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with other profiling and debugging options, such as INVARIANTS, WITNESS, kernel profiling, etc. They all interfere with each other nastily and will generate fairly useless results.
191 lines
6.4 KiB
Groff
191 lines
6.4 KiB
Groff
.\"-
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.\" Copyright (c) 2004 Dag-Erling Coïdan Smørgrav
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.\" Copyright (c) 2005 Robert N. M. Watson
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 7, 2005
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.Dt MUTEX_PROFILING 9
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.Os
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.Sh NAME
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.Nm MUTEX_PROFILING
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.Nd kernel mutex profiling support
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.Sh SYNOPSIS
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.Cd "options MUTEX_PROFILING"
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.Sh DESCRIPTION
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The
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.Dv MUTEX_PROFILING
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kernel option adds support for measuring and reporting mutex use and
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contention statistics.
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These statistics are collated by
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.Dq acquisition point .
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Acquisition points are
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distinct places in the kernel source code (identified by source file
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name and line number) where a mutex is acquired.
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.Pp
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For each acquisition point, the following statistics are accumulated:
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.Bl -bullet
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.It
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The total number of non-recursive acquisitions.
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.It
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The total time the mutex was held after being acquired at this point.
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.It
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The longest time the mutex was ever continuously held after being
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acquired at this point.
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.It
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The total number of times the mutex was already held by another thread
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when this point was reached, requiring a spin or a sleep.
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.It
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The total number of time another thread tried to acquire the mutex
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while it was held after having been acquired at this point.
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.El
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.Pp
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In addition, the average hold time is derived from the total hold time
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and the number of acquisitions.
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.Pp
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The
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.Dv MUTEX_PROFILING
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kernel option also adds the following
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.Xr sysctl 8
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variables to control and monitor the profiling code:
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.Bl -tag -width indent
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.It Va debug.mutex.prof.enable
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Enable or disable the mutex profiling code.
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This defaults to 0 (off).
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.It Va debug.mutex.prof.reset
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Reset the current mutex profiling buffers.
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.It Va debug.mutex.prof.acquisitions
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The total number of mutex acquisitions recorded.
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.It Va debug.mutex.prof.records
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The total number of acquisition points recorded.
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Note that only active acquisition points (i.e., points that have been
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reached at least once) are counted.
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.It Va debug.mutex.prof.maxrecords
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The maximum number of acquisition points the profiling code is capable
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of monitoring.
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Since it would not be possible to call
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.Xr malloc 9
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from within the mutex profiling code, this is a static limit.
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The number of records can be changed with the
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.Dv MPROF_BUFFERS
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kernel option.
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.It Va debug.mutex.prof.rejected
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The number of acquisition points that were ignored after the table
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filled up.
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.It Va debug.mutex.prof.hashsize
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The size of the hash table used to map acquisition points to
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statistics records.
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The hash size can be changed with the
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.Dv MPROF_HASH_SIZE
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kernel option.
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.It Va debug.mutex.prof.collisions
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The number of hash collisions in the acquisition point hash table.
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.It Va debug.mutex.prof.stats
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The actual profiling statistics in plain text.
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The columns are as follows, from left to right:
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.Bl -tag -width ".Va cnt_hold"
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.It Va max
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The longest continuous hold time in microseconds.
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.It Va total
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The total (accumulated) hold time in microseconds.
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.It Va count
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The total number of acquisitions.
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.It Va avg
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The average hold time in microseconds, derived from the total hold time
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and the number of acquisitions.
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.It Va cnt_hold
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The number of times the mutex was held and another thread attempted to
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lock the mutex.
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.It Va cnt_lock
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The number of times the mutex was already locked when this point was
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reached.
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.It Va name
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The name of the acquisition point, derived from the source file name
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and line number, followed by the name of the mutex in parentheses.
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.El
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.El
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.Sh SEE ALSO
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.Xr sysctl 8 ,
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.Xr mutex 9
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.Sh HISTORY
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Mutex profiling support appeared in
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.Fx 5.0 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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code was written by
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.An Eivind Eklund Aq eivind@FreeBSD.org ,
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.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org
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and
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.An Robert Watson Aq rwatson@FreeBSD.org .
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This manual page was written by
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.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org .
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.Sh NOTES
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The
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.Dv MUTEX_PROFILING
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option increases the size of
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.Vt "struct mtx" ,
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so a kernel built with that option will not work with modules built
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without it.
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.Pp
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The
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.Dv MUTEX_PROFILING
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option also prevents inlining of the mutex code, which results in a
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fairly severe performance penalty.
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It should therefore only be enabled on systems where mutex profiling
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is actually needed.
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.Dv MUTEX_PROFILING
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will introduce a substantial performance overhead that is easily
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monitorable using other profiling tools, so combining profiling tools
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with
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.Dv MUTEX_PROFILING
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is not recommended.
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.Pp
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Measurements are made and stored in nanoseconds using
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.Xr nanotime 9 ,
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but are presented in microseconds.
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This should still be sufficient for the locks one would be most
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interested in profiling (those that are held long and/or acquired
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often).
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.Pp
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.Dv MUTEX_PROFILING
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should generally not be used in combination with other debugging options, as
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the results may be strongly affected by interactions between the features.
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In particular,
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.Dv MUTEX_PROFILING
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will report higher than normal
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.Xr uma 9
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lock contention when run with
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.Dv INVARIANTS
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due to extra locking that occurs when
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.Dv INVARIANTS
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is present; likewise, using it in combination with
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.Dv WITNESS
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with
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will lead to much higher lock hold times and contention in profiling output.
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