[petsc-users] Block size

[Natacha BEREUX]

Thanks a lot for your answer!
Regards,
Natacha


On Thu, Oct 9, 2014 at 10:19 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Oct 9, 2014, at 3:12 AM, Natacha BEREUX <natacha.bereux at gmail.com>
> wrote:
>
> > Dear PETSc users,
> >
> > I am a bit confused  about blocksizes in MPI matrices.
> >
> > I define a MPI matrix A with a certain blocksize, let say bs = 3.
> > As far as I understand, the sparse pattern of the matrix A is made of
> square blocks of size 3x3, regardless of the actual values of the terms of
> A.
> > Am I right ?
>
>    Only if you use the BAIJ or SBAIJ matrix format. If you use AIJ then it
> still stores the values without regard to the block size but it carries
> knowledge of the block size around and that gets used in some places such
> as the GAMG solver.
> >
> > The matrix A is distributed among several processors: does the local
> number of rows of A on each processor have to be a multiple of bs   ?
>
>   Correct
> >
> > I want to use a multigrid preconditionner to  solve the linear system of
> matrix A : A x = b
> > Is it mandatory to define the right hand side b as a vector with the
> same blocksize bs=3 ?
>
>   It probably doesn’t matter, but if you know something has an associated
> block size it is generally best to capture it as soon as you can in the
> problem.
>
>    Barry
>
> >
> > Thank you very much of your help,
> > Best regards,
> > Natacha
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20141010/8e51ac70/attachment.html>