Re: 8.4 open item: copy performance regression?

Simon Riggs <simon@2ndQuadrant.com> writes:
> On Sat, 2009-06-20 at 13:15 +0200, Stefan Kaltenbrunner wrote:
>> 8192 6m43.203s/6m48.293s
>> 16384 6m24.980s/6m24.116s
>> 32768 6m20.753s/6m22.083s
>> 65536 6m22.913s/6m22.449s
>> 1048576 6m23.765s/6m24.645s

> The rest of the patch should have had a greater effect on tables with
> thinner rows. Your results match my expectations, though I read from
> them that we should use 16384, since that provides some gain, not just a
> cancellation of the regression. 

+1 for using 16384 (ie, max ring buffer size 16MB).  Maybe even more.
It seems likely that other cases might have an even bigger issue than
is exhibited in the couple of test cases we have here, so we should
leave some margin for error.  Also, there's code in there to limit the
ring buffer to 1/8th of shared buffers, so we don't have to worry about
trashing the whole buffer arena in small configurations.  Any limitation
at all is still a step forward over previous releases as far as not
trashing the arena is concerned.

> I would suggest that we leave it as a tunable parameter in this release
> and remove it again once we have clear evidence of how to set it.

It's not going to become an exposed tunable.  There is not evidence to
justify that, and we are down to the point of the release cycle where
any more than one-liner changes have to be looked at with considerable
reluctance.
        regards, tom lane