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

[zakaryah]

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 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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