Kernel Profiling on FreeBSD

Configuration

Use "config -p" to configure the kernel with profiling support; make sure to build the kernel from scratch (i.e., do a make clean before make depend). "config -p" will configure for low-resolution timers; specifying "-pp" will configure for high-resolution timers.

Use

Boot the kernel, and use the kgmon(8) command to frob the kernel profiling state. The "-b" (-p) and "-B" (-pp) parameters turn on profiling; "-h" turns it off, and "-p" dumps the profiling buffers to a gmon.out file in the current working directory.

You can then use gprof against the kernel binary and profile buffer to generate call graphs and user-readable profile data. Typically, this will take the form of "gprof /boot/kernel/kernel gmon.out".

You can also use kernbb(8) to dump the profile buffers in a form to be used by the gcov(1) tool to generate coverage data.

Caveats

• Profiling may generate incorrect results for SMP kernels. For now, it is recommended to profile on UP kernels. This needs to be fixed ASAP.
• It's not clear to me how to tell gprof about kernel modules (and shared libraries generally). I recommend statically linking all modules into the kernel.
• Profiling "ticks" from hardclock won't run during critical sections as interrupts are disabled. They kick in as critical sections end, so the time spent in functions within the critical section will be billed to critical_exit() or cpu_critical_exit().

See Also

See also: gcov(1), gprof(1), config(8), kernbb(8), kgmon(8)

Some Files

supersmack_mutex_profile
up_prof.out