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Re: allow changing autovacuum_max_workers without restarting

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>, pgsql-hackers@lists.postgresql.org, "Imseih (AWS), Sami" <simseih@amazon.com>, Justin Pryzby <pryzby@telsasoft.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-01-06T23:36:43Z
Lists: pgsql-hackers 
Nathan Bossart <nathandbossart@gmail.com> writes:
> On Mon, Jan 06, 2025 at 05:36:17PM -0500, Tom Lane wrote:
>> You'd have to think about how
>> it should interact with initdb's probes for workable values of
>> max_connections.  My first thought about that is to have initdb
>> set autovacuum_worker_slots to max_connections / 8 or thereabouts
>> as it works down the list of max_connections values to try.  Or
>> you could do something more complicated, but I don't see a reason
>> to make it too complex.

> My first instinct was just to set it to the lowest default we'd consider
> during the max_connections tests (which I'm assuming is 3 due to the
> current default for autovacuum_max_workers).  That way, the max_connections
> default won't change from version to version on affected systems, but you
> might get some extra autovacuum slots.

My only objection to this algorithm is it adds cycles to initdb,
in the form of at least one additional "postgres --check" step.
Admittedly that's not hugely expensive, but it'll add up over time
in the buildfarm, and I'm not sure this issue is worth that.
We already changed the max_connections default for affected systems
as a consequence of 38da05346, so I don't think the argument about not
changing it holds much water.

			regards, tom lane

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Give up on running with NetBSD/OpenBSD's default semaphore settings.

  2. initdb: Do not report default autovacuum_worker_slots.

  3. Lower default value of autovacuum_worker_slots in initdb as needed.

  4. Allow changing autovacuum_max_workers without restarting.

  5. Reserve a PGPROC slot and semaphore for the slotsync worker process.

  6. Make archiver process an auxiliary process.

  7. Correct the formulas for System V IPC parameters SEMMNI and SEMMNS in docs.