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LOCK_PROFILING(9)      FreeBSD 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)	vari-
     ables 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 ta-
	     ble 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	pro-
     filing 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 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 com-
     bining 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 microsec-
     onds.  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.

FreeBSD	10.1			 March 7, 2012			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | AUTHORS | NOTES

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