[Dune] GridFactory and parallel grids (and boundary adaptation)

[Oliver Sander]

Hi Jö,

> With regard to loading a parallel grid with the GridFactory, the current
> informal standard seems to be inserting on rank 0 only, inserting nothing on
> the other ranks, and following that by a loadBalance().  I know that worked
> with the old ALUGrid, and apparently UG supports that too[1].  Is that still
> the way to go with dune-alugrid or are there better ways (so long as you want
> to keep using the GridFactory?).
>

for UG, this is still how it is.


> GmshReader seems seems to be be similar: it doesn't seem to know anything
> about MPI, so you have to create an empty GridFactory, fill it with GmshReader
> on rank 0 only, create the grid, and then loadBalance.  I'm assuming there are
> no specializations of GmshReader out the, please point me to them if I'm
> wrong.
> 
> Now about boundary adaptation: UG states[1]
> 
>   Warning:
>     To use a parametrized boundary on a parallel machine you need to hand over
>     the boundary segments to the grid factory on all processes.  This behavior
>     violates the Dune grid interface specification and will be corrected in
>     the future.

That's still the status quo.

> 
> I'm assuming that means create a GridFactory on all ranks and insert each
> boundary segment on each rank.  But do I have to insert the vertices (and
> elements) on each rank too, to do I just make sure to use the same vertex
> numbers as on rank 0?

It should be sufficient to insert the elements and vertices only on rank 0,
but I doubt anyone has ever actually tested that.

Unfortunately I have not had the manpower to extend UG's features in the last
year.  I hesitate to formulate some of the open issues as gsoc projects,
because they are all pretty difficult.

Cheers,
Oliver

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