LOCK_PROFILING(9) FreeBSD 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
kernel source code (identified by source file name and line number) where
a lock is acquired.
For each acquisition point, the following statistics are accumulated:
o The longest time the lock was ever continuously held after being
acquired at this point.
o The total time the lock was held after being acquired at this point.
o The total time that threads have spent waiting to acquire the lock.
o The total number of non-recursive acquisitions.
o The total number of times the lock was already held by another thread
when this point was reached, requiring a spin or a sleep.
o 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)
variables 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.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.
debug.lock.prof.rejected
The number of acquisition points that were ignored after the
table filled up.
debug.lock.prof.skipspin
Disable or enable the lock profiling code for the spin locks.
This defaults to 0 (do profiling for the spin locks).
debug.lock.prof.skipcount
Do sampling approximately every N lock acquisitions.
SEE ALSO
sysctl(8), mutex(9)
HISTORY
Mutex profiling support appeared in FreeBSD 5.0. Generalized lock
profiling support appeared in FreeBSD 7.0.
AUTHORS
The MUTEX_PROFILING code was written by Eivind Eklund
<eivind@FreeBSD.org>, Dag-Erling Smorgrav <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
Smorgrav <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
combining 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
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).
LOCK_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, 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.
FreeBSD 15.1-STABLE-HBSD March 7, 2012 LOCK_PROFILING(9)