dtrace.md

dtrace

dtrace is a powerful Mac OS X kernel instrumentation system that can be used to profile wakeups. This article provides a light introduction to it.

::: 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

dtrace must be invoked as the super-user. A good starting command for profiling wakeups is the following.

sudo dtrace -n 'mach_kernel::wakeup { @[ustack()] = count(); }' -p $FIREFOX_PID > $OUTPUT_FILE

Let's break that down further.

• The -n option combined with the mach_kernel::wakeup selects a probe point. mach_kernel is the module name and wakeup is the probe name. You can see a complete list of probes by running sudo dtrace -l.
• The code between the braces is run when the probe point is hit. The above code counts unique stack traces when wakeups occur; ustack is short for "user stack", i.e. the stack of the userspace program executing.

Run that command for a few seconds and then hit [Ctrl]{.kbd} + [C]{.kbd} to interrupt it. dtrace will then print to the output file a number of stack traces, along with a wakeup count for each one. The ordering of the stack traces can be non-obvious, so look at them carefully.

Sometimes the stack trace has less information than one would like. It's unclear how to improve upon this.

See also

dtrace is very powerful, and you can learn more about it by consulting the following resources:

• The DTrace one-liner tutorial from FreeBSD.
• DTrace tools, by Brendan Gregg.