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

[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?
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