[petsc-users] extract Mat data

[Matthew Knepley]

On Thu, Mar 31, 2011 at 12:38 PM, gouarin <loic.gouarin at math.u-psud.fr>wrote:

>  On 31/03/2011 19:06, Matthew Knepley wrote:
>
> On Thu, Mar 31, 2011 at 10:10 AM, gouarin <loic.gouarin at math.u-psud.fr>wrote:
>
> Thanks for the reply.
>
>
> On 31/03/2011 16:48, Hong Zhang wrote:
>
> Loic,
>
>  I want to do the same for MATMPI. This is what I do:
>
> MatGetLocalMat(A,MAT_INITIAL_MATRIX,&Aloc);
> MatGetRowIJ(Aloc, 1, PETSC_FALSE, PETSC_FALSE,&(Acsr.n),&(Acsr.ia),
> &(Acsr.ja),&done);
> MatGetArray(Aloc,&(Acsr.a));
>
> Now, I need to know on what process the non local points are.
>
> Each process get its own local submatrix. See
>
> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatCreateMPIAIJ.html
> on data structure of MPIAIJ format. Applying your statements above to
> the example listed on the page (a 8x8 matrix), Proc0 will get
> Aloc=A(0:2,:), ..., Porc2 gets Aloc=A(6:7,:).
>
> Is there a petsc function that gives this partition ?
>
> MatGetOwnershipRange()
>
>  I use this function to know the local points index. In the same way, with
> this function I know the non local index.
>
> But this function doesn't give the process. I give you an example. I have
> this matrix:
>
> 1 0 | 0 1
> 1 2 | 1 0
> -----------
> 0 0 | 1 0
> 0 2 | 0 1
>
> I want this kind of format
>
> for proc 0
> a = {1,1,1,2,1}
> ia = {0,2,5}
> ja = {0,3,0,1,2}
> rank = {0,0,1,1}
>
> for proc 1
> a = {1,2,1}
> ia = {0,1,3}
> ja = {0,2,1}
> rank = {1,1,0}
>
> rank give me the process for each non zero column. Local rows should have
> numbers 1,...N and offdiagonal entries with j>N.
>
>
>  The ownership range is the row partition. However, MPIAIJ matrices are
> not stored as you indicate above.
>
>  Different storage formats make the process you describe fragile. WHy do
> you need the CSR form of the matrix? If you really
> do, use MatGetSubMatrix() to get a  SEQAIJ matrix on each process and pull
> out the values.
>
>   I want to use this format because I try to create a new multigrid PC
> with this method:
>
> http://homepages.ulb.ac.be/~ynotay/AGMG/userguide/index.html
>
> I don't understand why it's not possible to have the process partition.
> When you create a MPIAIJ matrix and you use a KSP to solve the system, you
> know what values to exchange. No? Where is this information ?
>

1) It is possible to get the partition of rows across processes. This is
EXACTLY what MatGetOwnershipRange() returns, as I said in the last email.

2) As I explained, matrices can be stored in a variety of formats. The
MPIAIJ format stores 2 local matrices per partition in order to overlap
communication with computation.

I can also use MatGetOwnershipRange() and MPI_Allgather to have this
> information.
>

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatGetOwnershipRanges.html

   Matt


> Loic
>
>
>
>
>      Matt
>
>
> I suppose that I have no information about the structure of MPIAIJ.
>
> Loic
>
>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
>
>
> --
> Loic Gouarin
> Laboratoire de Mathématiques
> Université Paris-Sud
> Bâtiment 425
> 91405 Orsay Cedex
> France
> Tel: (+33) 1 69 15 60 14
> Fax: (+33) 1 69 15 67 18
>
>


-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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