider the problem of projecting a vector onto the simplex Δ = x ∈ ℝ+d : Σi=1d xi = 1 , using a Bregman projection. This is a common problem in first-order methods for convex optimization and online-learning algorithms, such as mirror descent. We derive the KKT conditions of the projection problem, and show that for Bregman divergences induced by ω-potentials, one can efficiently compute the solution using a bisection method. More precisely, an ω-approximate projection can be obtained in O(d log 1 ω). We also consider a class of exponential potentials for which the exact solution can be computed efficiently, and give a O(d log d) deterministic algorithm and O(d) randomized algorithm to compute the projection. In particular, we show that one can generalize the KL divergence to a Bregman divergence which is bounded on the simplex (unlike the KL divergence), strongly convex with respect to the l1 norm, and for which one can still solve the projection in expected linear time.