Machine Learning (Theory)


Learning Track of International Planning Competition

The International Planning Competition (IPC) is a biennial event organized in the context of the International Conference on Automated Planning and Scheduling (ICAPS). This year, for the first time, there will a learning track of the competition. For more information you can go to the competition web-site.

The competitions are typically organized around a number of planning domains that can vary from year to year, where a planning domain is simply a class of problems that share a common action schema—e.g. Blocksworld is a well-known planning domain that contains a problem instance each possible initial tower configuration and goal configuration. Some other domains have included Logistics, Airport, Freecell, PipesWorld, and many others. For each domain the competition includes a number of problems (say 40-50) and the planners are run on each problem with a time limit for each problem (around 30 minutes). The problems are hard enough that many problems are not solved within the time limit.

Given that the planners are asked to solve many problems from individual domains, and that problems within a domain generally have common solution structures, it makes sense to consider learning from previous problem-solving experience in a domain to better solve future problems in the same domain. Here “better solve” could mean either solve the problems significantly more quickly or finding better quality plans in a similar time frame. However, no planner in any of the competitions has included a learning component. Rather, to quote Foreigner, for these planners each problem “feels like the first time”.

Perhaps one reason that planners have not incorporated learning into the competition setting is that there has not been much overlap between the ICML and ICAPS communities, although that is changing. Another reason is perhaps that the structure of the competition would deduct any “learning time” from a planner’s 30mins per problem, which could reduce the potential benefits.

The learning track for the 2008 competition is being designed so that learning time is not counted against planners. Rather, there will be a distinct learning phase and a distinct evaluation phase. During the learning phase the planners will be provided with the set of domains to be used in the competition and example problems from each. The evaluation phase will be conducted like the current competition, with the exception that the learning-based planners will be allowed to read in a learned domain-specific “knowledge file” when solving the problems. This structure is designed to help answer the following question:

Do we have techniques that can leverage a learning period to outperform state-of-the-art non-learning techniques across a wide range of domains?

My current belief is that the answer is “no”. I certainly have never seen any such demonstration. This is not because of lack of work in the area of “learning to plan” as there has been a long history dating back to some of the early planners (see my horribly outdated resource page for a taste). While many of the learning approaches have shown some degree of success, the evaluations have typically been very narrow, focusing on only 2 to 3 domains and often only a few problems. My intuition, grounded in personal experience, is that most (all) of these systems would be quite brittle when taken to new domains. The hope is that the learning track of the competition will force us to take the issue of robustness seriously and soon lead to learning systems that convincingly outperform non-learning planners across a wide range of domains given proper training experience.

I hope to see a wide range of approaches entered into the competition. I’ll mention two styles of approaches that might be particular interesting to readers of this blog.

First, one might consider applying reinforcement learning to learn “generalized policies” that can be applied to any problem from a domain. Recall that here the domain model is provided to us, so applying RL would mean that the domain model is used as a sort of simulator in which an RL algorithm is run. RL is particularly difficult in these domains due to the challenges in developing an appropriate representation for learning value functions and/or policies—the states can be viewed as sets of ground relational atoms, rather than the more typical n-dimensional vectors common in RL. Another challenge is the extremely sparse reward, which is obtained only at goal states. There has been some work on applying RL to IPC-style domains (e.g. relational reinforcement learning, approximate policy iteration, policy gradient) but much improvement is needed to compete with non-learning planners.

Second, one might consider structured-classification techniques for this problem. Here one might view the planning problem as an input X and the plan as the structured output Y. Training data can be generated by solving example planning problems using state-of-the-art planners perhaps using significant resources. This approach has been studied under the name max-margin planning, but applied to a very different class of planning problems. Other work has considered applying the Learning as Search Optimization (LaSO) framework to IPC-style domains with some success. Some of the challenges here are to automatically produce an appropriate feature set given a planning domain and ambiguity in the training data. Ambiguity here refers to the fact that there are often a huge number of equally good plans for a given problem and the training data has only one or a small number of them, making the training data incomplete.

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