ECE 517: Reinforcement Learning in Artificial Intelligence Lecture 14: Planning and Learning October 27, 2015 Dr. Itamar Arel College of Engineering Department of Electrical Engineering and Computer Science The University of Tennessee Fall 2015 1
Final projects - logistics Projects can be done in groups of up to 3 students Details on projects will be posted soon Students are encouraged to propose a topic Please email me your top three choices for a project along with a preferred date for your presentation Presentation dates: Nov. 17, 19, 24 and Dec. 1 + additional time slot (TBD) Format: 20 min presentation + 5 min Q&A ~5 min for background and motivation ~15 for description of your work, results, conclusions Written report due: Monday, Dec. 7 Format similar to project report ECE 517: Reinforcement Learning in AI 2
Final projects sample topics DQN Playing Atari games using RL Teris player using RL (and NN) Curiosity based TD learning* Reinforcement Learning of Local Shape in the Game of Go AIBO learning to walk Study of value function definitions for TD learning Imitation learning in RL ECE 517: Reinforcement Learning in AI 3
Outline Introduction Use of environment models Integration of planning and learning methods ECE 517: Reinforcement Learning in AI 4
Introduction Earlier we discussed Monte Carlo and temporal-difference methods as distinct alternatives Then showed how they can be seamlessly integrated by using eligibility traces such as in TD(l) Planning methods: e.g. Dynamic Programming and heuristic search Rely on knowledge of a model Model any information that helps the agent predict the way the environment will behave Learning methods: Monte Carlo and Temporal Difference Learning Do not require a model Our goal: Explore the extent to which the two methods can be intermixed ECE 517: Reinforcement Learning in AI 5
The original idea ECE 517: Reinforcement Learning in AI 6
The original idea (cont.) ECE 517: Reinforcement Learning in AI 7
Models Model: anything the agent can use to predict how the environment will respond to its actions Distribution models: provide description of all possibilities (of next states and rewards) and their probabilities e.g. Dynamic Programming Example - sum of a dozen dice produce all possible sums and their probabilities of occurring Sample models: produce just one sample experience In our example - produce individual sums drawn according to this probability distribution Both types of models can be used to (mimic) produce simulated experience Often sample models are much easier to come by ECE 517: Reinforcement Learning in AI 8
Planning Planning: any computational process that uses a model to create or improve a policy Planning in AI: State-space planning (such as in RL) search for policy Plan-space planning (e.g., partial-order planner) e.g. evolutionary methods We take the following (unusual) view: All state-space planning methods involve computing value functions, either explicitly or implicitly They all apply backups to simulated experience ECE 517: Reinforcement Learning in AI 9
Planning (cont.) Classical DP methods are state-space planning methods Heuristic search methods are state-space planning methods Planning methods rely on real experience as input, but in many cases they can be applied to simulated experience just as well Example: a planning method based on Q-learning: Random-Sample One-Step Tabular Q-Planning ECE 517: Reinforcement Learning in AI 10
Learning, Planning, and Acting Two uses of real experience: Model learning: to improve the model Direct RL: to directly improve the value function and policy Improving value function and/or policy via a model is sometimes called indirect RL or model-based RL. Here, we call it planning. Q: What are the advantages/disadvantages of each? ECE 517: Reinforcement Learning in AI 11
Direct vs. Indirect RL Indirect methods: make fuller use of experience: get better policy with fewer environment interactions Direct methods simpler not affected by bad models But they are very closely related and can be usefully combined: planning, acting, model learning, and direct RL can occur simultaneously and in parallel Q: Which scheme do you think applies to humans? ECE 517: Reinforcement Learning in AI 12
The Dyna-Q Architecture (Sutton 1990) ECE 517: Reinforcement Learning in AI 13
The Dyna-Q Algorithm Random-sample single-step tabular Q-planning method direct RL model learning (update) planning ECE 517: Reinforcement Learning in AI 14
Dyna-Q on a Simple Maze rewards = 0 until goal reached, when reward = 1 ECE 517: Reinforcement Learning in AI 15
Dyna-Q Snapshots: Midway in 2 nd Episode Recall that in a planning context Exploration trying actions that improve the model Exploitation Behaving in the optimal way given the current model Balance between the two is always a key challenge! ECE 517: Reinforcement Learning in AI 16
Variations on the Dyna-Q agent (Regular) Dyna-Q Soft exploration/exploitation with constant rewards Dyna-Q+ Encourages exploration of state-action pairs that have not been visited in a long time (in real interaction with the environment) If n is the number of steps elapsed between two consecutive visits to (s,a), then the reward is larger as a function of n Dyna-AC Actor-Critic learning rather that Q-learning ECE 517: Reinforcement Learning in AI 17
More on Dyna-Q+? Uses an exploration bonus : Keeps track of time since each state-action pair was tried for real An extra reward is added for transitions caused by state-action pairs related to how long ago they were tried: the longer unvisited, the more reward for visiting The agent (indirectly) plans how to visit long unvisited states ECE 517: Reinforcement Learning in AI 18
When the Model is Wrong: Blocking Maze (cont.) The maze example was oversimplified In reality many things could go wrong Environment could be stochastic Model can be imperfect (local minimum, stochasticity or no convergence) Partial experience could be misleading When the model is incorrect, the planning process will compute a suboptimal policy This is actually a learning opportunity Discovery and correction of the modeling error ECE 517: Reinforcement Learning in AI 19
When the Model is Wrong: Blocking Maze (cont.) The changed environment is harder ECE 517: Reinforcement Learning in AI 20
Shortcut Maze The changed environment is easier ECE 517: Reinforcement Learning in AI 21
Prioritized Sweeping In the Dyna agents presented, simulated transitions are started in uniformly chosen state-action pairs Probably not optimal Which states or state-action pairs should be generated during planning? Work backwards from states whose values have just changed: Maintain a queue of state-action pairs whose values would change a lot if backed up, prioritized by the size of the change When a new backup occurs, insert predecessors according to their priorities Always perform backups from first in queue Moore and Atkeson 1993; Peng and Williams, 1993 ECE 517: Reinforcement Learning in AI 22
Prioritized Sweeping ECE 517: Reinforcement Learning in AI 23
Prioritized Sweeping vs. Dyna-Q Both use N = 5 backups per environmental interaction ECE 517: Reinforcement Learning in AI 24
Trajectory Sampling Trajectory sampling: perform backups along simulated trajectories This samples from the on-policy distribution Distribution constructed from experience (visits) Advantages when function approximation is used Focusing of computation: can cause vast uninteresting parts of the state space to be (usefully) ignored: Initial states Reachable under optimal control Irrelevant states ECE 517: Reinforcement Learning in AI 25
Summary Discussed close relationship between planning and learning Important distinction between distribution models and sample models Looked at some ways to integrate planning and learning synergy among planning, acting, model learning Distribution of backups: focus of the computation prioritized sweeping trajectory sampling: backup along trajectories ECE 517: Reinforcement Learning in AI 26