Re: [HACKERS] Block level parallel vacuum

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.

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