Horton

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

Laurenz Albe <laurenz.albe@cybertec.at>

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From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Andres Freund <andres@anarazel.de>, Masahiko Sawada <masahiko.sawada@2ndquadrant.com>, David Rowley <dgrowleyml@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Darafei Komяpa Praliaskouski <me@komzpa.net>, pgsql-hackers@lists.postgresql.org, Alvaro Herrera <alvherre@2ndquadrant.com>, Michael Banck <mbanck@gmx.net>
Date: 2020-03-17T21:22:44Z
Lists: pgsql-hackers 
On Tue, 2020-03-17 at 16:07 -0500, Justin Pryzby wrote:
> > Assume a scale factor >= 1, for example 2, and n live tuples.
> > The table has just been vacuumed.
> > 
> > Now we insert m number tuples (which are live).
> > 
> > Then the condition
> > 
> >    threshold + scale_factor * live_tuples < newly_inserted_tuples
> > 
> > becomes
> > 
> >    10000000 + 2 * (n + m) < m
> > 
> > which can never be true for non-negative n and m.
> > 
> > So a scale factor >= 1 disables the feature.
> 
> No, this is what we mailed about privately yesterday, and I demonstrated that
> autovac can still run with factor=100.  I said:

I remember.
Can you point out where exactly the flaw in my reasoning is?

> > It's a multiplier, not a percent out of 100 (fraction is not a great choice of
> > words).
> > 
> >                 &autovacuum_vac_scale,
> >                 0.2, 0.0, 100.0,
> > 
> > The default is 0.2 (20%), so 100 means after updating/deleting 100*reltuples.

Yes, exactly.

> If 1.0 disabled the feature, it wouldn't make much sense to allow factor up to
> 100.

True, we could set the upper limit to 2, but it doesn't matter much.

Note that this is different from autovacuum_vacuum_scale_factor,
because inserted tuples are live, while dead tuples are not.

Yours,
Laurenz Albe

Commits

  1. Further improve stability fix for partition_aggregate test.

  2. Improve stability fix for partition_aggregate test.

  3. Attempt to stabilize partitionwise_aggregate test

  4. Fix race condition in statext_store().

  5. Attempt to fix unstable regression tests, take 2

  6. Attempt to fix unstable regression tests

  7. Trigger autovacuum based on number of INSERTs

  8. Fix upper limit for vacuum_cleanup_index_scale_factor