Lawrence Berkeley National Laboratory

With rapidly increasing parallelism, DRAM performance and power have surfaced as primary constraints from consumer electronics to high performance computing (HPC) for a variety of applications, including bulk-synchronous data-parallel applications which are key drivers for multi-core, with examples including image processing, climate modeling, physics simulation, gaming, face recognition, and many others. We present the last-level collective prefetcher (LLCP), a purely hardware last-level cache (LLC) prefetcher that exploits the highly correlated prefetch patterns of data-parallel algorithms that would otherwise not be recognized by a prefetcher that is oblivious to data parallelism. LLCP generates prefetches on behalf of multiple cores in memory address order to maximize DRAM efficiency and bandwidth, and can prefetch from multiple memory pages without expensive translations. Compared to well-established other prefetchers, LLCP improves execution time by 5.5% on average (10% maximum), increases DRAM bandwidth by 9% to 18%, decreases DRAM rank energy by 6%, produces 27% more timely prefetches, and increases coverage by 25% at minimum.