[petsc-users] Changing the operator for PCMG while leaving interpolation matrices unchanged

[Ben Yee]

Yes, the nonzero structure changes in a way that cannot easily be
predicted.  I could overallocate, add some extra entries to the first
matrix, and reuse that matrix, but I would have to roughly double the
number of allocated entries, leading to an undesirable increase in memory.
I think that one alternative would be to just reset the KSP object when I
change the operator, but, if I'm not mistaken, this would destroy all the
interpolation operators and I'd have to redefine all of them.

On Wed, Jan 24, 2018 at 10:41 AM, Smith, Barry F. <bsmith at mcs.anl.gov>
wrote:

>
>   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 <bcyee at umich.edu> 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
>
>


-- 
Ben Yee

NERS PhD Candidate, University of Michigan
B.S. Mech. & Nuclear Eng., U.C. Berkeley
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