[petsc-users] Testing MatGetSubMatrices

[Bikash Kanungo]

Thanks for the suggestion Matthew. The issue was that MatDenseGetArray
stores the value in a compressed column format rather than a row format. So
while setting the values into XTemp I must carefully swap between row and
column indices.

Regards,
Bikash


On Tue, Oct 20, 2015 at 11:58 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Oct 20, 2015 at 10:53 AM, Bikash Kanungo <bikash at umich.edu> wrote:
>
>> Hi,
>>
>> I was trying to perform a simple test on MatGetSubMatrices. I have an
>> MPIDENSE matrix X which has been populated and assembled and another matrix
>> XTemp of the same type and size. Since X is MPIDENSE in order to extract a
>> sub-matrix from it, the IS must be sorted. This can be done easily through
>> ISSort. Once the sub-matrix is obtained, I'm trying to set the values in
>> XTemp by using the sorted IS as the global indices and the sub-matrix as
>> the values to be set. Once XTemp is set and assembled, I evaluate XTemp - X
>> and see if the norm of the resultant matrix is zero or not. The norm turned
>> out to be as high as 1.0e6.
>>
>
> Make the problem very small (10 rows maybe) and starting printing things
> out.
>
>    Matt
>
>
>>
>> I'm providing a simplified version of the code that performs the above
>> test. Kindly let me know if I have done any mistake is designing the test.
>>
>>
>>       //
>>       // Given an initialized and assembled MPIDENSE Mat X and
>>       // an uninitialized MPIDENSE Mat XTemp of the same size as that of
>> X
>>       //
>>       IS  * ISRowIds, * ISColIds;
>>       PetscMalloc(1*sizeof(IS), &ISRowIds);
>>       PetscMalloc(1*sizeof(IS), &ISColIds);
>>
>>       //
>>       // globalRowIds and globalColIds are STL vectors containing
>>       // the global node ids for which the sub-matrix from X need
>>       // to be extracted
>>       //
>>       const PetscInt * rowIdsPtr = globalRowIds.data();
>>       const PetscInt * colIdsPtr = globalColIds.data();
>>
>>       ISCreateGeneral(PETSC_COMM_WORLD, globalRowIds.size(), rowIdsPtr,
>> PETSC_COPY_VALUES, &ISRowIds[0]);
>>       ISCreateGeneral(PETSC_COMM_WORLD, globalColIds.size(), colIdsPtr,
>> PETSC_COPY_VALUES, &ISColIds[0]);
>>       ISSort(ISRowIds[0]);
>>       ISSort(ISColIds[0]);
>>
>>       Mat * subMat;
>>       MatGetSubMatrices(X, 1, ISRowIds, ISColIds, MAT_INITIAL_MATRIX,
>> &subMat);
>>
>>       PetscScalar * subMatValues;
>>       MatDenseGetArray(*subMat, &subMatValues);
>>
>>       const PetscInt * rowIndicesSorted, * colIndicesSorted;
>>       ISGetIndices(ISRowIds[0], &rowIndicesSorted);
>>       ISGetIndices(ISColIds[0], &colIndicesSorted);
>>
>>       MatSetValues(XTemp,
>>                    globalRowIds.size(),
>>                    rowIndicesSorted,
>>                    globalColIds.size(),
>>                    colIndicesSorted,
>>                    subMatValues,
>>                    INSERT_VALUES);
>>
>>       //
>>       // commencing assembly
>>       //
>>       MatAssemblyBegin(XTemp,
>>                          MAT_FINAL_ASSEMBLY);
>>
>>       //
>>       // concluding assembly
>>       //
>>       MatAssemblyEnd(XTemp,
>>                MAT_FINAL_ASSEMBLY);
>>
>>       MatAXPY(XTemp, -1.0, X, DIFFERENT_NONZERO_PATTERN);
>>       double diffNorm;
>>       MatNorm(XTemp, NORM_1, &diffNorm);
>>
>>
>>
>> Thanks,
>> Bikash
>> --
>> Bikash S. Kanungo
>> PhD Student
>> Computational Materials Physics Group
>> Mechanical Engineering
>> University of Michigan
>>
>>
>
>
> --
> 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
>



-- 
Bikash S. Kanungo
PhD Student
Computational Materials Physics Group
Mechanical Engineering
University of Michigan
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