Re: Berserk Autovacuum (let's save next Mandrill)

From: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Justin Pryzby <pryzby(at)telsasoft(dot)com>, Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Darafei Komяpa Praliaskouski <me(at)komzpa(dot)net>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Michael Banck <mbanck(at)gmx(dot)net>
Subject: Re: Berserk Autovacuum (let's save next Mandrill)
Date: 2020-03-17 00:14:02
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On Mon, 2020-03-16 at 14:34 -0700, Andres Freund wrote:
> > > In particularl, I think it'd make sense to *not* have a lower freezing
> > > horizon for insert vacuums (because it *will* cause problems), but if
> > > the page is dirty anyway, then do the freezing even if freeze_min_age
> > > etc would otherwise prevent us from doing so?
> >
> > I don't quite see why freezing tuples in insert-only tables will cause
> > problems - are you saying that more WAL will be written compared to
> > freezing with a higher freeze_min_age?
> As far as I understand the patch may trigger additional vacuums e.g. for
> tables that have some heavily updated parts / key ranges, and otherwise
> are largely insert only (as long as there are in total considerably more
> inserts than updates). That's not at all uncommon.
> And for the heavily updated regions the additional vacuums with a 0 min
> age could prove to be costly. I've not looked at the new code, but it'd
> be particularly bad if the changes were to trigger the
> lazy_check_needs_freeze() check in lazy_scan_heap() - it'd have the
> potential for a lot more contention.

I think I got it.

Here is a version of the patch that does *not* freeze more tuples than
normal, except if a prior tuple on the same page is already eligible for freezing.

lazy_check_needs_freeze() is only called for an aggressive vacuum, which
this isn't.

Does that look sane?

Laurenz Albe

Attachment Content-Type Size
0001-Autovacuum-tables-that-have-received-only-inserts.v6.patch text/x-patch 29.2 KB

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