Horton

Re: Improve LWLock tranche name visibility across backends

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-07-11T21:58:30Z
Lists: pgsql-hackers 
On Fri, Jul 11, 2025 at 04:32:13PM -0500, Sami Imseih wrote:
> Now, what I think will be a good API is to provide an alternative to
> LWLockRegisterTranche,
> which now takes in both a tranche ID and tranche_name. The new API I propose is
> LWLockRegisterTrancheWaitEventCustom which takes only a tranche_name
> and internally
> calls WaitEventCustomNew to add a new wait_event_info to the hash
> table. The wait_event_info
> is made up of classId = PG_WAIT_LWLOCK and LWLockNewTrancheId().
> 
> I prefer we implement a new API for this to make it explicit that this
> API will both register
> a tranche and create a custom wait event. I do not think we should get
> rid of LWLockRegisterTranche
> because it is used by CreateLWLocks during startup and I don't see a
> reason to change that.
> See the attached v1.

Hm.  I was thinking we could have LWLockNewTrancheId() take care of
registering the name.  The CreateLWLocks() case strikes me as a special
path.  IMHO LWLockRegisterTranche() should go away.

> Is there a concern with a custom wait event to be created implicitly
> via the GetNamed* APIs?

I'm not sure I see any particular advantage to using custom wait events
versus a dedicated LWLock tranche name table.  If anything, the limits on
the number of tranches and the lengths of the names gives me pause.

-- 
nathan

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().