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

[Roland Richter]

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?

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
>> <mailto: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/>
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