On 2013-04-22 11:27:25 -0400, Robert Haas wrote:
> On Wed, Apr 17, 2013 at 8:21 PM, Greg Smith <g...@2ndquadrant.com> wrote:
> >> The more I read of this thread, the more unhappy I get.  It appears that
> >> the entire design process is being driven by micro-optimization for CPUs
> >> being built by Intel in 2013.
> >
> > And that's not going to get anyone past review, since all the tests I've
> > been doing the last two weeks are on how fast an AMD Opteron 6234 with OS
> > cache >> shared_buffers can run this.  The main thing I'm still worried
> > about is what happens when you have a fast machine that can move memory
> > around very quickly and an in-memory workload, but it's hamstrung by the
> > checksum computation--and it's not a 2013 Intel machine.
> This is a good point.  However, I don't completely agree with the
> conclusion that we shouldn't be worrying about any of this right now.
> While I agree with Tom that it's far too late to think about any
> CPU-specific optimizations for 9.3, I have a lot of concern, based on
> Ants's numbers, that we've picked a checksum algorithm which is hard
> to optimize for performance.  If we don't get that fixed for 9.3,
> we're potentially looking at inflicting many years of serious
> suffering on our user base.  If we at least get the *algorithm* right
> now, we can worry about optimizing it later.  If we get it wrong,
> we'll be living with the consequence of that for a really long time.
> I wish that we had not scheduled beta quite so soon, as I am sure
> there will be even more resistance to changing this after beta.  But
> I'm having a hard time escaping the conclusion that we're on the edge
> of shipping something we will later regret quite deeply.  Maybe I'm
> wrong?

I don't see us changing away from CRCs anymore either by now. But I
think at least changing the polynom to something that
a) has higher error detection properties
b) can noticeably sped up on a a good part of the hardware pg is run on

If we are feeling really adventurous we can switch to a faster CRC
implementation, there are enough ones around and I know that at least my
proposed patch from some years ago (which is by far not the fastest that
is doable) is in production usage some places.


Andres Freund

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

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