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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)
     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) 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), mutex(9)

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), 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) 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.