This paper demonstrates that performance on a hardware multithreaded processor is sensitive to the set of jobs that are coscheduled by the operating system jobscheduler. The full benefits of SMT hardware can only be achieved if the scheduler is aware of thread interactions. Here, a mechanism is presented that allows the scheduler to significantly raise the performance benefits of SMT architectures. This is done without any advance knowledge of an application's characteristics, using sampling to identify jobs which run well together.
We demonstrate an SMT jobscheduler called SOS. SOS combines an overhead-free
sample phase which collects information about various possible schedules,
and a symbiosis phase which uses that information to predict which schedule
will provide the best performance. We show that a small sample of
the possible schedules is sufficient to identify a good schedule quickly.
On a system with random job arrivals and departures, response time is improved
as much as 17% over a schedule which does not incorporate symbiosis.