Paul Bone

Calculating likely Parallelism within Dependant Conjunctions for Logic Programs

Paul Bone
Honours Report
Department of Computer Science, School of Engineering, The University of Melbourne
October, 2008


The rate at which computers are becoming faster at sequential execution has dropped significantly. Instead parallel processing power is increasing, and multicore computers are becoming more common. Automatically parallelising programs is becoming much more desirable. Parallelising programs written in imperative programming languages is difficult and often leads to unreliable software. Parallelising programs in declarative languages is easy, to the extent that compilers are able to do this automatically. However this often leads to cases where the overheads of parallel execution outweigh the speedup that might have been available by parallelising the program.

This thesis describes a new implicit parallelism implementation that calculates the speedup due to parallelism in dependent conjunctions for Mercury — a purely declarative logic programming language. This is done by analysing profiling data and a representation of the program in order to determine when during the execution of a parallel conjunct variables are likely to be produced and consumed. This enables us to calculate how long a conjunct may have to wait for a variable to be produced, and how much parallelism is actually available in a parallel conjunction. This information should enable the compiler to parallelise a program only in places where doing so is profitable.

We expect that two of the components we implemented for our implicit parallelism analysis, coverage profiling and a generic feedback framework, will also be quite useful in other applications. For example this can help the compiler select the best calls to inline.