Predictable performance in SMT processors

Abstract

Current instruction fetch policies in SMT processors are oriented towards optimization of overall throughput and/or fairness. However, they provide no control over how individual threads are executed, leading to performance unpredictability, since the IPC of a thread depends on the workload it is executed in and on the fetch policy used. From the point of view of the Operating System (OS), it is the job scheduler that determines how jobs are executed. However, when the OS runs on an SMT processor, the job scheduler cannot guarantee execution time constraints of any job due to this performance unpredictability. In this paper we propose a novel kind of collaboration between the OS and the SMT hardware that enables the OS to enforce that a high priority thread runs at a specific fraction of its full speed. We present an extensive evaluation using many different workloads, that shows that this mechanism gives the required performance in more than 97% of all cases considered, and even more than 99% for the less extreme cases. At the same time, our mechanism does not need to trade off predictability against overall throughput, as it maximizes the IPC of the remaining low priority threads, giving 94% on average (and 97.5% on average for the less extreme cases) of the throughput obtained using instruction fetch policies oriented toward throughput maximization, such as icount.