On Wed, Apr 6, 2011 at 11:36, Thomas Witkowski < [email protected]> wrote:

This brings me back to the starting point of my current problem: how to set splits for MatNest correctly? PCFieldSplitSetFields seems not to work in my case.

PCFieldSplitSetFields() is a special case that only works when fields are interlaced. I would like to get rid of it, but there are still some cases where it can provide a faster implementation in the special case so we'll wait until the faster implementation can be accessed through the generic interface.

When I want to make use of PCFieldSplitSetIS for splitting MatNest, which indices should I provide? The MatNest I've created consists of the to main diagonal blocks with indices 0 to n and 0 to m.

This does not make sense because these index sets overlap. Maybe you want to use (0:m) and (m:m+n), then these are the index sets you pass to PCFieldSplitSetIS(). With two processes, you would usually have is0 = [(0:m0), (m0+n0:m0+n0+m1)] is1 = [(m0:m0+n0), (m0+n0+m1:m0+n0+m1+n1)] This is the decomposition that is set up automatically by MatCreateNest when you pass PETSC_NULL for the index sets. You should be able to retrieve the index sets that MatNest created using MatNestGetSubMats(), but this is not implemented (Dave and I discussed it and agree, but a patch has not been pushed yet).