[petsc-users] Problems in use of VecNest in Petsc?

[Barry Smith]

It is in the loop below that it errors due to an unassembled vector. T

static PetscErrorCode VecCopy_Nest(Vec x, Vec y)
{
  Vec_Nest *bx = (Vec_Nest *)x->data;
  Vec_Nest *by = (Vec_Nest *)y->data;
  PetscInt  i;

  PetscFunctionBegin;
  VecNestCheckCompatible2(x, 1, y, 2);
  for (i = 0; i < bx->nb; i++) PetscCall(VecCopy(bx->v[i], by->v[i]));
  PetscFunctionReturn(PETSC_SUCCESS);
}

thus it is one of the individual vectors that make up the full vectors that is marked as not assembled

In ex70.c I see 

PetscErrorCode StokesExactSolution(Stokes *s)
{
  PetscInt    row, start, end, i, j;
  PetscScalar val;
  Vec         y0, y1;

  PetscFunctionBeginUser;
  /* velocity part */
  PetscCall(VecGetSubVector(s->y, s->isg[0], &y0));
  PetscCall(VecGetOwnershipRange(y0, &start, &end));
  for (row = start; row < end; row++) {
    PetscCall(StokesGetPosition(s, row, &i, &j));
    if (row < s->nx * s->ny) {
      val = StokesExactVelocityX(j * s->hy + s->hy / 2);
    } else {
      val = 0;
    }
    PetscCall(VecSetValue(y0, row, val, INSERT_VALUES));
  }
  PetscCall(VecRestoreSubVector(s->y, s->isg[0], &y0));

VecSetValues() is called on y0 but VecAssemblyBegin/End is never called hence that vector is not assembled and will result in the error reported. 
All the vectors in this example on which VecSetValue[s] is called need to have VecAssemblyBegin/End. 

  Barry

For sequential vectors where entries set with VecSetValues() are inserted directly in the array it is possible to "get away" with not calling VecAssemblyBegin/End but
on should never do that so even in the sequential case this example is technically buggy


> On Jul 15, 2023, at 7:08 PM, Rochan Upadhyay <rochan.upadhyay at gmail.com> wrote:
> 
> Dear Petsc developers,
> Is the use of VecNest format supported for field-split type preconditioner or for use with MatNest formats ?
> A quick change to example snes/tutorials/ex70.c seems to suggest that it is not possible.
> 
> For example the changes :
> Vec subx[2], subb[2]; (declared in Stokes struct)
> In routine StokesSetupVectors(Stokes *s) :
> MatCreateVecs(s->subA[0], &s->subx[0], &s->subb[0]);
> MatCreateVecs(s->subA[3], &s->subx[1], &s->subb[1]);
> //
> IS is_rows[2];
> MatNestGetISs(s->A, is_rows, NULL);
> //
> VecCreateNest(PETSC_COMM_WORLD, 2, is_rows, s->subx, &s->x);
> VecCreateNest(PETSC_COMM_WORLD, 2, is_rows, s->subb, &s->b);
> VecAssemblyBegin(s->x);
> VecAssemblyEnd(s->x);
> VecAssemblyBegin(s->b);
> VecAssemblyEnd(s->b);
> ....
> The serial case runs fine but the parallel with np = 2 fails with the error message copied below. Could you find out what I am doing wrong ? In th error message, which vector(s) needs assembling ?
> I would normally not use VecNest but many finite element packages that allow hooking up with petsc seem to have their block vectors of Petsc VecType VecNest
> Any help/insights would be appreciated.
> Regards,
> Rochan
> 
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: Object is in wrong state
> [0]PETSC ERROR: Not for unassembled vector, did you call VecAssemblyBegin()/VecAssemblyEnd()?
> [0]PETSC ERROR: See https://petsc.org/release/faq/ for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.19.3, Jun 29, 2023
> [0]PETSC ERROR: ./ex70 on a  named localhost.localdomain by user Sat Jul 15 17:51:01 2023
> [0]PETSC ERROR: Configure options --with-clean=yes --with-debugging=no --with-mpi=1 --with-mpi-dir=/home/user/software_installs/mpich-4.0.2/ --prefix=/home/user/software_installs/petsc-3.19.3/ --PETSC_ARCH=linux-gnu-opt --PETSC_DIR=/home/user/software_downloads/petsc-3.19.3/ --with-cxx-dialect=C++11 --with-hypre=1 --download-hypre=yes --with-suitesparse=1 --download-suitesparse=yes --with-metis=1 --download-metis=yes --with-shared-libraries=1 --with-zlib=1 --download-zlib=no --with-superlu_dist=1 --download-superlu_dist=yes --with-mumps=1 --download-mumps=yes --with-parmetis=1 --download-parmetis=yes --with-hdf5=yes --download-hdf5=1 --with-chaco=1 --download-chaco=yes --with-scalapack=1 --download-scalapack=yes --with-p4est=1 --download-p4est=yes --download-fblaslapack=1 --with-strumpack=1 --download-strumpack=yes
> [0]PETSC ERROR: #1 VecCopy() at /home/user/software_downloads/petsc-3.19.3/src/vec/vec/interface/vector.c:1684
> [0]PETSC ERROR: #2 VecCopy_Nest() at /home/user/software_downloads/petsc-3.19.3/src/vec/vec/impls/nest/vecnest.c:58
> [0]PETSC ERROR: #3 VecCopy() at /home/user/software_downloads/petsc-3.19.3/src/vec/vec/interface/vector.c:1723
> [0]PETSC ERROR: #4 KSPInitialResidual() at /home/user/software_downloads/petsc-3.19.3/src/ksp/ksp/interface/itres.c:60
> [0]PETSC ERROR: #5 KSPSolve_GMRES() at /home/user/software_downloads/petsc-3.19.3/src/ksp/ksp/impls/gmres/gmres.c:226
> [0]PETSC ERROR: #6 KSPSolve_Private() at /home/user/software_downloads/petsc-3.19.3/src/ksp/ksp/interface/itfunc.c:898
> [0]PETSC ERROR: #7 KSPSolve() at /home/user/software_downloads/petsc-3.19.3/src/ksp/ksp/interface/itfunc.c:1070
> [0]PETSC ERROR: #8 main() at /home/user/Projects/Misc/Petsc/SNES/ex70.c:684
> [0]PETSC ERROR: PETSc Option Table entries:
> [0]PETSC ERROR: -ksp_monitor (source: command line)
> [0]PETSC ERROR: -nx 4 (source: command line)
> [0]PETSC ERROR: -ny 8 (source: command line)
> [0]PETSC ERROR: ----------------End of Error Message -------send entire error message to petsc-maint at mcs.anl.gov----------
> Abort(73) on node 0 (rank 0 in comm 16): application called MPI_Abort(MPI_COMM_SELF, 73) - process 0

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