Automatic vectorization of tree traversals

Abstract

Both on-chip resource contention and off-chip latencies have a significant impact on memory requests in large-scale chip multiprocessors. We propose a memory-side prefetcher, which brings data on-chip from DRAM, but does not proactively further push this data to the cores/caches. Sitting close to memory, it avails close knowledge of DRAM state and memory channels to leverage DRAM row buffer locality and channel state to bring data (from the current row buffer) on-chip ahead of need. This not only reduces the number of off-chip accesses for demand requests, but also reduces row buffer conflicts, effectively improving DRAM access times. At the same time, our prefetcher maintains this data in a small buffer at each memory controller instead of pushing it into the caches to avoid on-chip resource contention. We show that the proposed memory-side prefetcher outperforms a state-of-the-art core-side prefetcher and an existing memory-side prefetcher. More importantly, our prefetcher can also work in tandem with the core-side prefetcher to amplify the benefits. Using a wide range of multiprogrammed and multi-threaded workloads, we show that this memory-side prefetcher provides IPC improvements of 6.2% (maximum of 33.6%), and 10% (maximum of 49.6%), on an average when running alone and when combined with a core-side prefetcher, respectively. By meeting requests midway, our solution reduces the off-chip latencies while avoiding the on-chip resource contention caused by inaccurate and ill-timed prefetches.