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In-Reply-To: 
 <CAHg+QDehxaJEd1Yp1MpW8UO71xmbasy7t2GZGvqOYwkr0md8DQ@mail.gmail.com>
From: Daniil Davydov <3danissimo@gmail.com>
Date: Mon, 30 Mar 2026 15:44:22 +0700
Message-ID: 
 <CAJDiXgi73x7h0=UoXriFjskRB6htZ-uqXKqvWN3RefuxbP93gA@mail.gmail.com>
Subject: Re: POC: Parallel processing of indexes in autovacuum
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	Masahiko Sawada <sawada.mshk@gmail.com>, Sami Imseih <samimseih@gmail.com>,
	Alexander Korotkov <aekorotkov@gmail.com>,
 Matheus Alcantara <matheusssilv97@gmail.com>,
	Maxim Orlov <orlovmg@gmail.com>,
 Postgres hackers <pgsql-hackers@lists.postgresql.org>
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Archived-At: 
 <https://www.postgresql.org/message-id/CAJDiXgi73x7h0%3DUoXriFjskRB6htZ-uqXKqvWN3RefuxbP93gA%40mail.gmail.com>
Precedence: bulk

Hi,

On Mon, Mar 30, 2026 at 7:17=E2=80=AFAM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Thank you for working on this, very useful feature. Sharing a few thought=
s:
>
> 1. Shouldn't we also cap by max_parallel_workers to avoid wasting DSM res=
ources in parallel_vacuum_compute_workers?

Actually, autovacuum_max_parallel_workers is already limited by
max_parallel_workers. It is not clear for me why we allow setting this GUC
higher than max_parallel_workers, but if this happens, I think it is a user=
's
misconfiguration.

> 2. Is it intentional that other autovacuum workers not yield cost limits =
to the parallel auto vacuum workers? Cost limits are distributed first equa=
lly to the autovacuum workers.
> and then they share that. Therefore, parallel workers will be heavily thr=
ottled. IIUC, this problem doesn't exist with manual vacuum.
>  If we don't fix this, at least we should document this.

Parallel a/v workers inherit cost based parameters (including the
vacuum_cost_limit) from the leader worker. Do you mean that this can be too
low value for parallel operation? If so, user can manually increase the
vacuum_cost_limit reloption for those tables, where parallel a/v sleeps too
much (due to cost delay).

BTW, describing the cost limit propagation to the parallel a/v workers is
worth mentioning in the documentation. I'll add it in the next patch versio=
n.

> 3. Additionally, is there a point where, based on the cost limits, launch=
ing additional workers becomes counterproductive compared to running fewer =
workers and preventing it?

I don't think that we can possibly find a universal limit that will be
appropriate for all possible configurations. By now we are using a pretty
simple formula for parallel degree calculation. Since user have several way=
s
to affect this formula, I guess that there will be no problems with it (exc=
ept
my concerns about opt-out style).

> 4. Would it make sense to add a table level override to disable paralleli=
sm or set parallel worker count?

We already have the "autovacuum_parallel_workers" reloption that is used as
an additional limit for the number of parallel workers. In particular, this
reloption can be used to disable parallelism at all.

>
> I ran some perf tests to show the improvements with parallel vacuum and s=
hared below.

Thank you very much!

> Observations:
>
> 1. Parallel autovacuum provides consistent speedup. With cost_limit=3D200=
 and
>    7 workers, vacuum completes 1.41x faster (71s -> 50s). With cost_limit=
=3D60,
>    the speedup is 1.25x (194s -> 154s).
> 2. I see the benefit comes from parallelizing index vacuum. With 8 indexe=
s totaling
>    ~530 MB, parallel workers scan indexes concurrently instead of the lea=
der
>    scanning them one by one. The leader's CPU user time drops from ~3s to
>    ~0.8s as index work is offloaded
>

1.41 speedup with 7 parallel workers may not seem like a great win, but it =
is
a whole time of autovacuum operation (not only index bulkdel/cleanup) with
pretty small indexes.

May I ask you to run the same test with a higher table's size (several doze=
n
gigabytes)? I think the results will be more "expressive".

--
Best regards,
Daniil Davydov