Ben
This is a good question, I'll take a look at how difficult it would be to
add this feature.
I take it the nonzero structure of the outer matrix changes over time?
Barry
> On Jan 24, 2018, at 9:12 AM, Ben Yee <[email protected]> wrote:
>
> Hi,
>
> I have multiple matrices (with different nonzero structures) for which I
> would like to use the same KSP operator (KSPRICHARDSON with PCMG, Galerkin
> process for generating coarse grid operator). I want to use the same PCMG
> structure for all the matrices (same interpolation, same restriction, same
> number of levels, same smoothing techniques, etc.), so I tried using
> KSPSetOperators to just change the operator. My interpolation matrices are
> MPIAIJ matrices that I have defined myself and provided to PCMG. My hope was
> that, with the new operator I provided, PETSc would recompute all of the
> coarse/intermediate grid operators using the Galerkin process with the old
> interpolation matrices and the new fine-grid operator.
>
> However, I get the wrong solution after changing the operator with
> KSPSetOperators. After some extensive digging around the PETSc source code,
> it seems that the operators in mg.c are not updated by KSPSetOperators. When
> I do a MatView on mglevels->A or the operator in mglevels->smoothu or
> mglevels->smoothd in PCMGMCycle_Private() of mg.c, it has the matrix values
> of the old matrix. From what I understand, the only place where mglevels->A
> (used to compute the residual) is updated is in PCApplyRIchardson_MG() of
> mg.c, from the following lines of code:
> 74: for
> (i=0; i<levels; i++) {
>
> 75: if
> (!mglevels[i]->A) {
>
> 76: KSPGetOperators
> (mglevels[i]->smoothu,&mglevels[i]->A,NULL);
>
> 77: PetscObjectReference((PetscObject
> )mglevels[i]->A);
>
> 78:
> }
>
> 79: }
> It seems to me that the pointer mglevels[i]->A is only set once, when it is
> undefined at the beginning of the iteration scheme. I think there are more
> issues as well, but this is the easiest one to point out. (For example, it
> doesn't seem like it is possible to update the operator for
> mglevels[i]->smoothd from KSPSetOperators on the outermost KSP object, and,
> if I try to set it manually via PCMGGetSmoother, I get errors if the nonzero
> structures of the coarse-grid operators have changed, which tends to be the
> case when the fine-grid operator's nonzero structure changes).
>
> Is there something I'm doing wrong? Is there a way to do this so that I can
> reuse PCMG with a new operator?
>
> Thanks!
>
> --
> Ben Yee
>
> NERS PhD Candidate, University of Michigan
> B.S. Mech. & Nuclear Eng., U.C. Berkeley
