Compiler Support for Optimizing Memory Bank-Level Parallelism

Abstract

Many prior compiler-based optimization schemes focused exclusively on cache data locality. However, cache locality is only one part of the overall performance of applications running on emerging multicores or many cores. For example, memory stalls could constitute a very large fraction of execution time even in cache-optimized codes, and one of the main reasons for this is lack of memory-level parallelism. Motivated by this, we propose a compiler-based Bank-Level Parallelism (BLP) optimization scheme that uses loop tile scheduling. More specifically, we first use Cache Miss Equations to predict where the last-level cache miss will happen in each tile, and then identify the set of memory banks that will be accessed in each tile. Using this information, two tile scheduling algorithms are proposed to maximize BLP, each targeting a different scenario. We further discuss how our compiler-based scheme can be enhanced to consider memory controller-level parallelism and row-buffer locality. Our experimental evaluation using 11 multithreaded applications shows that the proposed BLP optimization can improve average BLP by 17.1% on average, resulting in a 9.2% reduction in average memory access latency. Furthermore, considering memory controller-level parallelism and row-buffer locality (in addition to BLP) takes our average improvement in memory access latency to 22.2%.