gistfile1.diff

diff --git a/firedrake/dmplex.pyx b/firedrake/dmplex.pyx
index 576405f..42760d2 100644
--- a/firedrake/dmplex.pyx
+++ b/firedrake/dmplex.pyx
@@ -104,7 +104,7 @@ def facet_numbering(PETSc.DM plex, kind,
         # Second cell
         if facet_cells.shape[1] > 1:
             cell = facet_cells[f,1]
-            if cell > 0:
+            if cell >= 0:
                 fi = 0
                 for c in range(nclosure):
                     if cell_closures[cell, c] == facets[f]:
@@ -458,7 +458,7 @@ def get_facet_nodes(np.ndarray[np.int32_t, ndim=2] facet_cells,
         # Second parent cell for internal facets
         if ncells > 1:
             cell = facet_cells[f, 1]
-            if cell > 0:
+            if cell >= 0:
                 for i in range(ndofs):
                     facet_nodes[f, ndofs+i] = cell_nodes[cell, i]
             else:
diff --git a/firedrake/mesh.py b/firedrake/mesh.py
index 9ff547c..f8ebcb2 100644
--- a/firedrake/mesh.py
+++ b/firedrake/mesh.py
@@ -258,22 +258,29 @@ class Mesh(object):
     def _from_dmplex(self, plex, geometric_dim=0, periodic_coords=None):
         """ Create mesh from DMPlex object """
 
-        self._plex = plex
         self.uid = utils._new_uid()
 
         if geometric_dim == 0:
-            geometric_dim = self._plex.getDimension()
-
-        # Mark exterior and interior facets
-        dmplex.label_facets(self._plex)
+            geometric_dim = plex.getDimension()
 
         # Distribute the dm to all ranks
         if op2.MPI.comm.size > 1:
-            self.parallel_sf = self._plex.distribute(overlap=1)
+            self.parallel_sf = plex.distribute(overlap=1)
+        else:
+            # For non-1D meshes in serial, reorder using RCM
+            if plex.getDimension() > 1:
+                perm = plex.getOrdering(PETSc.Mat.OrderingType.RCM)
+                plex = plex.permute(perm)
+
+        self._plex = plex
+
+        # Mark exterior and interior facets
+        dmplex.label_facets(self._plex)
 
         # Mark OP2 entities and derive the resulting Plex renumbering
         dmplex.mark_entity_classes(self._plex)
-        self._plex_renumbering = dmplex.plex_renumbering(plex)
+        self._plex_renumbering = dmplex.plex_renumbering(self._plex)
 
         cStart, cEnd = self._plex.getHeightStratum(0)  # cells
         cell_vertices = self._plex.getConeSize(cStart)