DofMap::set and DofMap::set_x fails on non-ghosted vector

[Jan Blechta]

Following code

from dolfin import *

mesh = UnitSquareMesh(30, 30)
V = FunctionSpace(mesh, 'CG', 1)

# Make vector with and without ghosts
xg = Function(V).vector()
x = Vector()
x.init(mpi_comm_world(), V.dofmap().ownership_range())

# Check that range is the same
assert xg.local_range() == x.local_range()

# Test DofMap::set,set_x
V.dofmap().set(xg, 1.0)
V.dofmap().set_x(xg, 1.0, 0, mesh)

V.dofmap().set(x, 1.0) # fail
V.dofmap().set_x(x, 1.0, 0, mesh) # fail

fails and investigation shows that DofMap::set and Dof::set_x try to set also non-owned DOFs which is refused by PETSc for vector without ghost.

Possible resolutions:

1. WONTFIX because non-ghosted vector has no connection to DofMap so that nobody can guarantee that DofMap functions will work on it.

2. Little bit disturbing is that there is a lot of setting of non-owned DOFs. Not sure what happens behind the scene in PETSc but at least DOLFIN is trying to do a lot of non-local assignments where not necessary. Isn't it also the case in more critical DOLFIN routines?

[Jan Blechta]

When thinking about it, these assignments to non-owned indices are probably just local assignments to ghost without MPI communication. So no danger at all.

@garth-wells Can you confirm it so that it can be closed as INVALID?

[Jan Blechta]

1. It is not logical to use DofMap functions to vector which has no connection to DofMap.

2. Setting ghost values is really without communication. DofMap::set communicates just with final apply("insert"). DofMap::set_x does not communicate as it does not call apply("insert").