[petsc-dev] Bug introduced in MatGetSubmatrices()

[Hong]

Matt:
On mastter, we have

  if (C->submat_singleis) {
      /* flag is set in PCSetUp_ASM() to skip several MPIU_Allreduce() */
      MatGetSubMatrices_MPIAIJ_SingleIS(C,ismax,isrow,iscol,scall,submat);
    PetscFunctionReturn(0);
  }
...
MatGetSubMatrices_MPIAIJ_Local(C,...)

i.e., MatGetSubMatrices_MPIAIJ_SingleIS() is called when ismax=1 for each
process.
General case is handled by MatGetSubMatrices_MPIAIJ_Local(C,...)

I've optimized this routine and will merge it to master.

Hong

It comes from here:
>
>   https://bitbucket.org/petsc/petsc/commits/c10200c1442b553b7ad65c70101560
> db4fa22e78
>
> If we ask for more than 1 matrix, it dispatches to
>
>   MatGetSubMatrices_MPIAIJ_SingleIS()
>
> but then fails here
>
>   https://bitbucket.org/petsc/petsc/annotate/
> 2e559809f9aee9c95ee79eb0939630cfe5502c8d/src/mat/impls/aij/
> mpi/mpiov.c?at=master&fileviewer=file-view-default#mpiov.c-1306
>
> because ismax > 1. I think the ismax check needs to move up to here
>
>   https://bitbucket.org/petsc/petsc/annotate/
> 2e559809f9aee9c95ee79eb0939630cfe5502c8d/src/mat/impls/aij/
> mpi/mpiov.c?at=master&fileviewer=file-view-default#mpiov.c-2012
>
> but I don't know for sure. Please fix this since it is breaking PyLith.
>
>   Matt
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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