bionic (9) LOCK_PROFILING.9freebsd.gz

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.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.

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

     debug.lock.prof.skipspin
             Disable or enable the lock profiling code for the spin locks.  This defaults to 0 (do profiling for
             the spin locks).

     debug.lock.prof.skipcount
             Do sampling approximately every N lock acquisitions.

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.