Hi Matt, all,
> On 21 Feb 2019, at 18:17, Matthew Knepley <knep...@gmail.com> wrote:
>
> Here is why I did this. I wanted this to work
>
>
> https://bitbucket.org/petsc/petsc/src/4dbc1805575afffed4e440f1353fcfccbc893081/src/snes/examples/tutorials/ex62.c#lines-1062
>
> The DMKSP is initialized by SNES to have a function which says "get your
> matrix from SNES". Without this, the subproblem could
> not make the matrix. Now I changed the description of exactly what matrix it
> is making and stored that in the subDM.
>
> What is the right way to think about fixing this? I think that if the DM can
> create a field decomposition, it should also be able to transfer user
> callbacks from the parent to the subdms.
>
> That way, the DM (IOW me) can control how that happens.
>
> Yes, that is what I thought I was doing above. Preserving the callbacks that
> had been set in the original DM.
> Maybe clarify what you think should happen.
>
> Or do I have the way this is supposed to work all wrong. Is the
> ComputeOperators callback I set meant to be completely agnostic, and it
> should get all the problem-specific information from the DM?
>
> I am doing this, but it does not have to be the only way things work. What I
> do not understand is how this change breaks your stuff.
> If you were setting the DMKSP compute callback, it should just override what
> I am doing above. If not, why does me setting it here
> screw it up?
I've done some more digging, and the setup that breaks is:
SNES with pcfieldsplit inside and rediscretised multigrid inside the fieldsplit.
What is going on is, I think, the following:
SNESSolve sets up a ComputeOperators on the KSP:
KSP ksp;
ierr = SNESGetKSP(snes,&ksp);CHKERRQ(ierr);
ierr =
KSPSetComputeOperators(ksp,KSPComputeOperators_SNES,snes);CHKERRQ(ierr);
ierr =
DMCoarsenHookAdd(snes->dm,DMCoarsenHook_SNESVecSol,DMRestrictHook_SNESVecSol,snes);CHKERRQ(ierr);
How does this callback know where to linearise around? Well,
KSPComputeOperators_SNES does this:
if (dmsave == snes->dm) X = snes->vec_sol; /* We are on the finest level */
else { /* We are on a coarser level, this
vec was initialized using a DM restrict hook */
ierr = DMGetNamedGlobalVector(snes->dm,"SNESVecSol",&Xnamed);CHKERRQ(ierr);
And the DMCoarsenHookAdd and DMRestrictHook_SNESVecSol ensure that /if we
coarsen the DM associated with the SNES/ that state vector is transferred to
the coarser levels.
However, when we do PCFieldsplit and DMCreateFieldDecomposition, this
restriction hook and connection to the state vector is gone.
As a result, when I rediscretise on the coarse grids, I'm always linearising
around a zero state.
So I suppose that what should happen is that somewhere in fieldsplit, we need
to transfer SNES state over and setup the appropriate restrict hooks.
Cheers,
Lawrence
