Re: Enabling Checksums

On Apr22, 2013, at 21:14 , Jeff Davis <pgsql@j-davis.com> wrote:
> On Mon, 2013-04-22 at 20:04 +0200, Florian Pflug wrote:
>> The one downside of the fnv1+shift approach is that it's built around
>> the assumption that processing 64-bytes at once is the sweet spot. That
>> might be true for x86 and x86_64 today, but it won't stay that way for
>> long, and quite surely isn't true for other architectures. That doesn't
>> necessarily rule it out, but it certainly weakens the argument that
>> slipping it into 9.3 avoids having the change the algorithm later...
> 
> I think you are setting the bar way too high. Right now, we have a slow
> algorithm. According to Ants's tests, FNV-1a is much, much faster. Do
> you think that it will still be such a bottleneck that we will want to
> change it again later for purely performance reasons?

To clarify, it wasn't my intent to argue against shipping FNV1+SHIFT
in 9.3 - in fact I'd like to see us do exactly that. I was merely trying
to be unbiased, and hence stated not only arguments in favour or FNV1+SHIFT
(the ones about CRCs theoretical advantages in error detection being 
not really relevant to us), but also the one downside of FNV1+SHIFT.

Seems like I could have done a better job expressing myself though.

> The biggest problem now is getting one of these faster algorithms (FNV
> or even a faster CRC) into shape that is acceptable to
> reviewers/committers. If we don't do that, we will be missing out on a
> lot of potential checksum users for whom the existing CRC algorithm is
> just too slow.

Assuming that we only ship a plain C implementation with 9.3, what
are we missing on that front? The C implementation of FNV1+SHIFT is
only a few dozen lines or so.

best regards,
Florian Pflug