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: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-19T20:52:33Z
Lists: pgsql-hackers 
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > On Tue, Aug 19, 2025 at 02:37:19PM -0400, Andres Freund wrote:
> >> Sure, but we don't need to support a large number of tranches. Just make it,
> >> idk, 128 entries long. Adding a dynamically allocated dsm to every server
> >> seems like a waste - ever shared mapping makes fork / exit slower...
>
> > The other issue is that there's presently no limit on the length of a
> > tranche name registered via LWLockRegisterTranche().  Life would become
> > much simpler if we're willing to put a limit on both that and the number of
> > tranches, but thus far we've been trying to avoid it.
>
> I can hardly imagine a reason why it wouldn't be okay to limit the
> lengths of tranche names.  But especially so if an unlimited length
> causes practical problems.

If we limit the tranche name to NAMEDATALEN and also limit the
number of tranches an extension can register, we can put this
all in static shared memory (We would still need to have a backend local
cache to allow lookups to avoid going to shared memory).

However, I am not sure what the limit would be for the number of tranches,
and if we do impose something, will it break anything that is out there?

--
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().