Re: [HACKERS] Block level parallel vacuum

On Fri, Jan 17, 2020 at 11:34 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jan 17, 2020 at 11:00 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Fri, Jan 17, 2020 at 10:44 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Jan 17, 2020 at 9:36 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > I have few small comments.
> > > >
> > > > 1.
> > > > logical streaming for large in-progress transactions+
> > > > + /* Can't perform vacuum in parallel */
> > > > + if (parallel_workers <= 0)
> > > > + {
> > > > + pfree(can_parallel_vacuum);
> > > > + return lps;
> > > > + }
> > > >
> > > > why are we checking parallel_workers <= 0, Function
> > > > compute_parallel_vacuum_workers only returns 0 or greater than 0
> > > > so isn't it better to just check if (parallel_workers == 0) ?
> > > >
> > >
> > > Why to have such an assumption about
> > > compute_parallel_vacuum_workers()? The function
> > > compute_parallel_vacuum_workers() returns int, so such a check
> > > (<= 0) seems reasonable to me.
> >
> > Okay so I should probably change my statement to why
> > compute_parallel_vacuum_workers is returning "int" instead of uint?
> >
>
> Hmm, I think the number of workers at most places is int, so it is
> better to return int here which will keep it consistent with how we do
> at other places. See, the similar usage in compute_parallel_worker.

Okay, I see.

>
> I
> > mean when this function is designed to return 0 or more worker why to
> > make it return int and then handle extra values on caller. Am I
> > missing something, can it really return negative in some cases?
> >
> > I find the below code in "compute_parallel_vacuum_workers" a bit confusing
> >
> > +static int
> > +compute_parallel_vacuum_workers(Relation *Irel, int nindexes, int nrequested,
> > + bool *can_parallel_vacuum)
> > +{
> > ......
> > + /* The leader process takes one index */
> > + nindexes_parallel--; --> nindexes_parallel can become -1
> > +
> > + /* No index supports parallel vacuum */
> > + if (nindexes_parallel == 0) . -> Now if it is 0 then return 0 but
> > if its -1 then continue. seems strange no? I think here itself we can
> > handle if (nindexes_parallel <= 0), that will make code cleaner.
> > + return 0;
> > +
>
> I think this got recently introduce by one of my changes based on the
> comment by Mahendra, we can adjust this check.

Ok
>
> > > >
> > > > I don't like the idea of first initializing the
> > > > VacuumSharedCostBalance with lps->lvshared->cost_balance and then
> > > > uninitialize if nworkers_launched is 0.
> > > > I am not sure why do we need to initialize VacuumSharedCostBalance
> > > > here? just to perform pg_atomic_write_u32(VacuumSharedCostBalance,
> > > > VacuumCostBalance);?
> > > > I think we can initialize it only if nworkers_launched > 0 then we can
> > > > get rid of the else branch completely.
> > > >
> > >
> > > No, we can't initialize after nworkers_launched > 0 because by that
> > > time some workers would have already tried to access the shared cost
> > > balance. So, it needs to be done before launching the workers as is
> > > done in code. We can probably add a comment.
> > I don't think so, VacuumSharedCostBalance is a process local which is
> > just pointing to the shared memory variable right?
> >
> > and each process has to point it to the shared memory and that we are
> > already doing in parallel_vacuum_main. So we can initialize it after
> > worker is launched.
> > Basically code will look like below
> >
> > pg_atomic_write_u32(&(lps->lvshared->cost_balance), VacuumCostBalance);
> > pg_atomic_write_u32(&(lps->lvshared->active_nworkers), 0);
> >
>
> oh, I thought you were telling to initialize the shared memory itself
> after launching the workers. However, you are asking to change the
> usage of the local variable, I think we can do that.

Okay.

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