Ordered Landmarks in Planning

TL;DR

This paper introduces a domain-independent method for identifying and utilizing ordering constraints among landmarks and sub-goals in planning tasks to improve planning efficiency.

Contribution

It extends the concept of goal ordering to include landmarks and sub-goals, enabling better decomposition and guiding search in planning.

Findings

Significant runtime improvements observed with the approach

Method is domain- and planner-independent

Effective in guiding sub-optimal planning systems

Abstract

Many known planning tasks have inherent constraints concerning the best order in which to achieve the goals. A number of research efforts have been made to detect such constraints and to use them for guiding search, in the hope of speeding up the planning process. We go beyond the previous approaches by considering ordering constraints not only over the (top-level) goals, but also over the sub-goals that will necessarily arise during planning. Landmarks are facts that must be true at some point in every valid solution plan. We extend Koehler and Hoffmann's definition of reasonable orders between top level goals to the more general case of landmarks. We show how landmarks can be found, how their reasonable orders can be approximated, and how this information can be used to decompose a given planning task into several smaller sub-tasks. Our methodology is completely domain- and planner-independent. The implementation demonstrates that the approach can yield significant runtime performance improvements when used as a control loop around state-of-the-art sub-optimal planning systems, as exemplified by FF and LPG.