Re: [HACKERS] Block level parallel vacuum

On Wed, 20 Nov 2019 at 17:02, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Nov 20, 2019 at 11:01 AM Masahiko Sawada
> <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
> >
> > I've attached the latest version patch set. The patch set includes all
> > discussed points regarding index AM options as well as shared cost
> > balance. Also I added some test cases used all types of index AM.
> >
> > During developments I had one concern about the number of parallel
> > workers to launch. In current design each index AMs can choose the
> > participation of parallel bulk-deletion and parallel cleanup. That
> > also means the number of parallel worker to launch might be different
> > for each time of parallel bulk-deletion and parallel cleanup. In
> > current patch the leader will always launch the number of indexes that
> > support either one but it would not be efficient in some cases. For
> > example, if we have 3 indexes supporting only parallel bulk-deletion
> > and 2 indexes supporting only parallel index cleanup, we would launch
> > 5 workers for each execution but some workers will do nothing at all.
> > To deal with this problem, I wonder if we can improve the parallel
> > query so that the leader process creates a parallel context with the
> > maximum number of indexes and can launch a part of workers instead of
> > all of them.
> >
>
> Can't we choose the number of workers as a maximum of
> "num_of_indexes_that_support_bulk_del" and
> "num_of_indexes_that_support_cleanup"? If we can do that, then we can
> always launch the required number of workers for each phase (bulk_del,
> cleanup). In your above example, it should choose 3 workers while
> creating a parallel context. Do you see any problem with that?

I might be missing something but if we create the parallel context
with 3 workers the leader process always launches 3 workers. Therefore
in the above case it launches 3 workers even in cleanup although 2
workers is enough.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
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