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From: David Rowley <dgrowleyml@gmail.com>
Date: Sun, 12 Oct 2025 19:27:10 +1300
Message-ID: 
 <CAApHDvqrd=SHVUytdRj55OWnLH98Rvtzqam5zq2f4XKRZa7t9Q@mail.gmail.com>
Subject: Re: another autovacuum scheduling thread
To: Robert Haas <robertmhaas@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>,
 Jeremy Schneider <schneider@ardentperf.com>,
	Sami Imseih <samimseih@gmail.com>, pgsql-hackers@postgresql.org
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On Sat, 11 Oct 2025 at 07:43, Robert Haas <robertmhaas@gmail.com> wrote:
> I think this is a reasonable starting point, although I'm surprised
> that you chose to combine the sub-scores using + rather than Max.

Adding up the component scores doesn't make sense to me either. That
means you could have 0.5 for inserted tuples, 0.5 for dead tuples and,
say 0.1 for analyze threshold, which all add up to 1.1, but neither
component score is high enough for auto-vacuum to have to do anything
yet. With Max(), we'd clearly see that there's nothing to do since the
overall score isn't >= 1.0.

> - The wraparound score can't be more than about 10, but the bloat
> score could be arbitrarily large, especially for tables with few
> tuples, so there may be lots of cases in which the wraparound score
> has no impact on the behavior.

That's a good point. I think we definitely do want to make it so
tables in near danger of causing the database to stop accepting
transactions are dealt with ASAP.

Maybe the score calculation could change when the relevant age() goes
above vacuum_failsafe_age / vacuum_multixact_failsafe_age and start
scaling it very aggressively beyond that. There's plenty to debate,
but at a first cut, maybe something like the following (coded in SQL
for ease of result viewing):

select xidage as "age(relfrozenxid)",case xidage::float8 <
current_setting('vacuum_failsafe_age')::float8 when true then xidage /
current_setting('autovacuum_freeze_max_age')::float8 else power(xidage
/ current_setting('autovacuum_freeze_max_age')::float8,xidage::float8
/ 100_000_000) end xid_age_score from
generate_series(0,2_000_000_000,100_000_000) xidage;

which gives 1e+20 for age of 2 billion. It would take quite an
unreasonable amount of bloat to score higher than that.

I guess someone might argue that we should start taking it more
seriously before the table's relfrozenxid age gets to
vacuum_failsafe_age. Maybe that's true. I just don't know what. In any
case, if a table's age gets that old, then something's probably not
configured very well and needs attention. I did think maybe we could
keep the addressing of auto-vacuum being configured to run too slowly
as a separate thread.

David