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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: · The longest time the lock was ever continuously held after being acquired at this point. · The total time the lock was held after being acquired at this point. · The total time that threads have spent waiting to acquire the lock. · The total number of non-recursive acquisitions. · The total number of times the lock was already held by another thread when this point was reached, requiring a spin or a sleep. · 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.acquisitions The total number of lock acquisitions recorded. debug.lock.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.lock.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 lock profiling code, this is a static limit. The number of records can be changed with the LPROF_BUFFERS kernel option. debug.lock.prof.rejected The number of acquisition points that were ignored after the table filled up. debug.lock.prof.hashsize 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. debug.lock.prof.collisions The number of hash collisions in the acquisition point hash table. 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.
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 Smørgrav <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 Smørgrav <des@FreeBSD.org>.
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.