Re: Avoiding repeated snapshot computation

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Avoiding repeated snapshot computation
Date: 2011-11-29 15:19:15
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Pavan Deolasee wrote:
> On Sun, Nov 27, 2011 at 12:26 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com> writes:
> >> On Sat, Nov 26, 2011 at 10:43 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >>> Furthermore, it's
> >>> hard to understand how this could be a net improvement in the general
> >>> case, because now both B and F are copying everything twice (once to
> >>> the shared area and one to backend-local memory) instead of just once
> >>> (to backend-local memory) and C and D are sleeping (yikes!).
> >
> >> Yes, B and F pay a price of double copy. But I think it can be a net
> >> saving because C and D (and many more hopefully) don't need to
> >> recompute the snapshot again by going over a potentially large
> >> ProcArray.
> >
> > Like Robert, I'm finding it hard to believe that this isn't going to be
> > a net pessimization in as many or more cases as it'd be a win. ?Also,
> > didn't we just get rid of most of the issue of "going over a large
> > ProcArray" with the move of hot members to a separate array?
> >
> Yeah, separating the PGPROC members has helped a lot. But that does
> not reduce the number of GetSnapshotData calls. It only makes each
> call less expensive. As I said, I had seen 15-20% improvement with not
> even a slightly tuned patch, so I am optimistic about the potential of
> the approach.

Agreed. I think there is great potential here. We have been stumped at
how to reduce the overhead of this for years, and it seems you are now
on a promising path.

Bruce Momjian <bruce(at)momjian(dot)us>

+ It's impossible for everything to be true. +

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