Horton

Re: Improve LWLock tranche name visibility across backends

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Sami Imseih <samimseih@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-19T20:26:41Z
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.

			regards, tom lane

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().