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LOCK_PROFILING -- kernel lock profiling support
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
The LOCK_PROFILING kernel option also adds the following sysctl(8)
variables to control and monitor the profiling code:
Enable or disable the lock profiling code. This defaults to 0
Reset the current lock profiling buffers.
The total number of lock 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 lock profiling code, this is a static
limit. The number of records can be changed with the
LPROF_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
LPROF_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.
wait_max The longest continuous wait time in microseconds.
total The total (accumulated) hold time in microseconds.
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.
Mutex profiling support appeared in FreeBSD 5.0. Generalized lock
profiling support appeared in FreeBSD 7.0.
The MUTEX_PROFILING code was written by Eivind Eklund
<eivind@FreeBSD.org>, Dag-Erling Smorgrav <des@FreeBSD.org> and Robert
Watson <rwatson@FreeBSD.org>. The LOCK_PROFILING code was written by Kip
Macy <kmacy@FreeBSD.org>. This manual page was written by Dag-Erling
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
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
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.