Concurrent Execution of Mutually Exclusive Alternatives

We examine the task of concurrently computing alternative solutions to a problem. We restrict our interest to the case where only one solution is needed: in this case we need some rule for selecting between the solutions. We use "fastest first," where the first successful alternative is selected. For problems where the required execution time is unpredictable this method can show substantial execution time performance increases. These increases are dependent on the mean execution time of the alternatives, the fastest execution time, the overhead involved in concurrent computation, and the overhead of selecting and deleting alternatives. Rather than using the traditional approach of multiple computers cooperating on the solution to a problem, this method achieves a solution competitively. Among the problems with exploring multiple alternatives in parallel are side-effects and combinatorial explosion in the amount of state which must be preserved. These are solved by process management and an application of "copy-on-write" virtual memory management. The side effects resulting from interprocess communication are handled by a specialized message layer which interacts with process management. We show how the scheme for parallel execution can be applied to several application areas. The applications are distributed execution of recovery blocks, OR-parallelism in Prolog, and polynomial root-finding