Accurate predictions of parallel program execution time

It has been difficult to develop simple formulations to predict the execution time of parallel programs due to the complexity of characterizing parallel hardware and software. In an attempt to clarify these characterizations, we introduce a methodology for applying a simple performance model based on Amdahl′s law. Our formulation results in accurate predictions of execution time on available systems, allowing programmers to select the optimal number of processors to apply to a particular problem or to select an appropriate problem size for the number of processors available. In short, we accurately quantify the scalability of a specific algorithm when it is run on a specific parallel computer. Our predictions are based on simple experiments that characterize machine performance and on a simple analysis of the parallel program. We illustrate our method for a program executed on a Sequent Symmetry multiprocessor with 20 processors. Our predictions closely match experimental results, differing by no more than 5% from the actual execution times. Our results illustrate key performance limitations of parallel systems, showing the impact of overhead and the scaling of problem size.