On Fri, Feb 10, 2012 at 3:58 PM, Dmitry Karpeev <karpeev@mcs.anl.gov> wrote:

On Fri, Feb 10, 2012 at 3:53 PM, Jed Brown <jedbrown@mcs.anl.gov> wrote:

On Fri, Feb 10, 2012 at 15:50, Dmitry Karpeev <karpeev@mcs.anl.gov> wrote:

Why not simply allow DM to return two matrices as SNES and KSP would want anyway? 

How do you specify the reduced stencil to use for the preconditioning matrix? Maybe it wouldn't add too many more if statements to the current code which is already polluted with them.

It seems to me that the sparsity pattern (and even the type of the matrices) is up to the particular DM type,

so it is, in principle, opaque.  DMDA can control it by manipulating stencil width or widths, while FEM-oriented DM 

types (e.g., libMesh) could control it via the element type, etc. The generic DM interface doesn't have a way to specify 

the sparsity pattern. As it should be, in my opinion.

This makes no sense to me unless the two matrices being returned have different sparsity patterns, and we have no

good way for specifying that, or even a good rationale for doing it.

   Matt

 

Dmitry.

--
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener