Hardware-software co-design to mitigate DRAM refresh overheads: A case for refresh-aware process scheduling

DRAM cells need periodic refresh to maintain data integrity. With high capacity DRAMs, DRAM refresh poses a significant performance bottleneck as the number of rows to be refreshed (and hence the refresh cycle time, tRFC) for each refresh command increases. Modern day DRAMs perform refresh at a rank-level, while LPDDRs used in mobile environments support refresh at a per-bank level. Rank-level refresh degrades the performance significantly since none of the banks in a rank can serve the on-demand requests. Perbank refresh alleviates some of the performance bottlenecks as the other banks in a rank are available for on-demand requests. Typical DRAM retention time is in the order of several milliseconds, viz, 64msec for environments operating in temperatures below 85 deg C and 32msec for environments operating above 85 deg C. With systems moving towards increased consolidation (e.g., virtualized environments), DRAM refresh becomes a significant bottleneck as it reduces the available overall DRAM bandwidth per task. In this work, we propose a hardware-software co-design to mitigate DRAM refresh overheads by exposing the hardware address-mapping and DRAM refresh schedule to the operating system (OS). In our co-design, we propose a novel per-bank refresh schedule in the hardware which augments memory partitioning in the OS. Supported by the novel per-bank refresh schedule and memory-partitioning, we propose a refresh-aware process scheduling algorithm in the OS which schedules applications on cores such that none of the on-demand requests from the applications are stalled by refreshes. The evaluation of our proposed co-design using multi-programmed workloads from the SPEC CPU2006, STREAM and NAS suites show significant performance improvements compared to the previously proposed hardware-only approaches.