[petsc-dev] parallel direct solvers for MG

[Mark Adams]

Parallel coarse grid solvers are a bit broken at large scale where you
don't want to use all processors on the coarse grid. The ideal thing might
be to create a sub communicator, but it's not clear how to integrate this
in (eg, check if the sub communicator exists before calling the coarse grid
solver and convert if necessary). A bit messy. It would be nice if a
parallel direct solver would not redistribute the matrix, but then it would
be asking too much for it to reorder also, so we could have a crappy
ordering. So maybe the first option would be best long term.

I see we have MUMPS and PaStiX. Do either of these not redistribute if
asked?
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