Re: parallel vacuum comments - Mailing list pgsql-hackers

On Thu, Dec 9, 2021 at 3:35 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Dec 6, 2021 at 10:17 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Fri, Dec 3, 2021 at 6:06 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > 2. The patch seems to be calling parallel_vacuum_should_skip_index
> > > > thrice even before starting parallel vacuum. It has a call to find the
> > > > number of blocks which has to be performed for each index. I
> > > > understand it might not be too costly to call this but it seems better
> > > > to remember this info like we are doing in the current code.
> > >
> > > Yes, we can bring will_vacuum_parallel array back to the code. That
> > > way, we can remove the call to parallel_vacuum_should_skip_index() in
> > > parallel_vacuum_begin().
> > >
> > > > We can
> > > > probably set parallel_workers_can_process in parallel_vacuum_begin and
> > > > then again update in parallel_vacuum_process_all_indexes. Won't doing
> > > > something like that be better?
> > >
> > > parallel_workers_can_process can vary depending on bulk-deletion, the
> > > first time cleanup, or the second time (or more) cleanup. What can we
> > > set parallel_workers_can_process based on in parallel_vacuum_begin()?
> > >
> >
> > I was thinking to set the results of will_vacuum_parallel in
> > parallel_vacuum_begin().
> >
>
> This point doesn't seem to be addressed in the latest version (v6). Is
> there a reason for not doing it? If we do this, then we don't need to
> call parallel_vacuum_should_skip_index() from
> parallel_vacuum_index_is_parallel_safe().
>

Few minor comments on v6-0001
==========================
1.
The array
+ * element is allocated for every index, even those indexes where
+ * parallel index vacuuming is unsafe or not worthwhile (i.g.,
+ * parallel_vacuum_should_skip_index() returns true).

/i.g/e.g

2.
static void update_index_statistics(LVRelState *vacrel);
-static void begin_parallel_vacuum(LVRelState *vacrel, int nrequested);
-static void end_parallel_vacuum(LVRelState *vacrel);
-static LVSharedIndStats *parallel_stats_for_idx(LVShared *lvshared,
int getidx);
-static bool parallel_processing_is_safe(Relation indrel, LVShared *lvshared);
+
+static int parallel_vacuum_compute_workers(LVRelState *vacrel, int nrequested,
+    bool *will_parallel_vacuum);

In declaration, parallel_vacuum_compute_workers() is declared after
update_index_statistics but later defined in reverse order. I suggest
to make the order of definitions same as their declaration. Similarly,
the order of definition of parallel_vacuum_process_all_indexes(),
parallel_vacuum_process_unsafe_indexes(),
parallel_vacuum_process_safe_indexes(),
parallel_vacuum_process_one_index() doesn't match the order of their
declaration. Can we change that as well?

-- 
With Regards,
Amit Kapila.