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Re: allow changing autovacuum_max_workers without restarting

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
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: 2025-01-07T02:44:37Z
Lists: pgsql-hackers

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On Mon, Jan 06, 2025 at 06:36:43PM -0500, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> 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.

I see.  Here's a version that uses your max_connections / 8 idea.  I've
lowered the initial default value of autovacuum_worker_slots to 12 to keep
the code as simple as possible.  I considered trying 16 in the first
iteration or constructing a complicated formula like

	autovacuum_worker_slots = (max_connections * 13) / 75 - 1

but this stuff is pretty fragile already, so I felt that simplicity was
desirable in this case.

-- 
nathan

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.