ACE: And/or-parallel copying-based execution of logic programs

Abstract

The stack copying approach, as exemplified by the MUSE system, has been shown to be a quite successful alternative for representing multiple environments during or-parallel execution of logic programs. In this paper we present a novel execution model for parallel implementation of logic programs which is capable of exploiting both independent and-parallelism and or-parallelism in an efficient way and which combines the stack copying approach in the implementation of or-parallelism with proven techniques in the implementation of independent and-parallelism, such as those used in &-Prolog. The and parallelism is expressed via &-Prolog's conditional graph expressions. We show how all solutions to non-deterministic and-parallel goals are found without repetitions. This is done through recomputation as in Prolog (and &-Prolog), i.e., solutions of and-parallel goals are not shared. We also propose a scheme for the efficient management of the address space in a way that is compatible with the apparently incompatible requirements of both and- and or-parallelism. The resulting system retains the advantages of both purely or-parallel systems as well as purely and-parallel systems.