NVSleep: Using non-volatile memory to enable fast sleep/wakeup of idle cores

Spin-transfer torque random access memory (STTRAM) is an emerging memory technology with several attractive properties including non-volatility, high density, low leakage, and high endurance. These characteristics make it a potential candidate for replacing SRAM structures on processor chips. This paper presents NVSleep, a low-power microprocessor framework that leverages STT-RAM to implement fast checkpointing that enables near-instantaneous shutdown of cores without loss of the execution state. NVSleep stores almost all processor state in STT-RAM structures that do not lose content when power-gated. Memory structures that require low-latency access are implemented in SRAM and backed-up by “shadow” STT-RAM structures that are used to implement fast checkpointing. This enables rapid shutdown of cores and low-overhead resumption of execution, which allows cores to be turned off frequently and for short periods of time to take advantage of idle execution phases and save power. We present two implementations of NVSleep: NVSleepMiss which turns cores off when last level cache misses cause pipeline stalls and NVSleepBarrier which turns cores off when blocked on barriers. Evaluation of a simulated 64-core system shows average energy savings of 21% for NVSleepMiss for SPEC2000 benchmarks and 34% for NVSleepBarrier in high barrier count multi-threaded workloads from PARSEC and SPLASH2 benchmarks.