[petsc-users] Create MATSEQAIJ from MATMPIAIJ

[Eda Oktay]

Oh okay, I misunderstood before. I will try that. Thanks!

Matthew Knepley <knepley at gmail.com>, 22 Haz 2020 Pzt, 19:10 tarihinde
şunu yazdı:
>
> On Mon, Jun 22, 2020 at 11:39 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>>
>> Matthew Knepley <knepley at gmail.com>, 22 Haz 2020 Pzt, 14:43 tarihinde
>> şunu yazdı:
>> >
>> > On Mon, Jun 22, 2020 at 7:14 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>> >>
>> >> Hello everyone,
>> >>
>> >> I am trying to find elements in off diagonal blocks of a parallel
>> >> sparse matrix. That is why, I want to use MatGetDiagonalBlock and from
>> >> the matrix I obtain, I want to obtain off diagonal elements by
>> >> subtracting it from my original matrix by using MatAXPY.
>> >>
>> >> However, since MatGetDiagonalBlock gives a sequential matrix and my
>> >> The original one is parallel, I can't use MatAXPY. That's why I want to
>> >> change the type of one of the matrices.
>> >>
>> >> How can I change a MATSEQAIJ to MATMPIAIJ or vice versa?
>> >
>> >
>> > I assume you want a parallel matrix with the element in the diagonal block removed. I can think
>> > of at least two ways to do this which sound easier to me:
>> >
>> >   1) Make a copy and then zero out the diagonal block is a way similar to MatChop: https://www.mcs.anl.gov/petsc/petsc-current/src/mat/utils/axpy.c.html#MatChop
>>
>> I read MatChop before, however I couldn't understand how to decide
>> tolerance. What I understood from this function is to eliminate
>> entries less than a number which is set to be the tolerance. But the
>> entries in the diagonal blocks can be less than ones in offdiagonal
>> blocks. What am I missing?
>
>
> You _change_ the code. MatChop decides to make an element zero by looking at how big it is. You would decide
> to make an element zero by looking at what column it is in.
>
>   Thanks,
>
>      Matt
>
>>
>> >
>> >   2) Use MatGetSubMatrix() and exclude any columns from the diagonal block on each process.
>> >
>> >   Thanks,
>> >
>> >      Matt
>> >
>> >>
>> >> Thanks!
>> >
>> >
>> >
>> > --
>> > 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/
>
>
>
> --
> 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/