Multitasking and Multithreading on a Multiprocessor with Virtual Shared Memory

In this paper we investigate the combination of multitasking and multithreading in a (virtual) shared memory parallel machine running a number of parallel applications. In particular, we investigate whether it is better to run related threads, or unrelated threads on each node to achieve the best system throughput and to complete a mix of applications as quickly as possible. The experiments provide results for a range of mixes of applications. One of our benchmarks has a clear preference to place its threads across the whole machine, while the others have a slight preference to run their threads on smaller partitions of the machine. The differences are mostly slight, suggesting that the system scheduler has considerable flexibility in thread placement without jeopardising performance.