Insights into Parallelism with Intensive Knowledge Sharing

Novel search space splitting techniques have recently been successfully exploited to paralleliz Constraint Programming and Mixed Integer Programming solvers. We first show how universal hashing can be used to extend one such interesting approach to a generalized setting that goes beyond discrepancy-based search, while still retaining strong theoretical guarantees. We then explain that such static or explicit splitting approaches are not as effective in the context of parallel combinatorial search with intensive knowledge acquisition and sharing such as parallel SAT, where implicit splitting through clause sharing appears to dominate. Furthermore, we show that in a parallel setting there exists a surprising tradeoff between the well-known communication cost for knowledge sharing across multiple compute nodes and the so far neglected cost incurred by the computational load per node. We provide experimental evidence that one can successfully exploit this tradeoff and achieve reasonable speedups in parallel SAT solving beyond 16 cores.