tools/power/rapl

tools/power/rapl (or rapl for short) is a command-line utility in the Mozilla tree that periodically reads and prints all available Intel RAPL power estimates. These are machine-wide estimates, so if you want to estimate the power consumption of a single program you should minimize other activity on the machine while measuring.

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.

Invocation

First, do a standard build of Firefox.

Mac

On Mac, rapl can be run as follows.

$OBJDIR/dist/bin/rapl

Linux

On Linux, rapl can be run as root, as follows.

sudo $OBJDIR/dist/bin/rapl

Alternatively, it can be run without root privileges by setting the contents of /proc/sys/kernel/perf_event_paranoid to 0. Note that if you do change this file, its contents may reset when the machine is next rebooted.

You must be running Linux kernel version 3.14 or later for rapl to work. Otherwise, it will fail with an error message explaining this requirement.

Windows

Unfortunately, rapl does not work on Windows, and porting it would be difficult because Windows does not have APIs that allow easy access to the relevant model-specific registers.

Output

The following is 10 seconds of output from a default invocation of rapl.

    total W = _pkg_ (cores + _gpu_ + other) + _ram_ W
#01  5.17 W =  1.78 ( 0.12 +  0.10 +  1.56) +  3.39 W
#02  9.43 W =  5.44 ( 1.44 +  1.20 +  2.80) +  3.98 W
#03 14.26 W = 10.21 ( 5.47 +  0.19 +  4.55) +  4.04 W
#04 10.02 W =  6.15 ( 2.62 +  0.43 +  3.10) +  3.86 W
#05 14.63 W = 10.43 ( 4.41 +  0.81 +  5.22) +  4.19 W
#06 11.16 W =  6.90 ( 1.91 +  1.68 +  3.31) +  4.26 W
#07  5.40 W =  1.97 ( 0.20 +  0.10 +  1.67) +  3.44 W
#08  5.17 W =  1.76 ( 0.07 +  0.08 +  1.60) +  3.41 W
#09  5.17 W =  1.76 ( 0.09 +  0.08 +  1.58) +  3.42 W
#10  8.13 W =  4.40 ( 1.55 +  0.11 +  2.74) +  3.73 W

Things to note include the following.

All measurements are in Watts.
The first line indicates the meaning of each column.
The underscores in _pkg_, _gpu_ and _ram_ are present so that each column's name has five characters.
The total power is the sum of the package power and the RAM power.
The package estimate is divided into three parts: cores, GPU, and "other". "Other" is computed as the package power minus the cores power and GPU power.
If the processor does not support GPU or RAM estimates then " n/a " will be printed in the relevant column instead of a number, and it will contribute zero to the total.

Once sampling is finished — either because the user interrupted it, or because the requested number of samples has been taken — the following summary data is shown:

10 samples taken over a period of 10.000 seconds

Distribution of 'total' values:
            mean =  8.85 W
         std dev =  3.50 W
  0th percentile =  5.17 W (min)
  5th percentile =  5.17 W
 25th percentile =  5.17 W
 50th percentile =  8.13 W
 75th percentile = 11.16 W
 95th percentile = 14.63 W
100th percentile = 14.63 W (max)

The distribution data is omitted if there was zero or one samples taken.

Options

-i --sample-interval. The length of each sample in milliseconds. Defaults to 1000. A warning is given if you set it below 50 because that is likely to lead to inaccurate estimates.
-n --sample-count. The number of samples to take. The default is 0, which is interpreted as "unlimited".

Combining with `powermetrics`

On Mac, you can use the mach power command to run rapl in combination with powermetrics in a way that gives the most useful summary measurements for each of Firefox, Chrome and Safari.