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

[Matthew Knepley]

On Wed, Jun 10, 2020 at 12:07 PM Eda Oktay <eda.oktay at metu.edu.tr> wrote:

> 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.
>

Was the global size of the vector you wrapped around the dense matrix 72*4?

If you use CreateToAll(), it will make a vector on each process which has
the global size of the original vector.

  Thanks,

    Matt


> 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/
>


-- 
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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