On Mon, Apr 22, 2013 at 6:27 PM, Robert Haas <robertmh...@gmail.com> 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 was just now writing up a generic C based patch based on the parallel FNV-1a + shift that we discussed with Florian with an added round of mixing. Testing the performance in isolation indicates that: 1) it is about an order of magnitude faster than the Sarwate CRC method used in Postgresql. 2) it is about 2x faster than fastest software based CRC method. 3) by using -msse4.1 -funroll-loops -ftree-vectorize compilation options the performance improves 5x. (within 20% of handcoded ASM) This leaves lingering doubts about the quality of the checksum. It's hard if not impossible to prove absence of interesting patterns that would trigger collisions. I do know the checksum quality is miles ahead of the Fletcher sum originally proposed and during the last week I haven't been able to think of a way to make the collision rate significantly differ from CRC. > 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? Its unfortunate that this got delayed by so long. The performance side of the argument was clear a month ago. Regards, Ants Aasma -- Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26 A-2700 Wiener Neustadt Web: http://www.postgresql-support.de -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers