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

[Klaus Burkart]

 The output is:
local_size   =  25, on process 0
rstart =  0, on process 0
rend =  25, on process 0
local_size   =  25, on process 1
rstart =  0, on process 1
rend =  25, on process 1
local_size   =  25, on process 2
rstart =  0, on process 2
rend =  25, on process 2
local_size   =  25, on process 3
rstart =  0, on process 3
rend =  25, on process 3

Using PETSC_COMM_SELF has no effect.

    Am Freitag, 23. November 2018, 18:02:45 MEZ hat zakaryah <zakaryah at gmail.com> Folgendes geschrieben:  
 
 What does your output look like?  With PETSC_COMM_WORLD, I think you will only get the output from the rank 0 process.  Try with PETSC_COMM_SELF.
On Fri, Nov 23, 2018, 11:56 AM Matthew Knepley via petsc-users <petsc-users at mcs.anl.gov wrote:

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 dothis 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/

  
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