Exploiting the DRAM Microarchitecture to Increase Memory-Level Parallelism

This paper summarizes the idea of Subarray-Level Parallelism (SALP) in DRAM, which was published in ISCA 2012, and examines the work's significance and future potential. Modern DRAMs have multiple banks to serve multiple memory requests in parallel. However, when two requests go to the same bank, they have to be served serially, exacerbating the high latency of on-chip memory. Adding more banks to the system to mitigate this problem incurs high system cost. Our goal in this work is to achieve the benefits of increasing the number of banks with a low-cost approach. To this end, we propose three new mechanisms, SALP-1, SALP-2, and MASA (Multitude of Activated Subarrays), to reduce the serialization of different requests that go to the same bank. The key observation exploited by our mechanisms is that a modern DRAM bank is implemented as a collection of subarrays that operate largely independently while sharing few global peripheral structures. Our three proposed mechanisms mitigate the negative impact of bank serialization by overlapping different components of the bank access latencies of multiple requests that go to different subarrays within the same bank. SALP-1 requires no changes to the existing DRAM structure, and needs to only reinterpret some of the existing DRAM timing parameters. SALP-2 and MASA require only modest changes (< 0.15% area overhead) to the DRAM peripheral structures, which are much less design constrained than the DRAM core. Our evaluations show that SALP-1, SALP-2 and MASA significantly improve performance for both single-core systems (7%/13%/17%) and multi-core systems (15%/16%/20%), averaged across a wide range of workloads. We also demonstrate that our mechanisms can be combined with application-aware memory request scheduling in multicore systems to further improve performance and fairness.