LOCK_PROFILING(9)

💅 LOCK_PROFILING — kernel lock profiling support

okay bestie, welcome to LOCK_PROFILING(9), a kernel developer's manual page, and lemme tell you, this one is so niche and so useful. it's the vibe check for your locks. 🔥

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✨ SYNOPSIS ✨

options LOCK_PROFILING

that's it. one line. iconic behavior honestly. 💅

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🦋 DESCRIPTION 🦋

so the LOCK_PROFILING kernel option adds support for measuring and reporting lock use and contention statistics. yes, it's giving "your locks are being watched and the receipts are being kept." 😌

these stats get collated by “acquisition point”. and what is an acquisition point, you ask? it's a distinct place in the kernel source code — identified by source file name and line number — where a lock is acquired. so like, not "the mutex," but "the exact line where somebody grabbed the mutex." very specific. very main character.

for each acquisition point, these stats pile up:

• 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.

and 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. math! ✨

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the LOCK_PROFILING kernel option also adds the following sysctl(8) variables so you can control and monitor the profiling code. here's the whole roster, no skips:

• 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. fresh start bestie 🧚
• 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. 😬 yeah. the table can fill 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.

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👀 SEE ALSO 👀

sysctl(8), mutex(9)

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📜 HISTORY 📜

• Mutex profiling support appeared in FreeBSD 5.0.
• Generalized lock profiling support appeared in FreeBSD 7.0.

glow up timeline fr fr ✨

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🫶 AUTHORS 🫶

The MUTEX_PROFILING code was written by:

• Eivind Eklund <eivind@FreeBSD.org>
• Dag-Erling Smørgrav <des@FreeBSD.org>
• 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>.

absolute legends, all of them 👑

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⚠️ NOTES ⚠️

bestie. please read these. i'm begging. 🙈

• 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. no cap. it just won't. 💀
• 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. 😭😭

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and that's the whole page, bestie!! 🎀 this one came from the Debian build of the page, dated March 7, 2012. go forth and profile your locks responsibly, i love you, bye 💖✨🦋