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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>, "Imseih (AWS), Sami" <simseih@amazon.com>, Justin Pryzby <pryzby@telsasoft.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2025-01-06T21:50:24Z
Lists: pgsql-hackers 
On Mon, Jan 06, 2025 at 04:29:37PM -0500, Tom Lane wrote:
> Unsurprisingly, this has completely broken buildfarm member sawshark:
> you added 13 new semaphores to the system's default requirements,
> and we only had headroom for about 4 (cf. 38da05346).

Oh wow, I missed that the defaults were so low on some systems.

> Now maybe we should just abandon the notion that we ought to be
> able to start up under OpenBSD/NetBSD's tiny default value of SEMMNS.
> If so I'd be inclined to go revert 38da05346, at least in HEAD.
> But I kind of wonder whether this feature actually brings value
> commensurate with causing installation problems on real-world OSes.

I'm obviously biased, but I think it would be unfortunate to block features
like this one because of low settings that would otherwise be unsuitable
for any reasonable production workload.  If we do want to at least support
check-world on these systems, another option could be to simply lower the
default of autovacuum_worker_slots to 7 (or maybe lower).  Of course, that
only helps until the next time more semaphores are required, but that's not
a new problem.

-- 
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.