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Subject: Re: Should vacuum process config file reload more often
From: Daniel Gustafsson <daniel@yesql.se>
In-Reply-To: <CA+TgmoY3EaCz3_7F3+Sb3FUHonf4nxdC-_UWnn6fWEPtXU7t5g@mail.gmail.com>
Date: Thu, 6 Apr 2023 20:55:02 +0200
Cc: Melanie Plageman <melanieplageman@gmail.com>,
 Andres Freund <andres@anarazel.de>,
 Tom Lane <tgl@sss.pgh.pa.us>,
 Masahiko Sawada <sawada.mshk@gmail.com>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Amit Kapila <amit.kapila16@gmail.com>
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To: Robert Haas <robertmhaas@gmail.com>
Archived-At: <https://www.postgresql.org/message-id/D0A88FB1-56A6-4CD5-B886-8539B7BA7F58%40yesql.se>
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> On 6 Apr 2023, at 19:18, Robert Haas <robertmhaas@gmail.com> wrote:
>=20
> On Thu, Apr 6, 2023 at 11:52=E2=80=AFAM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
>>> Gah, I think I misunderstood you. You are saying that only calling
>>> AutoVacuumUpdateCostLimit() after napping while vacuuming a table =
may
>>> not be enough. The frequency at which the number of workers changes =
will
>>> likely be different. This is a good point.
>>> It's kind of weird to call AutoVacuumUpdateCostLimit() only after =
napping...
>>=20
>> A not fully baked idea for a solution:
>>=20
>> Why not keep the balanced limit in the atomic instead of the number =
of
>> workers for balance. If we expect all of the workers to have the same
>> value for cost limit, then why would we just count the workers and =
not
>> also do the division and store that in the atomic variable. We are
>> worried about the division not being done often enough, not the =
number
>> of workers being out of date. This solves that, right?
>=20
> A bird in the hand is worth two in the bush, though. We don't really
> have time to redesign the patch before feature freeze, and I can't
> convince myself that there's a big enough problem with what you
> already did that it would be worth putting off fixing this for another
> year.

+1, I'd rather see we did a conservative version of the feature first =
and
expand upon it in the 17 cycle.

> Reading your newer emails, I think that the answer to my
> original question is "we don't want to do it at every
> vacuum_delay_point because it might be too costly," which is
> reasonable.

I think we kind of need to get to that granularity eventually, but it's =
not a
showstopper for this feature, and can probably benefit from being done =
in the
context of a larger av-worker re-think (the importance of which =
discussed
downthread).

--
Daniel Gustafsson