Re: [HACKERS] Block level parallel vacuum

On Fri, Dec 13, 2019 at 11:08 AM Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Fri, 13 Dec 2019 at 14:19, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > > > >
> > > > > How about adding an additional argument to ReinitializeParallelDSM()
> > > > > that allows the number of workers to be reduced? That seems like it
> > > > > would be less confusing than what you have now, and would involve
> > > > > modify code in a lot fewer places.
> > > > >
> > > >
> > > > Yeah, we can do that. We can maintain some information in
> > > > LVParallelState which indicates whether we need to reinitialize the
> > > > DSM before launching workers. Sawada-San, do you see any problem with
> > > > this idea?
> > >
> > > I think the number of workers could be increased in cleanup phase. For
> > > example, if we have 1 brin index and 2 gin indexes then in bulkdelete
> > > phase we need only 1 worker but in cleanup we need 2 workers.
> > >
> >
> > I think it shouldn't be more than the number with which we have
> > created a parallel context, no? If that is the case, then I think it
> > should be fine.
>
> Right. I thought that ReinitializeParallelDSM() with an additional
> argument would reduce DSM but I understand that it doesn't actually
> reduce DSM but just have a variable for the number of workers to
> launch, is that right?
>

Yeah, probably, we need to change the nworkers stored in the context
and it should be lesser than the value already stored in that number.

> And we also would need to call
> ReinitializeParallelDSM() at the beginning of vacuum index or vacuum
> cleanup since we don't know that we will do either index vacuum or
> index cleanup, at the end of index vacum.
>

Right.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com