Managing Static Leakage Energy in Microprocessor Functional Units

Static energy due to subthreshold leakage current is projected to become a major component of the total energy in high performance microprocessors. Many studies so far have examined and proposed techniques to reduce leakage in on-chip storage structures. In this study, static energy is reduced in the integer functional units by leveraging the unique qualities of dual threshold voltage domino logic. Domino logic has desirable properties that greatly reduce leakage current while providing fast propagation times. However, due to the energy cost of entering the low leakage current state (sleep mode), domino logic has thus far been used only for leakage reduction in the longterm standby mode. We examine the utility of the sleep mode (while considering the aforementioned costs) when idle times are relatively short, one to a few hundred cycles, as is often the case for functional units. Using an analytical energy model suitable for architecture-level analysis, we explore the interaction of the application and technology, and the effect on energy and performance as the underlying parameters are varied, on a set of benchmarks. Our results show that if the leakage approaches the magnitude as projected in the literature, even for short idle intervals as few as ten cycles, an aggressive policy of activating the sleep mode at every idle period performs well and a more complex control strategy may not be warranted. We also propose a simple design, called Gradual Sleep, to reduce the energy impact of using the sleep mode for smaller idle periods.