Hardware-Software Co-design to Mitigate DRAM Refresh Overheads

Abstract

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) with 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. Per-bank 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 several of milliseconds, viz, 64msec for environments operating in temperatures below 85 deg C and 32msec for environments operating above 85 deg C.