[petsc-users] Testing MatGetSubMatrices

[Matthew Knepley]

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
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