bug#12350: Composites identified as primes in factor.c (when HAVE_GMP)

[Pádraig Brady]

On 10/08/2012 01:34 PM, Jim Meyering wrote:
> Torbjorn Granlund wrote:
>
> >    > Please do, and let me and Niels know if it takes more than 45s.  Your
> >    > test case takes 28s on my 3.3 GHz Sandy bridge system with our current
> >    > code.  I'm a little disappointed the code doesn't beat the old code more
> >    > for small factorisations.
> >
> >    So on my 2.1GHz i3-2310M, running over the range 452,930,477 to 472,882,027.
> >
> >    old broken code = 14m
> >    old fixed code  = 18m
> >    new code        = 63s
> >
> > OK, this is about 60% slower than I would have expected.  Our code at
> > http://gmplib.org:8000/factoring/ should run at about 39s on your
> > system.  (I am using gcc 4.7.1.)
> >
> > I haven't looked at the final version that went into codeutils, so I
> > have no idea why it runs slower.  A wild guess is that its actual input
> > or tokenisation has been slowed down.  For smallish numbers, such things
> > can dominate over actually factoring the numbers.
> >
> > I think the current coreutils factor performance for smallish numbers
> > might be sufficient.  (Larger numbers than 2^100 need a bit too much
> > time, but we are working on a fix.)
>
> When I compare the factor built from http://gmplib.org:8000/factoring/, and
> the one in coreutils.git I see these single-trial times: (on a 3.2GHz i7-970)
>
>    $ seq 452930477 472882027|env time ./factor > k
>    29.28user 0.44system 0:30.85elapsed 96%CPU (0avgtext+0avgdata 
> 548maxresident)k
>    0inputs+1011088outputs (0major+162minor)pagefaults 0swaps
>    ...
>    $ seq 452930477 472882027|env time /cu/src/factor > k
>    26.47user 0.48system 0:28.07elapsed 96%CPU (0avgtext+0avgdata 
> 692maxresident)k
>    0inputs+1011088outputs (0major+205minor)pagefaults 0swaps
>
> I.e., a 9% improvement.  Compiled with bleeding edge gcc 4.8.0 20121007.

Thanks for checking that.

Torbjorn you seem to be interploating from your
3.3 GHz Sandy bridge to my 2.1GHz i3-2310M Sandy Bridge,
but it might not be linear due to cache, turbo boost,
mem bandwidth, ...

cheers,
Pádraig.