Poisson parallel matrix assembly

Question

I would like to extract the stiffness matrix from the Poisson demo in parallel.

I tried to assemble the a(V,V) form into a PETScMatrix in parallel and non-parallel, but unfortunately didn't get the same J matrix.

  PETScMatrix JJ;
  assemble(JJ,a);
  Mat J = JJ.mat();

  PetscViewer viewer;
  MatView(J,viewer);

For example on a 10x10 mesh:
non-parallel:

row 120: (118, -0.5) (119, -0.5) (120, 1)

parallel:

row 120: (0, -0.5) (115, -1) (116, -0.5) (119, 0) (120, 2)

What's the reason for that, and how could I extract the same matrix in parallel?

Answer 1

I would guess that this is most likely due to DOF reordering. i.e row 120 is now some other row (columns are swapped in the same manner).

You can use the function space dofmap to reorder the dofs back to their original position (NOTE: this may not be the same as the serial, it can have been reordered too!)