Terrible Twins: A Simple Scheme to Avoid Bad Co-Schedules

Co-scheduling processes on different cores in the same server might lead to excessive slowdowns if they use a shared resource, like the memory bus. If possible, processes with a high shared resource use should be allocated to different server nodes to avoid contention, thus avoiding slowdown. This paper introduces the simple scheme of avoiding to coschedule twins, i.e., several instances of the same program. The rational for this is that instances of the same program use the same resources and they are more likely to be either low or high resource users − high resource users should obviously not be combined, but a bit non-intuitively, it is also shown that low resource users should also not be combined in order to not miss out on better scheduling opportunities. This is verified using both a statistical argument as well as experimentally using ten programs from the NAS parallel benchmark suite. By using the simple rule of forbidding twins, the average slowdown is shown to decrease from 6.6% down to 5.9%, and the worst case slowdown is lowered from 12.7% to 9.0%, indicating a considerable improvement despite having no information about any programs’ resource usage or slowdown behavior.