[petsc-users] Incorrect local row ranges allocated to the processes i.e. rstart and rend are not what I expected

[Matthew Knepley]

On Fri, Nov 23, 2018 at 9:54 AM Klaus Burkart via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hello,
>
> I am trying to compute the local row ranges allocated to the processes
> i.e. rstart and rend of each process, needed as a  prerequisite for
> MatMPIAIJSetPreallocation using d_nnz and o_nnz.
>
> I tried the following:
>
> ...
>
>     PetscInitialize(0,0,PETSC_NULL,PETSC_NULL);
>
>     MPI_Comm_size(PETSC_COMM_WORLD,&size);
>     MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
>
>     MatCreate(PETSC_COMM_WORLD,&A);
>     MatSetType(A,MATMPIAIJ);
>     PetscInt local_size = PETSC_DECIDE;
>     PetscSplitOwnership(PETSC_COMM_WORLD, &local_size, &N);
>     MPI_Scan(&local_size, &rend, 1, MPIU_INT, MPI_SUM, PETSC_COMM_WORLD);
>

This looks right to me. Not sure what your problem is. However, you can
always use PetscLayout to do
this automatically.

  Thanks,

     Matt


>     rstart = rend - local_size;
>     PetscInt d_nnz[local_size], o_nnz[local_size];
> /*
>
> compute d_nnz and o_nnz here
>
>     MatMPIAIJSetPreallocation(A,0,d_nnz,0,o_nnz);
> */
>
>     for (rank = 0; rank < size; rank++) {
>     PetscPrintf(PETSC_COMM_WORLD,"local_size   =  %d, on process %d\n",
> local_size, rank);
>     PetscPrintf(PETSC_COMM_WORLD,"rstart =  %d, on process %d\n", rstart,
> rank);
>     PetscPrintf(PETSC_COMM_WORLD,"rend =  %d, on process %d\n", rend,
> rank);
>     }
>
>     PetscFinalize();
>
> The local size is 25 rows on each process but rstart and rend are 0 and 25
> on all processes, I expected 0 and 25, 25 and 50, 50 and 75 and 75 and 101.
> N = 100
>
> I can't spot the error. Any ideas, what's the problem?
>
> Klaus
>


-- 
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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