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: 2025-07-14T21:39:18Z
Lists: pgsql-hackers 
On Mon, Jul 14, 2025 at 03:45:02PM -0500, Sami Imseih wrote:
>> Ah, I missed the problem with postmaster.  Could we have the first backend
>> that needs to access the table be responsible for creating it and
>> populating it with the built-in/requested-at-startup entries?
> 
> We can certainly maintain a flag in the shared state that is set once
> the first backend loads all the tranches in shared memory. That did not
> cross my mind, but it feels wrong to offload such responsibility to a
> normal backend.

Well, we already need each backend to either initialize or attach to the
dshash table, and the initialization would only ever happen once on a
running server.  Adding a new initialization step to bootstrap the built-in
and registered-at-startup tranche names doesn't seem like that much of a
leap to me.

Another random thought: I worry that the dshash approach might be quite a
bit slower, and IIUC we just need to map an integer to a string.  Maybe we
should just use a DSA for LWLockTrancheNames.  IOW we'd leave it as a char
** but put it in shared memory.  

-- 
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().