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Re: Adding locks statistics

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Michael Paquier <michael@paquier.xyz>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Jeff Davis <pgsql@j-davis.com>, Greg Sabino Mullane <htamfids@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2026-03-30T16:11:17Z
Lists: pgsql-hackers 
Hi,

Isn't pgstat_lock_flush_cb a bit broken with nowait=true? It'll skip
flushing stats for that particular lock type, but then it'll happily
reset the pending stats anyway, forgetting the stats.

AFAIK it should keep the pending stats, and flush them sometime lager,
when the lock is not contended. That's what the other flush callbacks
do, at least. This probably means it needs to reset the entries one by
one, not the whole struct at once.

TBH I'm rather skeptical about having one lock per entry. Sure, it
allows two backends to write different entries concurrently. But is it
actually worth it? With nowait=true it might even be cheaper to try with
a single lock, and AFAICS that's the case where it matters.

I wouldn't be surprised if this behaved quite poorly with contended
cases, because the backends will be accessing the locks in exactly the
same order and synchronize. So if one lock is contended, won't that
"synchronize" the backends, making the other locks contended too?

Has anyone tested it actually improves the behavior? I only quickly
skimmed the thread, I might have missed it ...

If per-entry locks help, maybe a good micro-optimization would be to
check if there even is anything to sync before acquiring the lock. I
mean, if (waits==0), why obtain the lock?


regards

-- 
Tomas Vondra

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix injection point detach timing problem in TAP test for lock stats

  2. Use single LWLock for lock statistics in pgstats

  3. Add tests for lock statistics, take two

  4. Remove isolation test lock-stats

  5. Avoid including clog.h in proc.h

  6. Don't include storage/lock.h in so many headers

  7. Add support for lock statistics in pgstats

  8. Move some code blocks in lock.c and proc.c

  9. Move declarations related to locktags from lock.h to new locktag.h