[petsc-users] MatGetRow for global rows of a parallel matrix

[Eda Oktay]

Der Matt,

When I looked at the results, I found that there are some problems I
couldn't understand.

First of all, I am working on a 72*4 matrix and as I said before, I
want to have 72 different vectors having size 4 each, whose elements
consist of the elements in the same row. And of course, all vectors
should be in all processors (currently I am using 4 processors).

When I use your scatter code, the output vector is divided into 4
parts for 4 processors and each vector consists of 18 row vectors
whose elements are arranged in a way that if I want to find zeroth row
vector, its elements are located in 0th,18th,36th,54th elements.

So, isn't scatter's goal is to scatter all values to all processors?

Furthermore, I am trying to use my vectors in that way but isn't there
any possible way that I can reach my goal entirely?

Thanks so much for your help,

Eda

Matthew Knepley <knepley at gmail.com>, 10 Haz 2020 Çar, 18:11 tarihinde
şunu yazdı:
>
> On Wed, Jun 10, 2020 at 10:09 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>>
>> Dear Matt,
>>
>> I have one last question I believe. Up to creating a dense matrix I
>> did what you've suggested. Thank you so much for that.
>>
>> I created a new dense matrix. Now, how should I wrap each vector in a
>> MatDense again? I mean, what is wrapping vectors in a matrix? To put
>> each of them again as rows?
>
>
> I thought you need a dense matrix for something, since you started with one. If you
> do not, just do VecGetArray() on the vector from CreateToAll and use the values.
>
>   Thanks,
>
>      Matt
>
>>
>> Thanks!
>>
>> Eda
>>
>> Matthew Knepley <knepley at gmail.com>, 10 Haz 2020 Çar, 16:16 tarihinde
>> şunu yazdı:
>> >
>> > On Wed, Jun 10, 2020 at 9:08 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>> >>
>> >> Dear Matt,
>> >>
>> >> Matthew Knepley <knepley at gmail.com>, 10 Haz 2020 Çar, 16:03 tarihinde
>> >> şunu yazdı:
>> >> >
>> >> > On Wed, Jun 10, 2020 at 8:56 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>> >> >>
>> >> >> Hi all,
>> >> >>
>> >> >> I am trying to get all the rows of a parallel matrix as individual
>> >> >> vectors. For instance, if I have 72*4 matrix, I want to get 72
>> >> >> different vectors having size 4.
>> >> >>
>> >> >> As far as I understood, MatGetRow is only for local rows, so
>> >> >> MatGetOwnershipRange is used, however, when I tried this one, I
>> >> >> couldn't get the whole and desired row vectors.
>> >> >>
>> >> >> In MatGetRow explanation, it is written that I should use
>> >> >> MatCreateSubMatrices first, then use MatGetRow. But I couldn't
>> >> >> understand to which extent I should create submatrices. I just need to
>> >> >> have all 72 rows as 72 different vectors each having 4 elements.
>> >> >
>> >> >
>> >> > 1) For sparse matrices, the storage is always divided by row, so that values can only be retrieved for local rows with MatGetRow()
>> >> >
>> >> > 2) Is this matrix sparse? It sounds like it is dense.
>> >>
>> >> Matrix is dense.
>> >>
>> >> >
>> >> > 3) Are you asking to get all matrix values on all processes? If so, I think the easiest thing to do is first wrap a Vec around the
>> >> >     values, then use VecScatterToAll(), then wrap each one in a MatDense again.
>> >>
>> >> Yes, I want all row vectors on all processes. In a dense matrix,
>> >> should I still wrap a Vec around the values? I know I should use
>> >> scatter but I couldn't even wrap a Vec around them.
>> >
>> >
>> > I would do
>> >
>> >   MatGetSize(&N);
>> >   MatGetLocalSize(&m);
>> >   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatDenseGetArray.html
>> >   <create vector of local size m*N>
>> >   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecPlaceArray.html
>> >   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecScatterCreateToAll.html
>> >   <do scatter>
>> >   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecResetArray.html#VecResetArray
>> >   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateDense.html
>> >   <use it>
>> >   <destroy matrix>
>> >   <destroy vector from CreateToAll>
>> >
>> >   Thanks,
>> >
>> >      Matt
>> >
>> >>
>> >> Thanks so much!
>> >>
>> >> Eda
>> >>
>> >> >
>> >> >   Thanks,
>> >> >
>> >> >      Matt
>> >> >
>> >> >>
>> >> >> Thanks!
>> >> >>
>> >> >> Eda
>> >> >
>> >> >
>> >> >
>> >> > --
>> >> > 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/
>
>
>
> --
> 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/