Re: [HACKERS] Block level parallel vacuum

On Sat, 11 Jan 2020 at 13:18, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Sat, Jan 11, 2020 at 9:23 AM Masahiko Sawada
> <masahiko.sawada@2ndquadrant.com> wrote:
> >
> > On Fri, 10 Jan 2020 at 20:54, Mahendra Singh Thalor <mahi6run@gmail.com> wrote:
> > >
> > > On Fri, 10 Jan 2020 at 15:51, Sergei Kornilov <sk@zsrv.org> wrote:
> > > >
> > > > Hi
> > > > Thank you for update! I looked again
> > > >
> > > > (vacuum_indexes_leader)
> > > > +               /* Skip the indexes that can be processed by parallel workers */
> > > > +               if (!skip_index)
> > > > +                       continue;
> > > >
> > > > Does the variable name skip_index not confuse here? Maybe rename to something like can_parallel?
> > >
> > > I also agree with your point.
> >
> > I don't think the change is a good idea.
> >
> > -               bool            skip_index = (get_indstats(lps->lvshared, i) == NULL ||
> > -                                                                 skip_parallel_vacuum_index(Irel[i],
lps->lvshared));
> > +               bool            can_parallel = (get_indstats(lps->lvshared, i) == NULL ||
> > +                                                                       skip_parallel_vacuum_index(Irel[i],
> > +
      lps->lvshared)); 
> >
> > The above condition is true when the index can *not* do parallel index vacuum. How about changing it to
skipped_indexand change the comment to something like “We are interested in only index skipped parallel vacuum”? 
> >
>
> Hmm, I find the current code and comment better than what you or
> Sergei are proposing.  I am not sure what is the point of confusion in
> the current code?

Yeah the current code is also good. I just thought they were concerned
that the variable name skip_index might be confusing because we skip
if skip_index is NOT true.

>
> > >
> > > >
> > > > Another question about behavior on temporary tables. Use case: the user commands just "vacuum;" to vacuum
entiredatabase (and has enough maintenance workers). Vacuum starts fine in parallel, but on first temporary table we
hit:
> > > >
> > > > +       if (RelationUsesLocalBuffers(onerel) && params->nworkers >= 0)
> > > > +       {
> > > > +               ereport(WARNING,
> > > > +                               (errmsg("disabling parallel option of vacuum on \"%s\" --- cannot vacuum
temporarytables in parallel", 
> > > > +                                               RelationGetRelationName(onerel))));
> > > > +               params->nworkers = -1;
> > > > +       }
> > > >
> > > > And therefore we turn off the parallel vacuum for the remaining tables... Can we improve this case?
> > >
> > > Good point.
> > > Yes, we should improve this. I tried to fix this.
> >
> > +1
> >
>
> Yeah, we can improve the situation here.  I think we don't need to
> change the value of params->nworkers at first place if allow
> lazy_scan_heap to take care of this.  Also, I think we shouldn't
> display warning unless the user has explicitly asked for parallel
> option.  See the fix in the attached patch.

Agreed. But with the updated patch the PARALLEL option without the
parallel degree doesn't display warning because params->nworkers = 0
in that case. So how about restoring params->nworkers at the end of
vacuum_rel()?

+                       /*
+                        * Give warning only if the user explicitly
tries to perform a
+                        * parallel vacuum on the temporary table.
+                        */
+                       if (params->nworkers > 0)
+                               ereport(WARNING,
+                                               (errmsg("disabling
parallel option of vacuum on \"%s\" --- cannot vacuum temporary tables
in parallel",
+
RelationGetRelationName(onerel))));

Regards,

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