Memory-Aware DVFS Governing Policy for Improved Energy-Saving in the Linux Kernel

Energy-aware computing is one of the most critical issues in modern computing systems. Linux has introduced the schedutil governor since its 4.7 kernel release, which dynamically scales the processor frequency level to reduce energy consumption. Although it has been widely used in most Linux-based systems, the governor often makes inaccurate decisions in frequency selection, thus leading to unnecessary energy consumption. In this paper, we propose an enhanced governor as an alternative. We first rigorously analyze the schedutil's policy and then find out that it does not take into account memory stalls when characterizing CPU performance via CPI (cycles per instruction). This yields inherent inaccuracy, particularly in modern SoCs where multiple CPU cores and accelerators incur a huge amount of memory traffic over the system bus. To rectify this problem, we reformulate the CPU performance estimation function of the schedutil governor via the memory stall cycle ratio so that it can dynamically reflect the effects of changes in the processor's frequency and the system's memory contention. We show that our CPU performance estimation function is easily integrated into schedutil. We also show that the memory stall cycle ratio, the key element of the function, can be efficiently calculated at runtime with performance monitoring units (PMU) commonly available in most modern SoCs. We have implemented our governor and conducted extensive experiments to validate its effectiveness. Experimental results show that our governor saves more energy than schedutil by up to 28.91% without noticeable performance degradation.