Simulating the performance of a multiprocessor operating system
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This paper examines the effect of contention for a single shared semaphore protecting critical regions in a multiprocessor operating system on system performance as the number of processors increases. A simple queueing model is used to make predictions as a function of the expected demand for the semaphore by each processor. Measurements are then presented for 2 and 4 processor configurations of the CRAY X-MP running an experimental version of UNICOS. The measurements indicate that demand is highly dependent on the workload, and that contention can be somewhat higher than predicted by the model due to process scheduling effects and request interdependencies. One interesting point which is revealed in the measurement results, is that the UNIX scheduler gives preferential treatment to jobs that make frequent requests for the semaphore.