MUTEX_PROFILING(9)
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MUTEX_PROFILING -- kernel mutex profiling support
options MUTEX_PROFILING
The MUTEX_PROFILING kernel option adds support for measuring and report-
ing 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.
For each acquisition point, the following statistics are accumulated:
+o The total number of non-recursive acquisitions.
+o The total time the mutex was held after being acquired at this point.
+o The longest time the mutex was ever continuously held after being
acquired at this point.
+o The total number of times the mutex was already held by another
thread when this point was reached, requiring a spin or a sleep.
+o The total number of time another thread tried to acquire the mutex
while it was held after having been acquired at this point.
In addition, the average hold time is derived from the total hold time
and the number of acquisitions.
The MUTEX_PROFILING kernel option also adds the following sysctl(8) vari-
ables to control and monitor the profiling code:
debug.mutex.prof.enable
Enable or disable the mutex profiling code. This defaults to 0
(off).
debug.mutex.prof.reset
Reset the current mutex profiling buffers.
debug.mutex.prof.acquisitions
The total number of mutex acquisitions recorded.
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.
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
malloc(9) from within the mutex profiling code, this is a static
limit. The number of records can be changed with the
MPROF_BUFFERS kernel option.
debug.mutex.prof.rejected
The number of acquisition points that were ignored after the ta-
ble filled up.
debug.mutex.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
MPROF_HASH_SIZE kernel option.
debug.mutex.prof.collisions
The number of hash collisions in the acquisition point hash ta-
ble.
debug.mutex.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.
total The total (accumulated) hold 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.
cnt_hold The number of times the mutex was held and another
thread attempted to lock the mutex.
cnt_lock The number of times the mutex was already locked 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 mutex in parentheses.
sysctl(8), mutex(9)
Mutex profiling support appeared in FreeBSD 5.0.
The MUTEX_PROFILING code was written by Eivind Eklund
<eivind@FreeBSD.org>, Dag-Erling Smorgrav <des@FreeBSD.org> and Robert
Watson <rwatson@FreeBSD.org>. This manual page was written by Dag-Erling
Smorgrav <des@FreeBSD.org>.
The MUTEX_PROFILING option increases the size of struct mtx, so a kernel
built with that option will not work with modules built without it.
The MUTEX_PROFILING option also prevents inlining of the mutex code,
which results in a fairly severe performance penalty. It should there-
fore only be enabled on systems where mutex profiling is actually needed.
Measurements are made and stored in nanoseconds using 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).