Parallel Portfolio Search for Gecode

Abstract: Constraint programming is used to solve hard combinatorial problems in a variety of domains, such as scheduling, networks and bioinformatics. Search for solving these problems in constraint programming is brittle and even slight variations in the problem data or search heuristic used can dramatically affect the runtime. But using portfolios of search engines on several variants of a problem by adding randomness to the heuristic used has been proved to counter this problem. A portfolio is defined as a collection of assets that combined gives it a desired return and risk. Unfortunately not all constraint programming systems have implementations of portfolio search, such as Gecode. Therefore were two portfolio search prototypes, sequential and parallel, designed and implemented for Gecode. The design is not system dependent and could easily be implemented in other constraint programming systems. The design and implementation is tested by both validity and performance tests to ensure its soundness. Validity is tested by finding all possible solutions on a moderately difficult combinatorial problem known as the N-Queens problem. Performance is tested by finding the first solution on a very difficult combinatorial problem known as the Latin Square Completion problem with different numbers of search engines. To compare against the same validity and performance tests were run with just one search engine. Results show that the design and implementation of portfolio search is sound. The parallel variant of portfolio search finds solutions faster with more search engines and outperforms the sequential variant. The sequential variant finds solutions about as fast as running with just one search engine. Successfully designing and implementing portfolio search in Gecode will help researchers and companies who use Gecode to save both time and money as they are now able to find better solutions faster by using this portfolio search. It may also contribute to the research within this area and the continued development of Gecode