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# 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](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.
## 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](tools_power_rapl.md) 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](http://robertovitillo.com/2014/02/04/idle-wakeups-are-evil/).
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