Enhancing DRAM Self-Refresh for Idle Power Reduction

DRAM can enter self-refresh mode to save power during idle periods. But self-refresh mode does not modify or reduce the number of refresh operations, therefore the refresh energy stays the same. We observe that in the self-refresh mode DRAM cells are in two distinct modes, static (idle) and dynamic (refreshing), and that the switching between these modes are predictable. In this paper, we propose two new self-refresh modes to improve the power efficiency of DRAM: Enhanced Self-Refresh (ESR) and Long latency Self-Refresh (LSR). The key idea behind our observation is to optimize the leakage current of DRAM cells by selectively applying different voltage levels to the DRAM cell transistors when they are active (accessed for refreshing) and idle (pre-charged) by adjusting both the word-line and body voltages. With our techniques, the retention time of DRAM cells is improved. In our SPICE and mathematical models, ESR and LSR modes result in a 39% and 48% DRAM self-refresh power reduction compared to the existing self-refresh mode, respectively. A workload analysis of ESR shows DRAM energy savings on average of 22%. In addition, for the long idle periods in server systems, the LSR mode can reduce DRAM idle power by nearly 50%, which results in a 6.5% total system idle power reduction.