Operating system support for mitigating software scalability bottlenecks on asymmetric multicore processors

Abstract

Asymmetric multicore processors (AMP) promise higher performance per watt than their symmetric counterparts, and it is likely that future processors will integrate a few fast out-of-order cores, coupled with a large number of simpler, slow cores, all exposing the same instruction-set architecture (ISA). It is well known that one of the most effective ways to leverage the effectiveness of these systems is to use fast cores to accelerate sequential phases of parallel applications, and to use slow cores for running parallel phases. At the same time, we are not aware of any implementation of this parallelism-aware (PA) scheduling policy in an operating system. So the questions as to whether this policy can be delivered efficiently by the operating system to unmodified applications, and what the associated overheads are remain open. To answer these questions we created two different implementations of the PA policy in OpenSolaris and evaluated it on real hardware, where asymmetry was emulated via CPU frequency scaling. This paper reports our findings with regard to benefits and drawbacks of this scheduling policy.