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: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-25T21:04:56Z
Lists: pgsql-hackers 
On Fri, Aug 22, 2025 at 03:01:53PM -0500, Sami Imseih wrote:
> I kept the local array to serve consecutive reads and to avoid having to
> take a shared lock on shared memory every time GetLWTrancheName is
> called. A new LWLock to protect this array is required.

I'm not seeing why we need this cache anymore.  This is an append-only
list, so we could instead keep a backend-local copy of LWLockCounter that
gets updated as needed.  As long as the ID is less than our backend-local
counter, we can go straight to the shared array.  If it is greater, we'll
have to first update our counter, which should be rare and inexpensive.

-- 
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().