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MUTEX_PROFILING - kernel mutex profiling support
The MUTEX_PROFILING kernel option adds support for measuring and
reporting mutex 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 mutex is acquired.
For each acquisition point, the following statistics are accumulated:
· The total number of non-recursive acquisitions.
· The total time the mutex was held after being acquired at this point.
· The longest time the mutex was ever continuously held after being
acquired at this point.
· The total number of times the mutex was already held by another
thread when this point was reached, requiring a spin or a sleep.
· 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)
variables to control and monitor the profiling code:
Enable or disable the mutex profiling code. This defaults to 0
Reset the current mutex profiling buffers.
The total number of mutex acquisitions recorded.
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.
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.
The number of acquisition points that were ignored after the
table filled up.
The size of the hash table used to map acquisition points to
statistics records. The hash size can be changed with the
MPROF_HASH_SIZE kernel option.
The number of hash collisions in the acquisition point hash
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.
Mutex profiling support appeared in FreeBSD 5.0.
The MUTEX_PROFILING code was written by Eivind Eklund
〈eivind@FreeBSD.org〉, Dag-Erling Smørgrav 〈des@FreeBSD.org〉 and Robert
Watson 〈rwatson@FreeBSD.org〉. This manual page was written by Dag-Erling
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
therefore only be enabled on systems where mutex profiling is actually
needed. MUTEX_PROFILING will introduce a substantial performance
overhead that is easily monitorable using other profiling tools, so
combining profiling tools with MUTEX_PROFILING is not recommended.
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).
MUTEX_PROFILING should generally not be used in combination with other
debugging options, as the results may be strongly affected by
interactions between the features. In particular, MUTEX_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.