[Paraview] Ghost data in parallel formats

[Berk Geveci]

Hi Renato,

> I think I'm missing something.... you said cells only?!
> If I understood this subject correctly, a cell should be considered ghost if
> it's held by more than one partition/process, isn't it?! In this case,
> there'll be an overlapped layer of elements. The problem is that my MPI
> solver does not make use of this overlapped layer of cells/elements.

Yep. You understood correctly. Ghost cells are very common for finite
difference calculations but not as common for finite elements.

> It only
> has nodes/points that are shared by processes. This explains why I asked by
> a ghost node (shared node would be more appropriated to define such kind of
> node).
> Can I consider a cell as ghost if it touches the parallel interface (without
> overlapping)? Would it work?

Nope. Then you'd start seeing gaps. The right thing to do is for
ParaView to support ghost points (nodes) better. However, this is
non-trivial in some cases. For removing internal interfaces, it is
sufficient to mark points as ghosts. However, for accurately
performing statistics, you need to make sure that you count all points
only once, which requires assigning ghost nodes to processes. So a
replicated node would be marked as ghost (a better word is shared) and
also owned by a particular process. We are going to improve VTK's
ghost level support. This is something we'll support. However, it will
be up to the simulation to produce the write output. Do you have the
ability to mark a node as "owned" by one partition and as "ghost" on
other partitions?

Best,
-berk