State-preserving vs. non-state-preserving leakage control in caches

This paper compares the effectiveness of state-preserving and non-state-preserving techniques for leakage control in caches by comparing drowsy cache and gated-V_ss for data caches using 70nm technology parameters. To perform the comparison, we introduce "HotLeakage", a new architectural model for subthreshold and gate leakage that explicitly models the effects of temperature, voltage, and parameter variations, and has the ability to recalculate leakage currents dynamically as temperature and voltage change at runtime due to operating conditions, DVS techniques, etc. By comparing drowsy-cache and gated-V_ss at different L2 latencies and different gate oxide thickness values, we are able to identify a range of operating parameters at which gated-V_ss is more energy efficient than drowsy-cache, even though gated-V_ss does not preserve data in cache lines that have been deactivated. We are also able to show potential further benefits of gated-V_ss if an effective dynamic adaptation technique can be found. These results debunk a fairly widespread belief that state-preserving techniques are inherently superior to non-state-preserving techniques.