[petsc-users] Question about MatCreateShell loca ros/colums

[Matthew Knepley]

On Wed, Apr 14, 2021 at 5:56 PM Sam Guo <sam.guo at cd-adapco.com> wrote:

> Dear PETSc dev team,
>    If I want PETSc to use my own parallel partition(instead of PETSc
> "evenly" partition rows), I assume I can provide local rows/columns as
> follows.
>
> MatCreateShell <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateShell.html#MatCreateShell>(MPI_Comm <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/MPI_Comm.html#MPI_Comm> comm,PetscInt <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> m,PetscInt <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> n,PetscInt <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> M,PetscInt <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscInt.html#PetscInt> N,void *ctx,Mat <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/Mat.html#Mat> *A)
>
>   If that's the case, what role does local columns play? Memory
> allocation? Should I use n (the global rows) or local rows or PETSC_DECIDE
> in this case?
>

The only role played by n is to match this matrix with parallel vectors
that you want it to act on. The local sizes should match. For square
matrices, this usually
means you match the local row size.

  Thanks,

     Matt


> Thanks,
> Sam
>


-- 
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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