Re: [HACKERS] Block level parallel vacuum

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).

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