A Predictor-Based Power-Saving Policy for DRAM Memories

Reducing power/energy consumption is an important goal for all computer systems, from servers to battery-driven hand-held devices. To achieve this goal, the energy consumption of all system components needs to be reduced. One of the most power-hungry components is the off-chip DRAM, even when it is idle. DRAMs support different power-saving modes, such as self-refresh and power-down, but employing them every time the DRAM is idle, reduces performance due to their power-up latencies. The self-refresh mode offers large power savings, but incurs a long power-up latency. The power-down mode, on the other hand, has a shorter power-up latency, but provides lower power savings. In this paper, we propose and evaluate a novel power-saving policy that combines the best of both power-saving modes in order to achieve significant power reductions with a marginal performance penalty. To accomplish this, we use a history-based predictor to forecast the duration of an idle period and then either employ self-refresh, or power-down, or a combination of both power saving modes. Significant refinements are made to the predictor to maximize the energy savings and minimize the performance penalty. The presented policy is evaluated using several applications from the multimedia domain and the experimental results show that it reduces the total DRAM energy consumption between 68.8% and 79.9% at a negligible performance penalty between 0.3% and 2.2%.