---
name: "MUTEX_PROFILING"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/MUTEX_PROFILING"
---

# MUTEX\_PROFILING(9)

kernel mutex profiling support

## NAME

**MUTEX\_PROFILING** – kernel mutex profiling support

## SYNOPSIS

**options MUTEX\_PROFILING**

## DESCRIPTION

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)](/sysctl/8.md) variables to control and monitor the profiling code:

- *debug.mutex.prof.enable*

  Enable or disable the mutex profiling code. This defaults to 0 (off).

- *debug.mutex.prof.reset*

  Reset the current mutex profiling buffers.

- *debug.mutex.prof.acquisitions*

  The total number of mutex acquisitions recorded.

- *debug.mutex.prof.records*

  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.

- *debug.mutex.prof.maxrecords*

  The maximum number of acquisition points the profiling code is capable of monitoring. Since it would not be possible to call [malloc(9)](/malloc/9.md) from within the mutex profiling code, this is a static limit. The number of records can be changed with the MPROF\_BUFFERS kernel option.

- *debug.mutex.prof.rejected*

  The number of acquisition points that were ignored after the table filled up.

- *debug.mutex.prof.hashsize*

  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.

- *debug.mutex.prof.collisions*

  The number of hash collisions in the acquisition point hash table.

- *debug.mutex.prof.stats*

  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.

## SEE ALSO

[sysctl(8)](/sysctl/8.md), [mutex(9)](/mutex/9.md)

## HISTORY

Mutex profiling support appeared in FreeBSD 5.0.

## AUTHORS

The **MUTEX\_PROFILING** code was written by Eivind Eklund ⟨eivind@FreeBSD.org⟩, Dag-Erling Smørgrav ⟨des@FreeBSD.org⟩ and Robert Watson ⟨rwatson@FreeBSD.org⟩. This manual page was written by Dag-Erling Smørgrav ⟨des@FreeBSD.org⟩.

## NOTES

The MUTEX\_PROFILING option increases the size of *struct mtx*, so a kernel built with that option will not work with modules built without it.

The MUTEX\_PROFILING option also prevents inlining of the mutex code, which results in a fairly severe performance penalty. It should therefore only be enabled on systems where mutex profiling is actually needed. MUTEX\_PROFILING will introduce a substantial performance overhead that is easily monitorable using other profiling tools, so combining profiling tools with MUTEX\_PROFILING is not recommended.

Measurements are made and stored in nanoseconds using [nanotime(9)](/nanotime/9.md), but are presented in microseconds. This should still be sufficient for the locks one would be most interested in profiling (those that are held long and/or acquired often).

MUTEX\_PROFILING should generally not be used in combination with other debugging options, as the results may be strongly affected by interactions between the features. In particular, MUTEX\_PROFILING will report higher than normal [uma(9)](/uma/9.md) lock contention when run with INVARIANTS due to extra locking that occurs when INVARIANTS is present; likewise, using it in combination with WITNESS will lead to much higher lock hold times and contention in profiling output.