A parallel prolog system for distributed memory

Abstract

This paper presents a parallel execution system (PDP: Prolog Distributed Processor) for efficiently supporting both Independent_AND OR parallelism on distributed-memory multiprocessors. The system is composed of a set of workers with a hierarchical structure scheduler. Each worker operates on its own private memory and interprocessor communication is performed only by the passing of messages. The execution model follows a multisequential approach in order to maintain the sequential optimizations. Independent AND_parallelism is exploited following a fork-join approach and OR_parallelism is exploited following a recomputation approach. PDP deals with OR_under_AND parallelism by producing the solutions of a set of parallel goals in a distributed way, that is, by creating a new task for each element of the cross product. This approach has the advantage of avoiding both storing partial solutions and synchronizing workers, resulting in a largely increased performance. Different scheduling policies have been studied, and granularity controls have been introduced for each kind of parallelism. PDP has been implemented on a network of transputers and performance results show that PDP introduces very little overhead into sequential programs, and provides a high speedup for coarse-grain parallel programs.