Re: [HACKERS] Block level parallel vacuum

On Thu, Oct 17, 2019 at 2:12 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Oct 17, 2019 at 5:30 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Oct 17, 2019 at 12:21 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Oct 17, 2019 at 10:56 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > > I guess that the concepts of vacuum delay contradicts the concepts of
> > > > parallel vacuum. The concepts of parallel vacuum would be to use more
> > > > resource to make vacuum faster. Vacuum delays balances I/O during
> > > > vacuum in order to avoid I/O spikes by vacuum but parallel vacuum
> > > > rather concentrates I/O in shorter duration.
> > > >
> > >
> > > You have a point, but the way it is currently working in the patch
> > > doesn't make much sense.
> > >
> >
> > Another point in this regard is that the user anyway has an option to
> > turn off the cost-based vacuum. By default, it is anyway disabled.
> > So, if the user enables it we have to provide some sensible behavior.
> > If we can't come up with anything, then, in the end, we might want to
> > turn it off for a parallel vacuum and mention the same in docs, but I
> > think we should try to come up with a solution for it.
>
> I finally got your point and now understood the need. And the idea I
> proposed doesn't work fine.
>
> So you meant that all workers share the cost count and if a parallel
> vacuum worker increase the cost and it reaches the limit, does the
> only one worker sleep? Is that okay even though other parallel workers
> are still running and then the sleep might not help?
>
I agree with this point. There is a possibility that some of the
workers who are doing heavy I/O continue to work and OTOH other
workers who are doing very less I/O might become the victim and
unnecessarily delay its operation.

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