Evaluating the Performance of Cache-Affinity Scheduling in Shared-Memory Multiprocessors

As a process executes on a processor, it builds up state in that processor s cache. In multiprogrammed workloads, the opportunity to reuse this state may be lost when a process gets rescheduled, either because intervening processes destroy its cache state or because the process may migrate to another processor. In this paper, we explore affinity scheduling, a technique that helps reduce cache misses by preferentially scheduling a process on a processor where it has run recently. Our study focuses on a bus-based multiprocessor executing a variety of workloads, including mixes of scientific, software development, and database applications. In addition to quantifying the performance benefits of exploiting affinity, our study is distinctive in that it provides low-level data from a hardware performance monitor that details why the workloads perform as they do. Overall, for the workloads studied, we show that affinity scheduling reduces the number of cache misses by 7-36%, resulting in execution time improvements of up to 10%. Although the overall improvements are small, modifying the operating system scheduler to exploit affinity appears worthwhile-affinity has no negative impact on the workloads and we show that it is extremely simple to add to existing schedulers.