Re: POC: Parallel processing of indexes in autovacuum - Mailing list pgsql-hackers

Hi,

On Mon, Mar 30, 2026 at 7:17 AM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Thank you for working on this, very useful feature. Sharing a few thoughts:
>
> 1. Shouldn't we also cap by max_parallel_workers to avoid wasting DSM resources 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
limitsare distributed first equally to the autovacuum workers. 
> and then they share that. Therefore, parallel workers will be heavily throttled. IIUC, this problem doesn't exist
withmanual 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 version.

> 3. Additionally, is there a point where, based on the cost limits, launching additional workers becomes
counterproductivecompared 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 ways
to affect this formula, I guess that there will be no problems with it (except
my concerns about opt-out style).

> 4. Would it make sense to add a table level override to disable parallelism 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 shared below.

Thank you very much!

> Observations:
>
> 1. Parallel autovacuum provides consistent speedup. With cost_limit=200 and
>    7 workers, vacuum completes 1.41x faster (71s -> 50s). With cost_limit=60,
>    the speedup is 1.25x (194s -> 154s).
> 2. I see the benefit comes from parallelizing index vacuum. With 8 indexes totaling
>    ~530 MB, parallel workers scan indexes concurrently instead of the leader
>    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 dozen
gigabytes)? I think the results will be more "expressive".

--
Best regards,
Daniil Davydov