Design and Evaluation of the Bandwidth Bandit

David Eklöv, Nikos Nikoleris, David Black-Schaffer, and Erik Hagersten

February 2012

Abstract:

Applications that are co-scheduled on a multicore compete for shared resources, such as cache capacity and memory bandwidth. The performance degradation resulting from this contention can be substantial, which makes it important to effectively manage these shared resources. This, however, requires an understanding of how applications are impacted by such contention.

While the effects of contention for cache capacity have been extensively studied, less is known about the effects of contention for memory bandwidth. This is in large due to its complex nature, as sensitivity to bandwidth contention depends on bottlenecks at several levels of the memory-system, the interaction and locality properties of the application s access stream. This paper explores the contention effects of increased latency and decreased memory parallelism at different points in the memory hierarchy, both of which cause decreases in available bandwidth. To understand the impact of such contention on applications, it also presents a method whereby an application s overall sensitivity to different degrees of bandwidth contention can be directly measured. This method is used to demonstrate the varying contention sensitivity across a selection of benchmarks, and explains why some of them experience substantial slowdowns long before the overall memory bandwidth saturates.

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