LOCK_PROFILING(9)					   BSD Kernel Developer's Manual					 LOCK_PROFILING(9)

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:

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

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

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

     o	 The total number of non-recursive acquisitions.

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

     o	 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 Smorgrav <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
     Smorgrav <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 with-
     out it.

     The LOCK_PROFILING option also prevents inlining of the mutex code, which can result in a fairly severe performance penalty.  This is, how-
     ever, 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 interac-
     tions 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.

BSD
								   March 7, 2012							       BSD