Flaw Selection Strategies for Partial-Order Planning

TL;DR

This paper reviews and experimentally compares flaw selection strategies in partial-order planning, highlighting the effectiveness of combined strategies and clarifying previous conflicting claims about their efficiency.

Contribution

It generalizes prior work by comparing flaw selection strategies, clarifies misconceptions, and proposes an effective combined strategy for reducing search space in POCL planning.

Findings

Combined flaw selection strategies can effectively reduce search space.

LCFR and ZLIFO strategies have different impacts on search efficiency.

Domain characteristics influence the effectiveness of flaw selection strategies.

Abstract

Several recent studies have compared the relative efficiency of alternative flaw selection strategies for partial-order causal link (POCL) planning. We review this literature, and present new experimental results that generalize the earlier work and explain some of the discrepancies in it. In particular, we describe the Least-Cost Flaw Repair (LCFR) strategy developed and analyzed by Joslin and Pollack (1994), and compare it with other strategies, including Gerevini and Schubert's (1996) ZLIFO strategy. LCFR and ZLIFO make very different, and apparently conflicting claims about the most effective way to reduce search-space size in POCL planning. We resolve this conflict, arguing that much of the benefit that Gerevini and Schubert ascribe to the LIFO component of their ZLIFO strategy is better attributed to other causes. We show that for many problems, a strategy that combines least-cost flaw selection with the delay of separable threats will be effective in reducing search-space size, and will do so without excessive computational overhead. Although such a strategy thus provides a good default, we also show that certain domain characteristics may reduce its effectiveness.