AgileWatts: An Energy-Efficient CPU Core Idle-State Architecture for Latency-Sensitive Server Applications
share
User-facing applications running in modern datacenters exhibit irregular request patterns and are implemented using a multitude of services with tight latency requirements. These characteristics render ineffective existing energy conserving techniques when processors are idle due to the long transition time from a deep idle power state (C-state). While prior works propose management techniques to mitigate this inefficiency, we tackle it at its root with AgileWatts (AW): a new deep C-state architecture optimized for datacenter server processors targeting latency-sensitive applications. AW is based on three key ideas. First, AW eliminates the latency overhead of saving/restoring the core context (i.e., micro-architectural state) when powering-off/-on the core in a deep idle power state by i) implementing medium-grained power-gates, carefully distributed across the CPU core, and ii) retaining context in the power-ungated domain. Second, AW eliminates the flush latency overhead (several tens of microseconds) of the L1/L2 caches when entering a deep idle power state by keeping L1/L2 cache content power-ungated. A minimal control logic also remains power-ungated to serve cache coherence traffic (i.e., snoops) seamlessly. AW implements sleep-mode in caches to reduce caches leakage power consumption and lowers a core voltage to the minimum operational voltage level to minimize the leakage power of the power-ungated domain. Third, using a state-of-the-art power efficient all-digital phase-locked loop (ADPLL) clock generator, AW keeps the PLL active and locked during the idle state, further cutting precious microseconds of wake-up latency at a negligible power cost. Our evaluation with an accurate simulator calibrated against an Intel Skylake server shows that AW reduces the energy consumption of Memcached by up to 71 (35
READ FULL TEXT