The Effects of Multiprogramming on Barrier Synchronization

One of the most common ways to share a multiprocessor among several applications is to give each application a set of dedicated processors. To ensure fairness, an application may receive fewer processors than it has processes. Unless an application can easily adjust the number of processes it employs during execution, several processes from the same application may have to share a processor. In this paper we quantify the performance penalty that arises when more than one process from the same application runs on a single processor of a NUMA (NonUniform Memory Access) multiprocessor. We consider programs that use coarse-grain parallelism and barrier synchronization because they are particularly sensitive to multiprogramming. We quantify the impact on the performance of an application of quantum size, frequency of synchronization, and the type of barrier used. We conclude that dedicating processors to an application, even without migration or dynamic adjustment of the number of processes, is an effective scheduling policy even for programs that synchronize frequently using barriers. (A shortened version of this paper appeared in Proc., 3rd IEEE Symposium on Parallel and Distributed Processing, 662--669, December 1991.