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

[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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