[petsc-users] extract Mat data

[gouarin]

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 
> <mailto:loic.gouarin at math.u-psud.fr>> wrote:
>
>     Thanks for the reply.
>
>
>     On 31/03/2011 16 <tel:31%2F03%2F2011%2016>: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 ?

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

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

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