[vtk-developers] MPI_Comm_split

[pat marion]

VTK wraps MPI_Comm_split with vtkMPIController::PartitionController(), which
calls vtkMPICommunicator::SplitInitialize().  Currently there is no support
for passing MPI_UNDEFINED as a color.  If you pass MPI_UNDEFINED as a color
you should get back a null communicator (MPI_COMM_NULL).

There is handling of null communicators in
vtkMPIController::CreateSubController() which wraps MPI_Comm_create.  So I
can follow the same pattern to make PartitionController support it too, no
problem.

But I do a lot of prototyping in python, and in python I can't easily use
MPI_UNDEFINED.  So I'm thinking about adding a static method called
vtkMPICommunicator::GetMPIUndefined() that returns MPI_UNDEFINED  (it's just
an int).  Objections?  My other thought was to have
vtkMPIController::PartitionController automatically convert negative color
values to MPI_UNDEFINED, but that behavior could cover up programming
errors, so I don't like that.

Pat
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