[petsc-users] local dimensions

[Massoud Rezavand]

Sorry, I forgot to say that my computational domain is decomposed with a
parallel library using MPI, and the particles are defined with a
distributed vector. So, the entries of the matrix are basically from that
distributed vector.

Thanks,
Massoud

On Mon, Dec 12, 2016 at 1:21 AM, Massoud Rezavand <msdrezavand at gmail.com>
wrote:

> Thanks,
>
> as I already discussed with you, the matrix is coming from SPH
> discretization, which is not fixed on a grid and is changing over time.
>
> On Mon, Dec 12, 2016 at 1:10 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> > On Dec 11, 2016, at 6:04 PM, Massoud Rezavand <msdrezavand at gmail.com>
>> wrote:
>> >
>> > Thank you very much,
>> >
>> > So, if I am using PetscSplitOwnership() and then MatGetOwnershipRange()
>> to be prepared for preallocation, then MatSetSizes(A, local_size,
>> local_size, N, N) should be called with the calculated  local_size from
>> PetscSplitOwnership() ?
>>
>>    Confusion from the two responses. You cannot use
>> MatGetOwnershipRange() for preallocation.
>>
>> Without preallocation:
>>
>> > > PetscInt local_size = PETSC_DECIDE;
>> > >
>> > > MatSetSizes(A, local_size, local_size, N, N);
>>
>>      MatGetOwnershipRanges(...)
>>
>> With preallocation:
>> > >
>> > >
>> > > 2)
>> > >
>> > > PetscInt local_size = PETSC_DECIDE;
>> > >
>> > > PetscSplitOwnership(PETSC_COMM_WORLD, &local_size, &N);
>> > >
>> > > MPI_Scan(&local_size, &end_row, 1, MPIU_INT, MPI_SUM,
>> PETSC_COMM_WORLD);
>> > > begin_row = end_row - local_size;
>>
>>     MatMPIAIJSetPreallocation(.....).
>>
>>
>> But note that normally if the matrix comes from a discretization on a
>> grid you would not use either approach above. The parallel layout of the
>> grid would determine the local sizes and you won't not obtain them with
>> PetscSplitOwnership() or local_size = PETSC_DECIDE;
>>
>> Where is your matrix coming from?
>>
>>    Barry
>>
>>
>>
>> > >
>> > >
>>
>>
>> >
>> > Thanks,
>> > Massoud
>> >
>> >
>> > On Mon, Dec 12, 2016 at 12:35 AM, Jed Brown <jed at jedbrown.org> wrote:
>> > Massoud Rezavand <msdrezavand at gmail.com> writes:
>> >
>> > > Dear PETSc team,
>> > >
>> > > What is the difference between the following two methods to get the
>> local
>> > > dimensions of a square matrix A? If they do the same, which one is
>> > > recommended? Should I use MPI_Scan after both?
>> >
>> > I would typically use 1 because it's fewer calls and automatically uses
>> > the correct communicator.  You can use MatGetOwnershipRange() instead of
>> > manually using MPI_Scan.
>> >
>> > > 1)
>> > >
>> > > PetscInt local_size = PETSC_DECIDE;
>> > >
>> > > MatSetSizes(A, local_size, local_size, N, N);
>> > >
>> > >
>> > > 2)
>> > >
>> > > PetscInt local_size = PETSC_DECIDE;
>> > >
>> > > PetscSplitOwnership(PETSC_COMM_WORLD, &local_size, &N);
>> > >
>> > > MPI_Scan(&local_size, &end_row, 1, MPIU_INT, MPI_SUM,
>> PETSC_COMM_WORLD);
>> > > begin_row = end_row - local_size;
>> > >
>> > >
>> > > Thanks in advance,
>> > > Massoud
>> >
>>
>>
>
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