[petsc-users] MatScale returns different results depending on matrix size

[Matthew Knepley]

On Wed, Jan 6, 2021 at 2:41 AM Roland Richter <roland.richter at ntnu.no>
wrote:

> Hei,
>
> I added one additional function to the code:
>
> *void test_scaling_petsc_pointer(const Mat &in_mat,*
> *                                Mat &out_mat,*
> *                                const PetscScalar &scaling_factor) {*
> *    MatCopy (in_mat, out_mat, SAME_NONZERO_PATTERN);*
> *    PetscScalar *mat_ptr;*
> *    MatDenseGetArray (out_mat, &mat_ptr);*
> *    PetscInt r_0, r_1;*
> *    MatGetLocalSize (out_mat, &r_0, &r_1);*
> *    for(int i = 0; i < r_0 * r_1; ++i)*
> *        *(mat_ptr + i) = (*(mat_ptr + i) * scaling_factor);*
>
> *    MatAssemblyBegin (out_mat, MAT_FINAL_ASSEMBLY);*
> *    MatAssemblyEnd (out_mat, MAT_FINAL_ASSEMBLY);*
> *}*
>
> When replacing test function *test_scaling_petsc()* with
> *test_scaling_petsc_pointer()* everything works as it should, but I do
> not understand why.
>
> Do you have any suggestions?
>
The easiest explanation is that you have a memory overwrite in the code
somewhere. Barry's suggestion to use
valgrind is good.

   Matt

> Thanks!
>
>
> Am 05.01.21 um 15:24 schrieb Roland Richter:
>
> Hei,
>
> the code I attached to the original mail should work out of the box, but
> requires armadillo and PETSc to compile/run. Armadillo stores the data in
> column-major order, and therefore I am transposing the matrices before and
> after transferring using .st().
>
> Thank you for your help!
>
> Regards,
>
> Roland
> Am 05.01.21 um 15:21 schrieb Matthew Knepley:
>
> On Tue, Jan 5, 2021 at 7:57 AM Roland Richter <roland.richter at ntnu.no>
> wrote:
>
>> Hei,
>>
>> I would like to scale a given matrix with a fixed scalar value, and
>> therefore would like to use MatScale(). Nevertheless, I observed an
>> interesting behavior depending on the size of the matrix, and currently
>> I am not sure why.
>>
>> When running the attached code, I intend to divide all elements in the
>> matrix by a constant factor of 10. If I have three or fewer rows and
>> 1024 columns, I get the expected result. If I have four or more rows
>> (with the same number of columns), suddenly my scaling factor seems to
>> be 0.01 instead of 0.1 for the PETSc-matrix. The armadillo-based matrix
>> still behaves as expected.
>>
>
> 1) It looks like you assume the storage in your armadillo matrix is row
> major. I would be surprised if this was true.
>
> 2) I think it is unlikely that there is a problem with MatScale, so I
> would guess either you have a memory overwrite
> or are misinterpreting your output. If you send something I can run, I
> will figure out which it is.
>
>   Thanks,
>
>      Matt
>
>
>> I currently do not understand that behavior, but do not see any problems
>> with the code either. Are there any possible explanations for that
>> behavior?
>>
>> Thank you very much,
>>
>> regards,
>>
>> Roland Richter
>>
>>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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