On Sun, Feb 19, 2012 at 4:17 AM, Robert Haas <robertmh...@gmail.com> wrote:

> Here's what's bugging me.  Greg seemed to be assuming that the
> business of the background writer might be the cause of the
> performance drop-off he measured on certain test cases.  But you and I
> both seem to feel that the business of the background writer is
> intentional and desirable.  Supposing we're right, where's the
> drop-off coming from?  *scratches head*

Any source of logical I/O becomes physical I/O when we run short of
memory. So if we're using more memory for any reason that will cause
more swapping. Or if we are doing things like consulting the vmap that
would also cause a problem.

I notice the issue is not as bad for 9.2 in the scale 4000 case, so it
seems more likely that we're just hitting the tipping point earlier on
9.2 and that scale 1000 is right in the middle of the tipping point.

What it does show quite clearly is that the extreme high end response
time variability is still there. It also shows that insufficient
performance testing has been done on this release so far. We may have
"solved" some scalability problems but we've completely ignored real
world performance issues and as Greg says, we now get to pray the
price for not having done that earlier.

I've argued previously that we should have a performance tuning phase
at the end of the release cycle, now it looks that has become a
necessity. Which will turn out to be a good thing in the end, I'm
sure, even if its a little worrying right now.

 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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