NAME

     LOCK_PROFILING — kernel lock profiling support

SYNOPSIS

     options LOCK_PROFILING

DESCRIPTION

     The LOCK_PROFILING kernel option adds support for measuring and reporting lock 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 lock is acquired.

     For each acquisition point, the following statistics are accumulated:

     •   The longest time the lock was ever continuously held after being acquired at this point.

     •   The total time the lock was held after being acquired at this point.

     •   The total time that threads have spent waiting to acquire the lock.

     •   The total number of non-recursive acquisitions.

     •   The total number of times the lock was already held by another thread when this point
         was reached, requiring a spin or a sleep.

     •   The total number of times another thread tried to acquire the lock while it was held
         after having been acquired at this point.

     In addition, the average hold time and average wait time are derived from the total hold
     time and total wait time respectively and the number of acquisitions.

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

     debug.lock.prof.enable
             Enable or disable the lock profiling code.  This defaults to 0 (off).

     debug.lock.prof.reset
             Reset the current lock profiling buffers.

     debug.lock.prof.acquisitions
             The total number of lock acquisitions recorded.

     debug.lock.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.lock.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 lock
             profiling code, this is a static limit.  The number of records can be changed with
             the LPROF_BUFFERS kernel option.

     debug.lock.prof.rejected
             The number of acquisition points that were ignored after the table filled up.

     debug.lock.prof.hashsize
             The size of the hash table used to map acquisition points to statistics records.
             The hash size can be changed with the LPROF_HASH_SIZE kernel option.

     debug.lock.prof.collisions
             The number of hash collisions in the acquisition point hash table.

     debug.lock.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.

             wait_max  The longest continuous wait time in microseconds.

             total     The total (accumulated) hold time in microseconds.

             wait_total
                       The total (accumulated) wait 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.

             wait_avg  The average wait time in microseconds, derived from the total wait time
                       and the number of acquisitions.

             cnt_hold  The number of times the lock was held and another thread attempted to
                       acquire the lock.

             cnt_lock  The number of times the lock was already held 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 lock in parentheses.

SEE ALSO

     sysctl(8), mutex(9)

HISTORY

     Mutex profiling support appeared in FreeBSD 5.0.  Generalized lock profiling support
     appeared in FreeBSD 7.0.

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>.  The LOCK_PROFILING code
     was written by Kip Macy <kmacy@FreeBSD.org>.  This manual page was written by Dag-Erling
     Smørgrav <des@FreeBSD.org>.

NOTES

     The LOCK_PROFILING option increases the size of struct lock_object, so a kernel built with
     that option will not work with modules built without it.

     The LOCK_PROFILING option also prevents inlining of the mutex code, which can result in a
     fairly severe performance penalty.  This is, however, not always the case.  LOCK_PROFILING
     can introduce a substantial performance overhead that is easily monitorable using other
     profiling tools, so combining profiling tools with LOCK_PROFILING is not recommended.

     Measurements are made and stored in nanoseconds using nanotime(9), (on architectures without
     a synchronized TSC) 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).

     LOCK_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,
     LOCK_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.