Horton

Re: Improve LWLock tranche name visibility across backends

Sami Imseih <samimseih@gmail.com>

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From: Sami Imseih <samimseih@gmail.com>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-06T16:54:33Z
Lists: pgsql-hackers 
Thanks for testing!

> which is what we expect.
>
> But if I look at LWLockTrancheNames.local (for the process that queried pg_stat_activity):
>
> (gdb) p LWLockTrancheNames.local[0]
> $1 = 0x0
> (gdb) p LWLockTrancheNames.local[1]
> $2 = 0x7b759a81b038 "BDT_Lck"
>
> It looks like there is an indexing issue, as we should start at index 0.

>
> Issue 2 / question --
>
> If I call LWLockNewTrancheId("BDT_play2") multiple times to ensure that
>
>
> Then if, in another backend, I call GetLWTrancheName by querying pg_stat_activity
> then I see "BDT_Lck" being reported as wait event name (which is good).
>
> The thing that worries me a bit is that the local cache is populated only for
> "BDT_Lck", but not for all the other "BDT_play2".

Issue #1 and issue #2 are related. In both cases you have a registered wait
event that only lives in shared memory, but was never required to be
accessed by pg_stat_activity, so it was never cached locally.


> When we need to populate the local cache, would it be better to populate
> it with the whole dsa content instead of just the current missing ID as done
> in GetLWTrancheName():

I do agree that we should just sync the local cache totally whenever we
can't find the tranche name in the local cache. Will fix in the next rev.

--
Sami

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().