Non-uniform time sharing in the concurrent execution of constraint solving

Abstract

Concurrent execution of multiple instances of a randomised search over a CSP is one of the techniques for improvement of performance. Hitherto, uniform time sharing is the dominant approach to the control of execution of such instances. This paper introduces a new modification of search algorithms—non-uniform time sharing with elimination (NUTSE)—and experimentally evaluates the efficiency of its combination with both the FC-MRV (Forward Checking with the Minimal-Remaining-Values heuristic) and the FC-B (FC with the Brelaz's heuristic). The experiments show that the NUTSE over FC-MRV can be in the underconstrained area many times faster than the singly-executed FC-B. This good behaviour is used in a hybrid CNUC algorithm (Combined Non-Uniform Concurrency) that combines the FC-B and the NFC-MRV algorithms to obtain a good algorithm across a wide range of problem instances. All the experiments in this paper use the graph three-colouring problem.