Re: [HACKERS] Block level parallel vacuum

On Wed, Nov 6, 2019 at 3:50 PM Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Wed, 6 Nov 2019 at 18:42, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Wed, Nov 6, 2019 at 2:01 PM Mahendra Singh <mahi6run(at)gmail(dot)com> wrote:
> > >
> > > Hi
> > > I took all attached patches(v32-01 to v32-4) and one Dilip's patch from "Questions/Observations related to Gist vacuum" mail thread. On the top of all these patches, I created one more patch to test parallel vacuum functionally for all existence test suite.
>
> Thank you for looking at this patch!
>
> > > For reference, I am attaching patch.
> > >
> > > What does this patch?
> > > As we know that if we give parallel option with vacuum, then only we are vacuuming using parallel workers. So to test, I used existence guc force_parallel_mode and tested parallel vacuuming.
> > >
> > > If force_parallel_mode is set as regress, then if parallel option is not given with vacuum, I am forcing to use parallel workers for vacuum. If there is only one index and parallel degree is not given with vacuum(or parallel option is not given), and force_parallel_mode = regress, then I am launching one parallel worker(I am not doing work by leader in this case), but if there is more than one index, then i am using leader as a worker for one index and launching workers for all other indexes.
> > >
> > > After applying this patch and setting force_parallel_mode = regress, all test cases are passing (make-check world)
> > >
> > > I have some questions regarding my patch. Should we do vacuuming using parallel workers even if force_parallel_mode is set as on, or we should use new GUC to test parallel worker vacuum for existence test suite?
> >
> > IMHO, with force_parallel_mode=on we don't need to do anything here
> > because that is useful for normal query parallelism where if the user
> > thinks that the parallel plan should have been selected by the planer
> > but planer did not select the parallel plan then the user can force
> > and check. But, vacuum parallelism is itself forced by the user so
> > there is no point in doing it with force_parallel_mode=on.
>
> Yeah I think so too. force_parallel_mode is a planner parameter and
> parallel vacuum can be forced by vacuum option.
>
> > However,
> > force_parallel_mode=regress is useful for testing the vacuum with an
> > existing test suit.
>
> If we want to control the leader participation by GUC parameter I
> think we would need to have another GUC parameter rather than using
> force_parallel_mode.
I think the purpose is not to disable the leader participation,
instead, I think the purpose of 'force_parallel_mode=regress' is that
without changing the existing test suit we can execute the existing
vacuum commands in the test suit with the worker. I did not study the
patch but the idea should be that if "force_parallel_mode=regress"
then normal vacuum command should be executed in parallel by using 1
worker.

And it's useful if we can use the parameter for
> parallel CREATE INDEX as well. But it should be a separate patch.
>

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com