Concurrent Action Execution with Shared Fluents

Michael Buro, Alexander Kovarsky

Concurrent action execution is important for plan-length minimization. However, action specifications are often limited to avoid conflicts arising from precondition/effect interactions. PDDL - the planning domain definition language - for example, implements the ``no moving targets'' rule, which means that no two actions can simultaneously make use of a value if one of the two is updating the value. This rule poses problems for resource allocation planning in which resource values are accessed in preconditions and effects. A simple example is construction actions that consume certain amounts of a resource. For speeding up plan execution, we would like to be able to dispatch several construction actions simultaneously. Because action preconditions depend on resource values and action effects change them, the ``no moving targets'' rule does not allow concurrent execution. However, if sufficient resources are available, executing actions simultaneously poses no problems. This paper addresses the problem of deciding whether a set of actions produced by a planning system can be executed concurrently in the presence of fluent variables that occur in both action preconditions and effects. We first motivate the concurrent action execution problem by introducing a fair action scheduling algorithm for real-time strategy (RTS) games. Then we prove that the general decision problem, when restricting effects and preconditions to polynomial time computations, is co-NP complete. Thereafter, we focus on problem restrictions based on commutative operators which allow us to specify sufficient conditions for concurrent executability that can be checked quickly if the number of shared fluents is small. Finally, we apply these findings to action execution with shared resources in RTS games.

Subjects: 1.11 Planning; 1.12 Scheduling

Submitted: Apr 23, 2007

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