[petsc-users] Near nullspace lost in fieldsplit

[Barry Smith]

   The new code is in https://gitlab.com/petsc/petsc/-/merge_requests/7293 and retains the null space on the submatrices for both MatZeroRows() and MatZeroRowsAndColumns() regardless of changes to the nonzero structure of the matrix.

  Barry


> On Feb 13, 2024, at 7:12 AM, Jeremy Theler (External) <jeremy.theler-ext at ansys.com> wrote:
> 
> Hi Barry
> 
> >   7279 does change the code for MatZeroRowsColumns_MPIAIJ(). But perhaps that does not resolve the problem you are seeing? If that is the case we will need a reproducible example so we can determine exactly what else is happening in your code to cause the difficulties. 
> >
> >Here is the diff for  MatZeroRowsColumns_MPIAIJ()
> >
> >@@ -1026,7 +1023,7 @@ static PetscErrorCode MatZeroRowsColumns_MPIAIJ(Mat A, PetscInt N, const PetscIn
> >
> >   PetscCall(PetscFree(lrows));
> >
> >   /* only change matrix nonzero state if pattern was allowed to be changed */
> > -  if (!((Mat_SeqAIJ *)(l->A->data))->keepnonzeropattern) {
> > +  if (!((Mat_SeqAIJ *)(l->A->data))->nonew) {
> >      PetscObjectState state = l->A->nonzerostate + l->B->nonzerostate;
> >      PetscCall(MPIU_Allreduce(&state, &A->nonzerostate, 1, MPIU_INT64, MPI_SUM, PetscObjectComm((PetscObject)A)));
> >    }
> 
> Fair enough, I might have overlooked this single line. But this change does not fix our issue. The if condition is still true. 
> I'll need some time to come up with a reproducible example because it involves setting up the PC of the KSP of an SNES of a TS before doing TSSolve() with the DM-allocated jacobian.
> 
> However, regarding bug #1, I do have a MWE. It is not CI-friendly. I tried to modify ex47.c to illustrate the issue but could not make it work.
> Anyway, the attached tarball:
> reads a global matrix A from a file.
> reads two ISs from another file
> sets up a field split PC 
> attaches a near nullspace (read from another file) to one of the sub-KSPs
> sets "dirichlet BCs" with MatZeroRowsColumns() or MatZeroRows() depending if -columns was given in the command line
> 
> The issue comes when calling MatZeroRowsColumns() or MatZeroRows().
> In the first case, the near nullspace is not lost but it is in the second:
> 
> $ make lost
> $ ./lost -columns -ksp_view | grep "near null"
>             has attached near null space
>         has attached near null space
> $ mpiexec -n 2 ./lost -columns -ksp_view | grep "near null"
>             has attached near null space
>         has attached near null space
> $ ./lost -ksp_view | grep "near null"
> $ mpiexec -n 2 ./lost -ksp_view | grep "near null"
> $ 
> 
> When using MatZeroRows(), the code passes through fieldsplit.c:692 which looses the near nullspace.
> 
> 
> Note that the original issue we see in our code is that there is a difference between the serial and parallel implementation of MatZeroRowsColumns().
> We loose the near nullspace only in parallel but not in serial because of a combination of bugs #1 and #2.
> 
> 
> --
> jeremy
> 
> <lost.tar.gz>

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