Copyright (c) 2004 Dag-Erling Co��dan Sm��rgrav
Copyright (c) 2005 Robert N. M. Watson
Copyright (c) 2006 Kip Macy
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$FreeBSD: releng/10.2/share/man/man9/LOCK_PROFILING.9 232658 2012-03-07 14:50:14Z maxim $
.Dd March 7, 2012 .Dt LOCK_PROFILING 9 .Os .Sh NAME .Nm LOCK_PROFILING .Nd kernel lock profiling support .Sh SYNOPSIS .Cd "options LOCK_PROFILING" .Sh DESCRIPTION The .Dv LOCK_PROFILING kernel option adds support for measuring and reporting lock use and contention statistics. These statistics are collated by .Dq 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.
p For each acquisition point, the following statistics are accumulated: l -bullet t The longest time the lock was ever continuously held after being acquired at this point. t The total time the lock was held after being acquired at this point. t The total time that threads have spent waiting to acquire the lock. t The total number of non-recursive acquisitions. t The total number of times the lock was already held by another thread when this point was reached, requiring a spin or a sleep. t The total number of times another thread tried to acquire the lock while it was held after having been acquired at this point. .El
p 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.
p The .Dv LOCK_PROFILING kernel option also adds the following .Xr sysctl 8 variables to control and monitor the profiling code: l -tag -width indent t Va debug.lock.prof.enable Enable or disable the lock profiling code. This defaults to 0 (off). t Va debug.lock.prof.reset Reset the current lock profiling buffers. t Va debug.lock.prof.stats The actual profiling statistics in plain text. The columns are as follows, from left to right: l -tag -width ".Va cnt_hold" t Va max The longest continuous hold time in microseconds. t Va wait_max The longest continuous wait time in microseconds. t Va total The total (accumulated) hold time in microseconds. t Va wait_total The total (accumulated) wait time in microseconds. t Va count The total number of acquisitions. t Va avg The average hold time in microseconds, derived from the total hold time and the number of acquisitions. t Va wait_avg The average wait time in microseconds, derived from the total wait time and the number of acquisitions. t Va cnt_hold The number of times the lock was held and another thread attempted to acquire the lock. t Va cnt_lock The number of times the lock was already held when this point was reached. t Va 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. .El t Va debug.lock.prof.rejected The number of acquisition points that were ignored after the table filled up. t Va 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). t Va debug.lock.prof.skipcount Do sampling approximately every N lock acquisitions. .El .Sh SEE ALSO .Xr sysctl 8 , .Xr mutex 9 .Sh HISTORY Mutex profiling support appeared in .Fx 5.0 . Generalized lock profiling support appeared in .Fx 7.0 . .Sh AUTHORS .An -nosplit The .Nm MUTEX_PROFILING code was written by .An Eivind Eklund Aq eivind@FreeBSD.org , .An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org and .An Robert Watson Aq rwatson@FreeBSD.org . The .Nm code was written by .An Kip Macy Aq kmacy@FreeBSD.org . This manual page was written by .An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org . .Sh NOTES The .Dv LOCK_PROFILING option increases the size of .Vt "struct lock_object" , so a kernel built with that option will not work with modules built without it.
p The .Dv 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. .Dv LOCK_PROFILING can introduce a substantial performance overhead that is easily monitorable using other profiling tools, so combining profiling tools with .Dv LOCK_PROFILING is not recommended.
p Measurements are made and stored in nanoseconds using .Xr 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).
p .Dv 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, .Dv LOCK_PROFILING will report higher than normal .Xr uma 9 lock contention when run with .Dv INVARIANTS due to extra locking that occurs when .Dv INVARIANTS is present; likewise, using it in combination with .Dv WITNESS will lead to much higher lock hold times and contention in profiling output.