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MUTEX_PROFILING - kernel mutex profiling support


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The MUTEX_PROFILING kernel option adds support for measuring and reporting mutex use and contention statistics. These statistics are collated by "acquisition point". Acquisition points are distinct places in the kernel source code (identified by source file name and line number) where a mutex is acquired.

For each acquisition point, the following statistics are accumulated:

  • The total number of non-recursive acquisitions.
  • The total time the mutex was held after being acquired at this point.
  • The longest time the mutex was ever continuously held after being acquired at this point.
  • The total number of times the mutex was already held by another thread when this point was reached, requiring a spin or a sleep.
  • The total number of time another thread tried to acquire the mutex while it was held after having been acquired at this point.

In addition, the average hold time is derived from the total hold time and the number of acquisitions.

The MUTEX_PROFILING kernel option also adds the following sysctl(8) variables to control and monitor the profiling code:

Enable or disable the mutex profiling code. This defaults to 0 (off).
Reset the current mutex profiling buffers.
The total number of mutex acquisitions recorded.
The total number of acquisition points recorded. Note that only active acquisition points (i.e., points that have been reached at least once) are counted.
The maximum number of acquisition points the profiling code is capable of monitoring. Since it would not be possible to call malloc(9) from within the mutex profiling code, this is a static limit. The number of records can be changed with the MPROF_BUFFERS kernel option.
The number of acquisition points that were ignored after the table filled up.
The size of the hash table used to map acquisition points to statistics records. The hash size can be changed with the MPROF_HASH_SIZE kernel option.
The number of hash collisions in the acquisition point hash table.
The actual profiling statistics in plain text. The columns are as follows, from left to right:
max The longest continuous hold time in microseconds.
total The total (accumulated) hold time in microseconds.
count The total number of acquisitions.
avg The average hold time in microseconds, derived from the total hold time and the number of acquisitions.
cnt_hold The number of times the mutex was held and another thread attempted to lock the mutex.
cnt_lock The number of times the mutex was already locked when this point was reached.
name The name of the acquisition point, derived from the source file name and line number, followed by the name of the mutex in parentheses.


sysctl(8), mutex(9)




nanotime(9), uma(9)

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