A High Performance OR-parallel Prolog System

P ROLOG, the most popular logic programming language, has been developed for single-processor computers. The implementations of sequential Prolog became eecient with the development of the Warren Abstract Machine (WAM) and are still improving. Today many parallel computers are commercially available and the question is how to utilize this opportunity to speed up the execution of Prolog programs. Our approach has been to study and develop eecient techniques for implementing OR-parallel systems for the full Prolog language on UMA and NUMA computers. Based on those techniques a high performance OR-parallel Prolog system has been designed and implemented on six diierent parallel computers. The system has a number of processes, called workers, consisting of two components: the engine, which does the actual Prolog work, and the scheduler. The schedulers, working together, divide the available work between the engines and support the sequential semantics of Prolog. We have extended the WAM, resulting in a Prolog engine with almost the same speed as the original WAM, i.e. the OR-parallel execution model preserves the high eeciency of the sequential implementation. We have designed and implemented a scheduler supporting dynamic load balancing and low overhead. We have also designed and implemented a clean interface between the engine and the scheduler, improving the modularity of the system and preserving its high eeciency. We have developed tools for debugging and evaluating the system. The evaluation of the system on Sequent Symmetry and on BBN Butterry machines I and II shows very promising results in absolute speed and also in comparison with results of the other OR-parallel systems. A layered approach for building a simple and eecient OR-parallel Prolog system by extending any sequential Prolog implementation based on the WAM has been designed.

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