PWU: Pre- Wakeup for CPU Idle to Reduce Latency and Power Consumption

Abstract

More and more services require high real-time per-formance, and to achieve high real-time performance, a server's power-saving features tend to be disabled to achieve strict latency requirements at the expense of power consumption. However, increased server power consumption becomes a problem. One of the power-saving functions of a server is central processing unit (CPU) idle, which reduces power consumption of a CPU core by transitioning the CPU core to the idle state when the CPU core continues to have no tasks. There are multiple idle states, and although the power-consumption reduction effect increases with transitions to deeper idle states, it is known that wake-up latency of several to 100 µs is required to recover from a deep CPU idle state, which is a problem that impairs real-time performance. This paper proposes a pre-wakeup (PWU) system that reduces wake-up latency by pre-waking the CPU core before assigning tasks to it, while transitioning to a deep idle state when there is enough time to sleep. To evaluate the effect of pre-wakeup latency reduction by the PWU and the power-consumption overhead by pre-wakeup, we conducted evaluations on an actual device using an Intel Xeon processor and showed that the power-consumption overhead by pre-wakeup is small and that pre-wakeup can reduce the recovery time from C6 state by 84%.