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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.

             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.