STRICT AND NONSTRICT INDEPENDENT AND PARALLELISM IN LOGIC PROGRAMS - CORRECTNESS, EFFICIENCY, AND COMPILE-TIME CONDITIONS

This paper presents some fundamental properties of independent and parallelism and extends its applicability by enlarging the class of goals eligible for parallel execution. A simple model of(independent) and-parallel execution is proposed and issues of correctness and efficiency are discussed in the light of this model. Two conditions, ''strict'' and ''non-strict'' independence, are defined and then proved sufficient to ensure correctness and efficiency of parallel execution: If goals which meet these conditions are executed in parallel, the solutions obtained are the same as those produced by standard sequential execution. Also, in the absence of failure, the parallel proof procedure does not generate any additional work (with respect to standard SLD resolution), while the actual execution time is reduced. Finally, in case of failure of any of the goals, no slowdown will occur. For strict independence, the results are shown to hold independently of whether the parallel goals execute in the same environment or in separate environments. In addition, a formal basis is given for the automatic compile-time generation of independent and-parallelism: Compile-time conditions to efficiently check goal independence at run time are proposed and proved sufficient. Also, rules are given for constructing simpler conditions if information regarding the binding context of the goals to be executed in parallel is available to the compiler.