Much recent work in satisficing planning has aimed at striking a balance between coverage - solving as many problems as possible - and plan quality. Current planners achieve near perfect coverage on the latest IPC benchmarks. It is therefore natural to investigate their scaling behavior on more difficult instances. Among state of the art planners, LAMA (Richter, Helmert, and Westphal 2008) is able to generate high quality plans, but its coverage drops off rapidly with increasing prob- lem complexity. The Arvand planner (Nakhost and Müller 2009) scales to much harder instances but generates lower quality plans. This paper introduces a new algorithm, Monte Carlo Random Walk-based Local Tree Search (MRW-LTS), which uses random walks to selectively build local search trees. Experiments demonstrate that MRW-LTS combines a scaling behavior that is better than LAMA’s with a plan quality that is better than Arvand’s.

A ubiquitous feature of planning problems — problems involving the automatic generation of action sequences for attaining a given goal — is the need to economize limited resources such as fuel or money. While heuristic search, mostly based on standard algorithms such as A*, is currently the superior method for most varieties of planning, its ability to solve critically resource-constrained problems is limited: current planning heuristics are bad at dealing with this kind of structure. To address this, one can try to devise better heuristics. An alternative approach is to change the nature of the search instead. Local search has received some attention in planning, but not with a specific focus on how to deal with limited resources. We herein begin to fill this gap. We highlight the limitations of previous methods, and we devise a new improvement (smart restarts) to the local search method of a previously proposed planner (Arvand). Systematic experiments show how performance depends on problem structure and search parameters. In particular, we show that our new method can outperform previous planners by a large margin.