Perf

perf is a powerful system-wide instrumentation service that is part of Linux. This article discusses how it can be relevant to power profiling.

Note: The power profiling overview is worth reading at this point if you haven’t already. It may make parts of this document easier to understand.

Energy estimates

perf can access the Intel RAPL energy estimates. The following example shows how to invoke it for this purpose.

sudo perf stat -a -r 1 \
    -e "power/energy-pkg/" \
    -e "power/energy-cores/" \
    -e "power/energy-gpu/" \
    -e "power/energy-ram/" \
    <command>

The -a is necessary; it means “all cores”, and without it all the measurements will be zero. The -r 1 means <command> is executed once; higher values can be used to get variations.

The output will look like the following.

Performance counter stats for 'system wide':

   51.58 Joules power/energy-pkg/     [100.00%]
   14.80 Joules power/energy-cores/   [100.00%]
    9.93 Joules power/energy-gpu/     [100.00%]
   27.38 Joules power/energy-ram/     [100.00%]

5.003049064 seconds time elapsed

It’s not clear from the output, but the following relationship holds.

energy-pkg >= energy-cores + energy-gpu

The measurement is in Joules, which is usually less useful than Watts.

For these reasons rapl is usually a better tool for measuring power consumption on Linux.

Wakeups {#Wakeups}

perf can also be used to do high-context profiling of wakeups.