Understanding cache hierarchy contention in CMPs to improve job scheduling

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In order to improve CMP performance, recent research has focused on scheduling to mitigate contention produced by the limited memory bandwidth. Nowadays, commercial CMPs implement multi-level cache hierarchies where last level caches are shared by at least two cache structures located at the immediately lower cache level. In turn, these caches can be shared by several multithreaded cores. In this microprocessor design, contention points may appear along the whole memory hierarchy. Moreover, this problem is expected to aggravate in future technologies, since the number of cores and hardware threads, and consequently the size of the shared caches increases with each microprocessor generation. In this paper we characterize the impact on performance of the different contention points that appear along the memory subsystem. Then, we propose a generic scheduling strategy for CMPs that takes into account the available bandwidth at each level of the cache hierarchy. The proposed strategy selects the processes to be co-scheduled and allocates them to cores in order to minimize contention effects. The proposal has been implemented and evaluated in a commercial single-threaded quad-core processor with a relatively small two-level cache hierarchy. Despite these potential contention limitations are less than in recent processor designs, compared to the Linux scheduler, the proposal reaches performance improvements up to 9% while these benefits (across the studied benchmark mixes) are always lower than 6% for a memory-aware scheduler that does not take into account the cache hierarchy. Moreover, in some cases the proposal doubles the speedup achieved by the memory-aware scheduler.