[Libmesh-devel] Questions regarding Submatrix Extraction

[Boris B. <bor...@bu...>]

Hello again all,

I've been working at getting GMG + fieldsplit functioning together and 
am running into some difficulties which I hope I could get some feedback on.

In short, I cant seem to properly extract the sub-projections needed for 
GMG when considering problems with mutiple variables. It seems to me 
that this is an issue with maintaining a consistent dof ordering when 
passing the row and column index vectors to 
PetscMatrix::create_submatrix(), an order which needs to be "in sync" 
with the ordering of dofs coming from System::projection_matrix().

For example if I wish to extract the velocity sub-block in a Stokes type 
problem, my first attempt was to simply use the 
DofMap::local_variable_indices(), concatenating the relevant variable 
dofs and passing them to create_submatrix() but this seemed to provide 
incorrectly sorted SubMats which later manifests as deteriorated 
convergence on coarser grid GMG levels.

I also tried looping over active local elem, and getting their parents 
old_dof_indicies but this seems to provide the same submat as the 
earlier attempts and appears to still be ordered by variable. I'll also 
note that sorting these vectors seems to only make matters worse.

Ive been playing with these ideas in a branch [1] which adds a unit test 
that attempts to add two vars to a system, and extract the whole 
"sub"matrix which should be equivalent to the global projection and I 
cant seem to find the right way to do this.  Does anyone have any 
suggestions on some other DoF ordering extraction I can try, or maybe Im 
going about this in the wrong way altogether?

Thanks as always for any information you can provide,

  - Boris

[1] : https://github.com/bboutkov/libmesh/tree/subproj_test