Energy Consumption of Software – Using CPU Time or CPU Utilization as a Proxy

Figure 1: Two processes that get scheduled the same CPU time by CFS but significantly differ in their energy consumption.

We demonstrate this discrepancy between CPU and energy consumption with a simple experiment. Figure 1 shows the energy consumption of two processes that are run on Linux with CFS, which allocates each of the processes roughly the same CPU time. The two processes that execute two Python programs. Process 1 is memory-intensive: it repeatedly reads and writes large byte-arrays into heap memory, while process 2 is CPU-intensive: it repeatedly computes SHA256 on long strings (see §5 for the full evaluation setup). We measure each process’s energy consumption using Wattmeter. The experiment shows that despite the fact that both processes are scheduled almost identical CPU time, process 2 consumes about 50% more energy than process 1. In general, variability in energy consumption can stem from factors such as the usage of different CPU instructions, memory consumption, and the cache hit rate [17]. Linux today has no awareness of this variability in energy consumption across processes.