Reinforcement Learning: a Comparison of UCB Versus Alternative Adaptive Policies

09/13/2019 ∙ by Wesley Cowan, et al. ∙ 1

In this paper we consider the basic version of Reinforcement Learning (RL) that involves computing optimal data driven (adaptive) policies for Markovian decision process with unknown transition probabilities. We provide a brief survey of the state of the art of the area and we compare the performance of the classic UCB policy of bkmdp97 with a new policy developed herein which we call MDP-Deterministic Minimum Empirical Divergence (MDP-DMED), and a method based on Posterior sampling (MDP-PS).

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1 Introduction

Reinforcement Learning (RL) refers to machine learning (ML) techniques designed for sequential decision making when an agent needs to “learn” a policy which maximizes a reward (or minimizes a cost) criterion when some parameters of the model are not known in advance, c.f.

Bertsekas [8], Sutton and Barto [43], Mohri et al. [34], Alpaydin [4], Tewari and Bartlett [46, 47], Ortner et al. [37]. Reinforcement learning is experiencing significant growth in recognition due to successful applications in many areas c.f. Wiering [52], Russo and Van Roy [39], Chang et al. [11], Neu et al. [36], Munos et al. [35], Szepesvári [44], Szepesvári [45], Filippi et al. [19], and Tewari and Bartlett [46, 47].

In this paper we consider the basic version of a probabilistic sequential decision system the discrete time, finite state and action Markovian decision process (MDP) cf. Dynkin and Yushkevich [17]. After a very brief survey of the state of the art of the area of computing optimal data driven (adaptive) policies for MDPs with unknown transition probabilities. Then, we compare the performance of the classic UCB policy of Burnetas and Katehakis [9] with a new policy developed herein which we call MDP-Deterministic Minimum Empirical Divergence (MDP-DMED), and a method based on Posterior sampling (MDP-PS). The MDP-DMED algorithm is inspired by the DMED method for the Multiarmed Bandit Problem developed in Honda and Takemura [24, 25]

and is based on estimating the optimal rates at which actions should be taken. The MDP-PS method is based on ideas of greedy posterior sampling that go back to

Thompson [48], cf. Osband and Van Roy [38]. Indeed many modern ideas of RL originate in work done for the multi-armed bandit problem cf. Gittins [22], Gittins et al. [23], Auer et al. [6] Whittle [51], Weber [50], Villar et al. [49] Sonin [41], Sonin and Steinberg [42], Mahajan and Teneketzis [33], Katehakis and Veinott Jr [30], Katehakis and Rothblum [29], Katehakis and Derman [28].

Some additional related work and areas of potential applications are contained in Cowan and Katehakis [12], Cowan and Katehakis [14] Azar et al. [7], Katehakis et al. [31], Cowan and Katehakis [15] Abbeel and Ng [1], Katehakis et al. [31], Ferreira et al. [18], Jaksch et al. [27], Asmussen and Glynn [5].

2 Formulation

A finite MPD is specified by a quadruple , where is the state space, is the action space, with being the set of admissible actions (or controls) in state , , is the reward structure and is the transition law. Here and are respectively the one step expected reward and transition probability from state to state under action For extensions regarding state and action spaces and continuous time we refer to [13] and references therein. Lerma [21], and Dynkin and Yushkevich [17].

When all elements of are known the model is said to be an MDP with complete information (CI-MDP). In this case, optimal polices can be obtained via the appropriate version of optimality equations, given the prevailing optimization criterion, state - action - time conditions and regularity assumptions c.f. Lerma [21], Dekker et al. [16], Dynkin and Yushkevich [17].

When some of the elements of are unknown the model is said to be an MDP with incomplete or partial information (PI-MDP).

For the body of the paper, we consider the following partial information model: the transition probability vector

is taken to be an element of parameter space

that is, the space of all

-dimensional probability vectors. The restriction that each transition probability be non-negative is simply to ensure that for any control policy, the resulting Markov chain is irreducible. Additionally, for the body of the paper we will take the reward structure

to be known, and constant. Unknown or probabilistic reward structures are to be considered in future work.

Under this model, we define a sequence of state valued random variables

representing the sequence of states of the MDP (taking as a given initial state), and action valued random variables as the action taken by the controller, action being taken at time when the MDP is in state . It is convenient to define a control policy as a (potentially random) history dependent sequence of actions such that . We may then define the value of a policy as the total expected reward over a given horizon of action:

(1)

Let be the set of all feasible MDP policies . We are interested in policies that maximize the expected reward from the MDP, in particular policies that are capable of maximizing the expected reward irrespective of the initial uncertainty that exists about the underlying MDP dynamics (i.e., for all possible under consideration). It is convenient then to define . We may then define the “regret” as the expected loss due to ignorance of the underlying dynamics,

(2)

We are interested in Uniformly Fast c.f. Burnetas and Katehakis [9] policies, that achieve for all feasible transition laws . In this case, despite the controller’s initial lack of knowledge about the underlying dynamics, she can be assured that her expected loss due to ignorance grows not only sub-linearly over time, but slower than any power of . It is shown in Burnetas and Katehakis [9] that any uniformly fast policy has a strict lower bound of logarithmic asymptotic growth of regret, with a bound on the order coefficient in terms of the unknown transition law and the known reward structure . Policies that achieve this lower bound are Asymptotically Optimal c.f. Burnetas and Katehakis [9]; see also Cowan and Katehakis [12], Cowan et al. [13], Burnetas and Katehakis [10], and references therein.

It is additionally convenient to define the following notation: with a given policy to be understood, we denote by the number of times the MDP has been in state in the first periods; we denote by the number of times the MDP has been in state and had action taken; we denote by the number of times the MDP has transitioned from to under action .

In the next subsection, we consider the case of the controller having complete information (the best possible case) and use this to motivate notation and machinery for the remainder of the paper. The body of the paper is devoted to presenting and discussing three control policies that are either provably asymptotically optimal, or at least appear to be. While no proofs are presented, the results of numerical experiments are presented demonstrating the efficacy of these policies.

2.1 The Optimal Policy Under Known Parameters

In this section, we consider the case of complete information, when and are known. In this case, it can be shown that there is a deterministic policy, one in which the action taken at any time depends only on the current state, that realizes the maximal long term average expected reward. Letting be the (finite) set of all such deterministic policies:

(3)

That there is such an optimal deterministic policy is a classical result cf. [6].

We may characterize this optimal policy in terms of the solution for of the following system of optimality equations:

(4)

Given the solution and vector to the above equations, the asymptotically optimal policy can be characterized as, whenever in state , take any action for which

(5)

We denote the set of such asymptotically optimal actions as . In general, should be taken to denote an action .

The solution above represents the maximal long term average expected reward. The vector , i.e., for any , represents in some sense the immediate value of being in state relative to the long term average expected reward. The value essentially encapsulates the future opportunities for value available due to being in state .

It will be convenient in what is to follow to define the following notation:

(6)

The function effectively represents the value of a given action in a given state, for a given transition vector - both the immediate reward, and the expected future value of whatever state the MDP transitions into. The value of an asymptotically optimal action for any state is thus given by . It can be shown that the “expected loss” due to an asymptotically sub-optimal action, taking action when the MDP is in state , is effectively in the limit given by

(7)

In the general (partial or complete information) case, it is shown in [6] that the regret of a given policy can be expressed asymptotically as

(8)

Note, the above formula justifies the description of as the “average loss due to sub-optimal activation of in state ’. Additionally, from the above it is clear that in the case of complete information, when is known and therefore the asymptotically optimal actions are computable, the total regret at any time is bound by a constant. Any expected loss at time is due only to finite horizon effects. In general, for the incomplete information case, we have the following bound due to [6], for any uniformly fast policy ,

(9)

where

represents the minimal Kullback-Leibler divergence between

and any such that substituting for in renders the unique optimal action for . Note, the Kullback-Leibler divergence is given by . Policies that achieve this lower bound, for all , are referred to as Asymptotically Optimal.

3 The UCB Algorithm for MDPs Under Unknown Transition Distributions

The policy we present here is a simplified version of the UCB-MDP policy developed in Burnetas and Katehakis [9]

. In this classical upper confidence MDP-UCB setting in each time instance estimates of the values of each available action are computed based on available data, inflated by a certain confidence interval (based on the Kullback-Leibler divergence). The more data on a given action that is available, the tighter the confidence interval will be, and therefore the less the corresponding estimate will be inflated.

At any time , let be the current (given) state of the MDP. We construct the following estimators:

  • Transition Probability Estimators: for each state and action , construct based on

    (10)

    Note, the biasing terms (the in the numerator, in the denominator) serve to force the estimated transition probabilities away from , and thus our estimates of will be in .

  • “Good” Action Sets: construct the following subset of the available actions ,

    (11)

    The set represents the actions available from state that have been sampled frequently enough that the estimates of the associated transition probabilities should be “good’. In the limit, we expect that sub-optimal actions will be taken only logarithmically, and hence for sufficiently large , will contain only actions that are truly optimal. If no actions have been taken sufficiently many times, we take to prevent it from being empty.

  • Value Estimates: having constructed these estimators, we compute and as the solution to the optimality equations in Eq. (4), essentially treating the estimated probabilities as correct and computing the optimal values and policy for the resulting estimated MDP.

At this point, we implement the following UCB index based decision rule: for each action , we compute the following index:

(12)

where is the Kullback-Leibler divergence, and take action

(13)

This is a natural extension of several classical KL-divergence based UCB policies for the multi-armed bandit problem cf. Cowan and Katehakis [12], Burnetas and Katehakis [10], and references therein, taking the view of the function as the “value” of taking a given action in a given state, estimated with the current data. In Burnetas and Katehakis [9], a version of the above policy is in fact shown to be asymptotically optimal. The modification is largely for analytical benefit however, the pure UCB index policy defined above shows excellent performance cf. Figure 1. Further discussion of the performance of this policy is given in the Comparison of Performance section.

An important and legitimate concern to the practical usage of this UCB policy that has been noted in Tewari and Bartlett [46] among others, is actually calculating the index in Eq. (12). Efficient formulations can be derived and this will be explored in depth in future work.

4 A DMED-Type Algorithm for MDPs Under Uncertain Transitions

In the classical DMED algorithm for Multi-armed Bandit Problems, the decision process proceeds by attempting to successively estimate the asymptotically minimal rates with which sub-optimal actions must be taken, and then attempting to take actions in such a way so as to realize the estimated minimal rates. As applied to MDPs, we have the following relationship from [6]. For any uniformly fast policy , for any state and sub-optimal action ,

(14)

where is, as before, the minimal Kullback-Leibler divergence between the true transition probability vector , and any transition probability vector such that substituting for in would render action uniquely optimal for state .

Computing the function is not easy. We consider the following substitute, then:

(15)

The function measures how far the transition vector associated with and must be perturbed (under the KL-divergence) to make the optimal action for . The function measures how far the transition vector associated with and must be perturbed (under the KL-divergence) to make the value of , as measured by the -function, greater than the value of an optimal action .

In this way, we have the following approximate MDP-DMED algorithm; see Honda and Takemura [24, 25] for a multi-armed bandit version of this policy.

At any time , let be the current state, and construct the estimators as in the UCB-MDP algorithm in section 3, , , and utilize these to compute the estimated optimal values, and .

Let be the estimated “best” action to take at time . For each , compute the discrepancies

If , take , otherwise, take

Following this algorithm, we perpetually reduce the discrepancy between the estimated sub-optimal actions, and the estimated rate at which those actions should be taken. The exchange from to sacrifices some performance in the pursuit of computational simplicity, however it also seems clear from computational experiments that DMED-MDP as above is not only computationally tractable, but also produces reasonable performance in terms of achieving small regret cf. Figure 1. Further discussion of the performance of this policy is given in the Comparison of Performances section.

5 A Posterior Sampling Algorithm for MDPs

In this section we introduce a Posterior Sampling (Thompson-Type ) policy for MDPs, or PS-MDP. This type of policy is also known as Thompson Sampling, or Probability matching. The basic idea is to generate estimates for the unknown parameters (transition probabilities) randomly, according to the posterior distribution for those unknown parameters, based on the current data. In particular, PS-MDP proceeds in the following way:

At any time , let be the current state of the MDP. As in UCB-MDP and DMED-MDP previously, construct the estimators . In addition, generate the following random vectors.

For each action , let be the vector of observed transition counts from state to under action . Generate the random vector according to

(16)

The are distributed according to the joint posterior distribution of with a uniform prior.

At this point, define the following values as posterior estimates of the potential value of each action:

(17)

and take action

6 Comparison of Performance

In this section we discuss the results of our simulation test of these policies on a small example with 3 states ( and ) with 2 available actions ( and ) in each state. Below we show the transition probabilities, as well as the reward, returned under each action.

0.04 0.69 0.27 0.88 0.01 0.11 0.02 0.46 0.52

0.28 0.68 0.04 0.26 0.33 0.41 0.43 0.35 0.22

0.13 0.47 0.89 0.18 0.71 0.63

If these transition probabilities were known, the optimal policy for this MDP would be and .

We simulated each policy 100 times over a time horizon of 10,000 and for each time step we computed the mean regret as well as the variance. In Figure

1, we plot the mean regret over time for each policy, [1] PS, [2] UCB, and [3] DMED, along with a confidence interval for all sample paths.

[scale=.15]simulation.pdf

Figure 1: Average cumulative regret over time for each policy

We can see that all policies seem to have logarithmic growth of regret. There are a few interesting differences that the plot highlights, at least for these specific parameter values:

DMED-MDP has not only the highest finite time regret, but also large variance that seems to increase over time. This seems primarily due to the “epoch” based nature of the policy, which results in exponentially long periods when the policy may get trapped taking sub-optimal actions, incurring large regret until the true optimal actions are discovered. The benefit of this epoch structure is that once the optimal actions are discovered, they are taken for exponentially long periods, to the exclusion of sub-optimal actions.

PS-MDP seems to perform best, exhibiting lowest finite time regret as well as the tightest variance. This seems largely in agreement with the performance of PS-type policies in other bandit problems as well, in which they are frequently asymptotically optimal cf. Agrawal S and Goyal N. [3, 2], Honda and Takemura [26], Kaufmann et al. [32] and references therein.

6.1 Policy Robustness - Inaccurate Priors

How do these policies respond to potentially “unlucky” or non-representative streaks of data? Can these policies be fooled, and what are the resulting costs before they recover?

To test the robustness of these policies, with respect to prior information, we “rigged” the first 60 actions and transitions, such that under the estimated transition probabilities the optimal policy would be to activate the sub-optimal action in each state. In more detail, let be the number of times we transitioned from state to state under action . Then we “rigged” so that it started like so,

8 1 1 1 1 8 8 1 1  ,

1 1 8 8 1 1 1 1 8

Under the resulting (bad) estimated transition probabilities, we have that the optimal policy is , and . Under these initial estimates, the assumed optimal policy chooses the sub-optimal action in each state.

The subsequent performances of the MDP policies are plotted in Figure 2. All policies still appear to have logarithmic growth in regret, suggesting they can all “recover” from the initial bad estimates. It is striking though, the extent to which the average regrets for DMED-MDP and PS-MDP are affected, increasing dramatically as a result, PS-MDP demonstrating an increase in variance as well. However, the UCB-MDP policy seems relatively stable: its average regret has barely increased, and maintains a small variance. Empirically, this phenomenon appears common for the UCB-MDP policy under other extreme conditions.

[scale=.15]robustness.pdf

Figure 2: Robustness test. UCB seems to be largely unaffected by the unlucky beginning.

Acknowledgments

We acknowledge support for this work from the National Science Foundation, NSF grant CMMI-1662629.

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