freebsd-nq/share/man/man9/MUTEX_PROFILING.9

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.Dd January 25, 2004
.Dt MUTEX_PROFILING 9
.Os
.Sh NAME
.Nm MUTEX_PROFILING
.Nd kernel mutex profiling support
.Sh SYNOPSIS
.Cd "options MUTEX_PROFILING"
.Sh DESCRIPTION
The
.Dv MUTEX_PROFILING
kernel option adds support for measuring and reporting mutex use and
contention statistics.
These statistics are collated by acquisition point, these being
distinct places in the kernel source code (identified by source file
name and line number) where a mutex is acquired.
.Pp
For each acquisition point, the following statistics are accumulated:
.Bl -bullet
.It
The total number of non-recursive acquisitions.
.It
The total time the mutex was held after being acquired at this point.
.It
The longest time the mutex was ever continuously held after being
acquired at this point.
.It
The total number of times the mutex was already held by another thread
when this point was reached, requiring a spin or a sleep.
.It
The total number of time another thread tried to acquire the mutex
while it was held after having been acquired at this point.
.El
.Pp
In addition, the average hold time is derived from the total hold time
and the number of acquisitions.
.Pp
The
.Dv MUTEX_PROFILING
kernel option also adds the following
.Xr sysctl 8
variables to control and monitor the profiling code:
.Bl -tag -width indent
.It Va debug.mutex.prof.enable
Enable or disable the mutex profiling code.
This defaults to 0 (off).
.It Va debug.mutex.prof.reset
Reset the current mutex profiling buffers.
.It Va debug.mutex.prof.acquisitions
The total number of mutex acquisitions recorded.
.It Va debug.mutex.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.
.It Va debug.mutex.prof.maxrecords
The maximum number of acquisition points the profiling code is capable
of monitoring.
Since it would not be possible to call
.Xr malloc 9
from within the mutex profiling code, this is a static limit.
.It Va debug.mutex.prof.rejected
The number of acquisition points that were ignored after the table
filled up.
.It Va debug.mutex.prof.hashsize
The size of the hash table used to map acquisition points to
statistics records.
.It Va debug.mutex.prof.collisions
The number of hash collisions in the acquisition point hash table.
.It Va debug.mutex.prof.stats
The actual profiling statistics in plain text.
The columns are as follows, from left to right:
.Bl -tag -width ".Va cnt_hold"
.It Va max
The longest continuous hold time.
.It Va total
The total (accumulated) hold time.
.It Va count
The total number of acquisitions.
.It Va avg
The average hold time, derived from the total hold time and the number
of acquisitions.
.It Va cnt_hold
The number of times the mutex was contested while being held.
.It Va cnt_lock
The number of times the mutex was already locked when this point was
reached.
.It Va name
The name of the acquisition point, derived from the source file name
and line number, followed by the name of the mutex in parentheses.
.El
.El
.Sh SEE ALSO
.Xr sysctl 8 ,
.Xr mutex 9
.Sh HISTORY
Mutex profiling support appeared in
.Fx 5.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
code was written by
.An Eivind Eklund Aq eivind@FreeBSD.org ,
.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org
and
.An Robert Watson Aq rwatson@FreeBSD.org .
This manual page was written by
.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org .
.Sh NOTES
The
.Dv MUTEX_PROFILING
option increases the size of
.Vt "struct mtx" ,
so a kernel built with that option will not work with modules built
without it.
.Pp
The
.Dv MUTEX_PROFILING
option also prevents inlining of the mutex code, which results in a
fairly severe performance penalty.
It should therefore only be enabled on systems where mutex profiling
is actually needed.
.Pp
Measurements are made and stored in nanoseconds using
.Xr nanotime 9 ,
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).