[petsc-users] MatCreateMPIAIJ with zero size on some ranks

[Matthew Knepley]

On Wed, Apr 25, 2012 at 7:42 AM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

>  Am 25.04.2012 13:21, schrieb Jed Brown:
>
> On Wed, Apr 25, 2012 at 02:21, Thomas Witkowski <
> thomas.witkowski at tu-dresden.de> wrote:
>
>> Is it possible to create a parallel matrix (MPIAIJ) with empty size on
>> some ranks? Even if this should work, does it make sense (with respect to
>> runtime and efficiency) to collect all ranks having a nonempty contribution
>> the the global matrix in an mpi group and to use this communicator for
>> matrix create/computations?
>
>
> It depends how many ranks are empty. For coarse levels of multigrid (where
> this situation arises frequently), it's sometimes worthwhile to also move
> the data to a part of the machine that is topologically "close".
> Identifying "closeness" is not always easy because MPI does not necessarily
> expose this information in a useful fashion.
>
>  What is the circumstance where this occurs for you?
>
> Also some coarse space method. When not all ranks contribute to the coarse
> space, a matrix defined on the coarse space leads to local empty matrices
> on these ranks. So the question is if this leads to technical troubles in
> PETSc. The share of nodes participating in the coarse space may vary.
>

No

  Matt


>
> Thomas
>



-- 
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
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