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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