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+# 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](power_profiling_overview.md) 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](/setup/index.rst).
+
+### Mac
+
+On Mac, `rapl` can be run as follows.
+
+``` {.brush: .bash}
+$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](http://unix.stackexchange.com/questions/14227/do-i-need-root-admin-permissions-to-run-userspace-perf-tool-perf-events-ar)
+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`.
+
+``` {.brush: .bash}
+    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:
+
+``` {.brush: .bash}
+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](powermetrics.html#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.