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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: 2025-01-07T03:29:07Z
Lists: pgsql-hackers 
Nathan Bossart <nathandbossart@gmail.com> writes:
> On Mon, Jan 06, 2025 at 06:36:43PM -0500, Tom Lane wrote:
>> 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.

+1 for simplicity ... but on reflection, what do you think about
using max_connections / 6?  That would keep autovacuum_worker_slots
at 100 / 6 = 16 for the vast majority of systems.  For the worst case
*BSD machines, we'd select 25 / 6 = 4 which results in consuming one
more semaphore than where we were yesterday.  I'm willing to accept
that outcome though, since we still have 3 or so to spare.

Other than the specific magic number, your patch LGTM.

			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.