Re: [HACKERS] Block level parallel vacuum

On Mon, 4 Nov 2019 at 17:26, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Mon, Nov 4, 2019 at 1:00 PM Masahiko Sawada
> <masahiko.sawada@2ndquadrant.com> wrote:
> >
> > On Mon, 4 Nov 2019 at 14:02, Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Fri, Nov 1, 2019 at 2:21 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > >
> > > > I think that two approaches make parallel vacuum worker wait in
> > > > different way: in approach(a) the vacuum delay works as if vacuum is
> > > > performed by single process, on the other hand in approach(b) the
> > > > vacuum delay work for each workers independently.
> > > >
> > > > Suppose that the total number of blocks to vacuum is 10,000 blocks,
> > > > the cost per blocks is 10, the cost limit is 200 and sleep time is 5
> > > > ms. In single process vacuum the total sleep time is 2,500ms (=
> > > > (10,000 * 10 / 200) * 5). The approach (a) is the same, 2,500ms.
> > > > Because all parallel vacuum workers use the shared balance value and a
> > > > worker sleeps once the balance value exceeds the limit. In
> > > > approach(b), since the cost limit is divided evenly the value of each
> > > > workers is 40 (e.g. when 5 parallel degree). And suppose each workers
> > > > processes blocks  evenly,  the total sleep time of all workers is
> > > > 12,500ms (=(2,000 * 10 / 40) * 5 * 5). I think that's why we can
> > > > compute the sleep time of approach(b) by dividing the total value by
> > > > the number of parallel workers.
> > > >
> > > > IOW the approach(b) makes parallel vacuum delay much more than normal
> > > > vacuum and parallel vacuum with approach(a) even with the same
> > > > settings. Which behaviors do we expect?
> > > >
> > >
> > > Yeah, this is an important thing to decide.  I don't think that the
> > > conclusion you are drawing is correct because it that is true then the
> > > same applies to the current autovacuum work division where we divide
> > > the cost_limit among workers but the cost_delay is same (see
> > > autovac_balance_cost).  Basically, if we consider the delay time of
> > > each worker independently, then it would appear that a parallel vacuum
> > > delay with approach (b) is more, but that is true only if the workers
> > > run serially which is not true.
> > >
> > > > I thought the vacuum delay for
> > > > parallel vacuum should work as if it's a single process vacuum as we
> > > > did for memory usage. I might be missing something. If we prefer
> > > > approach(b) I should change the patch so that the leader process
> > > > divides the cost limit evenly.
> > > >
> > >
> > > I am also not completely sure which approach is better but I slightly
> > > lean towards approach (b).
> >
> > Can we get the same sleep time as approach (b) if we divide the cost
> > limit by the number of workers and have the shared cost balance (i.e.
> > approach (a) with dividing the cost limit)? Currently the approach (b)
> > seems better but I'm concerned that it might unnecessarily delay
> > vacuum if some indexes are very small or bulk-deletions of indexes
> > does almost nothing such as brin.
>
> Are you worried that some of the workers might not have much I/O to do
> but still we divide the cost limit equally?

Yes.

> If that is the case then
> that is the case with the auto vacuum workers also right?

I think It is not right because we rebalance the cost after an
autovacuum worker finished. So as Amit mentioned on the new thread we
might need to make parallel vacuum workers notice to the leader once
exited so that it can rebalance the cost.

Regards,

--
Masahiko Sawada      http://www.2ndQuadrant.com/
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