Re: [Libmesh-devel] Questions regarding Submatrix Extraction

[Stogner, R. H <roy...@ic...>]

On Tue, 29 Jan 2019, Boris Boutkov wrote:

> 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().

How are the orders out of sync, exactly?

> 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.

Are the local_variable_indices results not in ascending order?  If
not, that would be a bug.  If so, what order would you want instead?
---
Roy