Discussion related to cado-nfs

[Pierrick Gaudry <pierrick.gaudry@loria.fr>]

On Tue, Aug 07, 2012 at 11:36:31AM +0430, hamid reza arkian wrote:
> Hi,
> 
> I tried GMP-ECM project (the branch that is currently under active
> development for GPU devices) on a GPU cluster and got good results.
> I want to replace ecm part of sieving step in CADO-NFS with GMP-ECM(that
> support GPU), but I have the following questions:
> 
> - Basically, it's a good idea or not?
> - Is the GMP-ECM faster than ecm part of CADO-NFS? and if so, why the
> developers don't do this replacement,yet?
> In addition, if all things is OK and it's a good idea, it would be pleasure
> if you give me some tips and points before doing that.
> 
> Thanks in advance,
> Hamid

Hi,

Using GPU for the ECM step in the so-called cofactorization step of the
sieving phase is something to consider. However, this is not as simple as
pluging gmp-ecm within Cado-nfs.

Here are a few thoughts on the topic of ECM for NFS:
- the size of the numbers to be tested within NFS is small, and few
  curves are tested. This is not the traditionnal target of GMP-ECM, and
  therefore another version of ECM was written in CADO-NFS (mostly by
  Alex Kruppa, who is also a developper of GMP-ECM).
- Kruppa's thesis contains many details on this topic.
  http://tel.archives-ouvertes.fr/tel-00477005/en/
- the main question for using GPU is how to take advantage of the
  parallelism. I think that GPU implem of GMP-ECM currently runs many
  curves in parallel to make an optimal use of the card. But for the
  application to NFS, only few cruves are needed, and we want a
  parallelization at the input number level.
- the question of the transfers between the card and the cpu must be
  addressed. I tend to think that it would be interesting to accumulate a
  list of numbers to be tested and then send all of them at once, and
  keep the CPU busy with sieving while the GPU tries to finish the
  cofactorization. This implies some reorganization of the sieving code
  that is not so simple.

Regards,
Pierrick