Re: New CRC algorithm: Slicing by 8

On Mon, 23 Oct 2006, Mark Kirkwood wrote:

> Tom Lane wrote:
> >
> >
> > Yah, I checked. Several times... but if anyone else wants to repeat
> > the experiment, please do. Or look for bugs in either my test case
> > or Gurjeet's.
>
>

Just for fun, I tried it out with both GCC and with Intel's C compiler
with some agressive platform-specific flags on my 2.8Ghz Xeon running
Gentoo.

Std crc Slice-8 crc

Intel P4 Xeon 2.8Ghz (Gentoo, gcc-3.4.5, -O2)

8192 bytes 4.697572 9.806341
1024 bytes 0.597429 1.181828
64 bytes 0.046636 0.086984

Intel P4 Xeon 2.8Ghz (Gentoo, icc-9.0.032, -O2 -xN -ipo -parallel)

8192 bytes 0.000004 0.001085
1024 bytes 0.000004 0.001292
64 bytes 0.000003 0.001078

So at this point I realize that intel's compiler is optimizing the loop
away, at least for the std crc and probably for both. So I make mycrc an
array of 2, and substript mycrc[j&1] in the loop.

Std crc Slice-8 crc

Intel P4 Xeon 2.8Ghz (Gentoo, gcc-3.4.5, -O2)

8192 bytes 51.397146 9.523182
1024 bytes 6.430986 1.229043
64 bytes 0.400062 0.128579

Intel P4 Xeon 2.8Ghz (Gentoo, icc-9.0.032, -O2 -xN -ipo -parallel)

8192 bytes 29.881708 0.001432
1024 bytes 3.750313 0.001432
64 bytes 0.238583 0.001431

So it looks like something fishy is still going on with the slice-8 with
the intel compiler.

I have attached my changed testcrc.c file.

> FWIW - FreeBSD and Linux results using Tom's test program on almost identical
> hardware[1]:
>
> Std crc Slice-8 crc
>
> Intel P-III 1.26Ghz (FreeBSD 6.2)
>
> 8192 bytes 12.975314 14.503810
> 1024 bytes 1.633557 1.852322
> 64 bytes 0.111580 0.206975
>
>
> Intel P-III 1.26Ghz (Gentoo 2006.1)
>
>
> 8192 bytes 12.967997 28.363876
> 1024 bytes 1.632317 3.626230
> 64 bytes 0.111513 0.326557
>
>
> Interesting that the slice-8 algorithm seems to work noticeably better on
> FreeBSD than Linux - but still not as well as the standard one (for these
> tests anyway)...
>
>
> Cheers
>
> Mark
>
> [1] Both boxes have identical mobos, memory and CPUs (same sspec nos).
>
>
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