LOCK_PROFILING man page on FreeBSD

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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 ker‐
     nel 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) 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.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 ta‐
	     ble filled up.

     debug.lock.prof.hashsize
	     The size of the hash table used to map acquisition points to sta‐
	     tistics 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 ta‐
	     ble.

     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 pro‐
     filing 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 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.

BSD			       November 11, 2006			   BSD
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