On applying or-parallelism and tabling to logic programs

Abstract

Logic Programming languages, such as Prolog, provide a high-level, declarative approach to programming. Logic Programming offers great potential for implicit parallelism, thus allowing parallel systems to often reduce a program's execution time without programmer intervention. We believe that for complex applications that take several hours, if not days, to return an answer, even limited speedups from parallel execution can directly translate to very significant productivity gains.It has been argued that Prolog's evaluation strategy -SLD resolution -often limits the potential of the logic programming paradigm. The past years have therefore seen widening efforts at increasing Prolog's declarativeness and expressiveness. Tabling has proved to be a viable technique to efficiently overcome SLD's susceptibility to infinite loops and redundant subcomputations.Our research demonstrates that implicit or-parallelism is a natural fit for logic programs with tabling. To substantiate this belief, we have designed and implemented an or-parallel tabling engine -OPTYap -and we used a shared-memory parallel machine to evaluate its performance. To the best of our knowledge, OPTYap is the first implementation of a parallel tabling engine for logic programming systems. OPTYap builds on Yap's efficient sequential Prolog engine. Its execution model is based on the SLG-WAM for tabling, and on the environment copying for or-parallelism.Preliminary results indicate that the mechanisms proposed to parallelize search in the context of SLD resolution can indeed be effectively and naturally generalized to parallelize tabled computations, and that the resulting systems can achieve good performance on shared-memory parallel machines. More importantly, it emphasizes our belief that through applying or-parallelism and tabling to logic programs the range of applications for Logic Programming can be increased.

Concurrent and Local Evaluation of Normal Programs

“…SLG C differs from [2] in not requiring an explicit table manager and in using the "optimistic" Usurpation operation to control concurrency, as well as in being a formalism sufficient for proving completeness and other properties. [9] presents algorithms for adding tabling to an or-parallel engine and implements these algorithms in YAP, with impressive results for definite programs. As mentioned above, unlike [9] Local SLG C does not address general table parallelism, although it addresses normal programs and is based on a formalization which permits a concise implementation.…”

“…[9] presents algorithms for adding tabling to an or-parallel engine and implements these algorithms in YAP, with impressive results for definite programs. As mentioned above, unlike [9] Local SLG C does not address general table parallelism, although it addresses normal programs and is based on a formalization which permits a concise implementation. Perhaps the closest work is [3] which allows threads to share answers when tables are not completed: Concurrent SLG differs from this work in using a simpler method of concurrency control, as well as in modeling normal rather than definite programs.…”

Mostly-Functional Behavior in Java Programs

“…We analyzed these programs with version 0.92 of the GNU Classpath library, using the Soot compiler framework [3] and the dimple + version of our dimple static analysis tool [4], running on version 5.1.3 of the Yap Prolog system [5]. We timed our analyses by running them on one core of a 2 ghz Opteron workstation with 16 gb of ram.…”

Retroactive Subsumption-Based Tabled Evaluation of Logic Programs

“…In order to support this, we introduce the following main extensions for subsumption-based tabling: (i) a new algorithm to efficiently traverse the table space searching for subsumed subgoals; (ii) a new table space organization, based on the ideas of the common global trie proposal [4], where answers are represented only once; and (iii) a new evaluation strategy to selectively prune and restart the evaluation of tabled nodes. We will focus our discussion on a concrete implementation, the YapTab system [5], but our proposals can be generalized and applied to other tabling systems. Notice that, in order to do this work, first we have ported, from XSB Prolog to YapTab, the full code that implements the TST design 1 .…”

“…Finally, we present some experimental results and conclusions. To support tabled evaluation, the YapTab design [5] extends the Warren's Abstract Machine (WAM) [6] execution model with the following four operations:…”