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  1. Remove check hooks for GUCs that contribute to MaxBackends.

  1. remove check hooks for GUCs that contribute to MaxBackends

    Nathan Bossart <nathandbossart@gmail.com> — 2024-06-19T19:04:58Z

    While working on an idea from another thread [0], I noticed that each of
max_connections, max_worker_process, max_autovacuum_workers, and
max_wal_senders have a check hook that verifies the sum of those GUCs does
not exceed a certain value.  Then, in InitializeMaxBackends(), we do the
same check once more.  Not only do the check hooks seem redundant, but I
think they might sometimes be inaccurate since some values might not yet be
initialized.  Furthermore, the error message is not exactly the most
descriptive:

	$ pg_ctl -D . start -o "-c max_connections=262100 -c max_wal_senders=10000"

	FATAL:  invalid value for parameter "max_wal_senders": 10000

The attached patch removes these hooks and enhances the error message to
look like this:

	FATAL:  too many backends configured
	DETAIL:  "max_connections" (262100) plus "autovacuum_max_workers" (3) plus "max_worker_processes" (8) plus "max_wal_senders" (10000) must be less than 262142.

The downside of this change is that server startup progresses a little
further before it fails, but that might not be too concerning given this
_should_ be a relatively rare occurrence.

Thoughts?

[0] https://postgr.es/m/20240618213331.ef2spg3nasksisbi%40awork3.anarazel.de

-- 
nathan

  2. Re: remove check hooks for GUCs that contribute to MaxBackends

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-06-19T19:09:36Z

    Nathan Bossart <nathandbossart@gmail.com> writes:
> While working on an idea from another thread [0], I noticed that each of
> max_connections, max_worker_process, max_autovacuum_workers, and
> max_wal_senders have a check hook that verifies the sum of those GUCs does
> not exceed a certain value.  Then, in InitializeMaxBackends(), we do the
> same check once more.  Not only do the check hooks seem redundant, but I
> think they might sometimes be inaccurate since some values might not yet be
> initialized.

Yeah, these per-variable checks are inherently bogus.  If we can get
of them and make the net user experience actually better, that's a
win-win.

It seems easier to do for these because they can't change after server
start, so there can be one well-defined time to apply the consistency
check.  IIRC, we have some similar issues in other hooks for variables
that aren't PGC_POSTMASTER, so it's harder to see how we might get rid
of their cross-checks.  That doesn't make them less bogus though.

			regards, tom lane

  3. Re: remove check hooks for GUCs that contribute to MaxBackends

    Tom Lane <tgl@sss.pgh.pa.us> — 2024-06-19T19:14:16Z

    Nathan Bossart <nathandbossart@gmail.com> writes:
> The attached patch removes these hooks and enhances the error message to
> look like this:

> 	FATAL:  too many backends configured
> 	DETAIL:  "max_connections" (262100) plus "autovacuum_max_workers" (3) plus "max_worker_processes" (8) plus "max_wal_senders" (10000) must be less than 262142.

BTW, I suggest writing it as "too many server processes configured",
or perhaps "too many server processes required".  "Backend" is too
much of an insider term.

			regards, tom lane

  4. Re: remove check hooks for GUCs that contribute to MaxBackends

    Nathan Bossart <nathandbossart@gmail.com> — 2024-06-19T19:24:08Z

    On Wed, Jun 19, 2024 at 03:14:16PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> The attached patch removes these hooks and enhances the error message to
>> look like this:
> 
>> 	FATAL:  too many backends configured
>> 	DETAIL:  "max_connections" (262100) plus "autovacuum_max_workers" (3) plus "max_worker_processes" (8) plus "max_wal_senders" (10000) must be less than 262142.
> 
> BTW, I suggest writing it as "too many server processes configured",
> or perhaps "too many server processes required".  "Backend" is too
> much of an insider term.

Will do, thanks for reviewing.

-- 
nathan

  5. Re: remove check hooks for GUCs that contribute to MaxBackends

    Nathan Bossart <nathandbossart@gmail.com> — 2024-07-05T19:50:39Z

    Committed.

-- 
nathan