Weeding out Front-End Stalls with Uneven Block Size Instruction Cache

Abstract

The core front-end remains a critical bottleneck in modern server workloads owing to their multi-MB instruction footprints stemming from deep software stacks. Prior work has mainly investigated instruction prefetching and cache replacement policies to mitigate this bottleneck. In this work, we take an orthogonal approach and analyze instruction cache storage efficiency. Our analysis shows that, on average, about 60% of the bytes in a cache block are never accessed before the block is evicted from the instruction cache. This represents a huge storage inefficiency that more than halves the effective cache capacity. We observe that this inefficiency is caused by the fixed cache block sizes which are unable to accommodate the varying spatial locality inherent in the instruction stream. To mitigate this inefficiency, we propose Uneven Block Size (UBS) instruction cache, which supports different cache block sizes in a cache set. Our evaluation shows that UBS cache improves the storage efficiency by 32 percentage points over the baseline instruction cache. Further, by supporting uneven block sizes, UBS cache accommodates more than twice the number of blocks than a conventional cache within a given storage budget. Overall, the additional blocks combined with the better storage efficiency result in UBS cache approaching the performance of a 64KB conventional cache on a set of server workloads while requiring a storage budget similar to a 32KB conventional cache.

Weeding out Front-End Stalls with Uneven Block Size Instruction Cache