Re: [HACKERS] Block level parallel vacuum

On Tue, 12 Nov 2019 at 20:29, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Tue, Nov 12, 2019 at 4:04 PM Masahiko Sawada
> <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
> >
> > On Mon, 11 Nov 2019 at 17:57, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Oct 29, 2019 at 12:37 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > I realized that v31-0006 patch doesn't work fine so I've attached the
> > > > updated version patch that also incorporated some comments I got so
> > > > far. Sorry for the inconvenience. I'll apply your 0001 patch and also
> > > > test the total delay time.
> > > >
> > > While reviewing the 0002, I got one doubt related to how we are
> > > dividing the maintainance_work_mem
> > >
> > > +prepare_index_statistics(LVShared *lvshared, Relation *Irel, int nindexes)
> > > +{
> > > + /* Compute the new maitenance_work_mem value for index vacuuming */
> > > + lvshared->maintenance_work_mem_worker =
> > > + (nindexes_mwm > 0) ? maintenance_work_mem / nindexes_mwm :
> > > maintenance_work_mem;
> > > +}
> > > Is it fair to just consider the number of indexes which use
> > > maintenance_work_mem? Or we need to consider the number of worker as
> > > well. My point is suppose there are 10 indexes which will use the
> > > maintenance_work_mem but we are launching just 2 workers then what is
> > > the point in dividing the maintenance_work_mem by 10.
> > >
> > > IMHO the calculation should be like this
> > > lvshared->maintenance_work_mem_worker = (nindexes_mwm > 0) ?
> > > maintenance_work_mem / Min(nindexes_mwm, nworkers) :
> > > maintenance_work_mem;
> > >
> > > Am I missing something?
> >
> > No, I think you're right. On the other hand I think that dividing it
> > by the number of indexes that will use the mantenance_work_mem makes
> > sense when parallel degree > the number of such indexes. Suppose the
> > table has 2 indexes and there are 10 workers then we should divide the
> > maintenance_work_mem by 2 rather than 10 because it's possible that at
> > most 2 indexes that uses the maintenance_work_mem are processed in
> > parallel at a time.
> >
> Right, thats the reason I suggested divide with Min(nindexes_mwm, nworkers).

Thanks! I'll fix it in the next version patch.

--
Masahiko Sawada http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services