//! Help PETSc identify the coarser DM dmc given the fine DM dmf
PetscErrorCode libmesh_petsc_DMCoarsen(DM dmf, MPI_Comm /*comm*/, DM * dmc)
{
libmesh_assert(dmc);
libmesh_assert(dmf);
PetscErrorCode ierr;
// Extract our context from the incoming dmf
void * ctx_f = nullptr;
ierr = DMShellGetContext(dmf, &ctx_f);LIBMESH_CHKERR(ierr);
libmesh_assert(ctx_f);
PetscDMContext * p_ctx = static_cast<PetscDMContext*>(ctx_f);
// First, ensure that there exists a coarse DM that we want to
// set. There ought to be as we created it while walking the
// hierarchy.
libmesh_assert(p_ctx->coarser_dm);
libmesh_assert(*(p_ctx->coarser_dm));
// In situations using fieldsplit we need to (potentially)
// provide a coarser DM which only has the relevant subfields in
// it. Since we create global DMs for each mesh level, we need
// to extract the section from the DM, and check the number of
// fields. When less than all the fields are used, we need to
// create the proper subsections.
// Get the number of fields and their names from the incomming
// fine DM and the global reference DM
PetscInt nfieldsf, nfieldsg;
char ** fieldnamesf;
char ** fieldnamesg;
libmesh_assert(p_ctx->global_dm);
DM * globaldm = p_ctx->global_dm;
ierr = DMCreateFieldIS(dmf, &nfieldsf, &fieldnamesf, nullptr);
LIBMESH_CHKERR(ierr);
ierr = DMCreateFieldIS(*globaldm, &nfieldsg, &fieldnamesg, nullptr);
LIBMESH_CHKERR(ierr);
// If the probed number of fields is less than the number of
// global fields, this amounts to PETSc 'indicating' to us we
// are doing FS. So, we must create subsections for the coarser
// DMs.
if ( nfieldsf < nfieldsg )
{
PetscSection section;
PetscSection subsection;
std::vector<PetscInt> subfields(nfieldsf); // extracted fields
// First, get the section from the coarse DM
#if PETSC_VERSION_LESS_THAN(3,10,0)
ierr = DMGetDefaultSection(*(p_ctx->coarser_dm), §ion);
#else
ierr = DMGetSection(*(p_ctx->coarser_dm), §ion);
#endif
LIBMESH_CHKERR(ierr);
// Now, match fine grid DM field names to their global DM
// counterparts. Since PETSc can internally reassign field
// numbering under a fieldsplit, we must extract
// subsections via the field names. This is admittedly
// gross, but c'est la vie.
for (int i = 0; i < nfieldsf ; i++)
{
for (int j = 0; j < nfieldsg ;j++)
if ( strcmp( fieldnamesg[j], fieldnamesf[i] ) == 0 )
subfields[i] = j;
}
// Next, for the found fields we now make a subsection and set it for the coarser DM
ierr = PetscSectionCreateSubsection(section, nfieldsf, subfields.data(), &subsection);
LIBMESH_CHKERR(ierr);
#if PETSC_VERSION_LESS_THAN(3,10,0)
ierr = DMSetDefaultSection(*(p_ctx->coarser_dm) , subsection);
#else
ierr = DMSetSection(*(p_ctx->coarser_dm) , subsection);
#endif
LIBMESH_CHKERR(ierr);
ierr = PetscSectionDestroy(&subsection);
LIBMESH_CHKERR(ierr);
}
// Finally, set the coarser DM
*(dmc) = *(p_ctx->coarser_dm);
return 0;
}
GEN
subfields0(GEN nf,GEN d)
{
return d? subfields(nf,d): subfieldsall(nf);
}
