Putting a peculiar multilevel scheme into DMMG?

Matthew Knepley knepley at gmail.com
Sun Dec 13 18:56:46 CST 2009

On Sun, Dec 13, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote:

> On Sun, 13 Dec 2009 18:06:04 -0600, Matthew Knepley <knepley at gmail.com>
> wrote:
> > I will explain the history since I am to blame for this one. I needed
> > coarsenHierarchy because repeated coarsening can cause degradation of
> > the meshes. Thus all coarse meshes must be made at once. I added the
> > hierarchy option, but only the coarsen branch was used.
> I suspected something like that.  So one real difference with hierarchy
> is that (in principle) DMRefine can move to a different communicator.
> Is this flexibility used (it seems like this could be quite powerful for
> additive multigrid, but I'm not aware of such a thing working now)?  If
> so, DMRefineHierarchy and DMCoarsenHierarchy should also take an array
> of communicators (passing NULL would mean to use the same communicator).
> > > DMCoarsenHierarchy is implemented for Mesh, but the array is currently
> > > leaking (DMMGSetDM forgets a PetscFree).  Is it indeed preferred that
> > > the caller does not allocate the array (despite knowing how much is
> > > needed) but is responsible for freeing it (I ask because this is clumsy
> > > for a single level of refinement).  Either way, I'll document the
> choice
> > > and fix the leak.
> > >
> >
> > Cool. I guess we could have caller allocation, but that is harder to
> > check for correctness.
> The alternative (unless we want two ways to do the same thing) seems
> much worse:
>  DM *tmp;
>  DMRefineHierarchy(dmc,1,&tmp);
>  dmf = *tmp;
>  PetscFree(tmp);

I am not sure why this is much worse. There are many things users are
required to destroy.


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