Couple more lines.

git-svn-id: https://svn.dealii.org/trunk@22625 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-40/step-40.cc b/deal.II/examples/step-40/step-40.cc
index 6b563e88ceeb5efea87eb89f655725e5876bed24..f0283eb77631162879c33ea445989d1f45c38a60 100644 (file)
--- a/deal.II/examples/step-40/step-40.cc
+++ b/deal.II/examples/step-40/step-40.cc
@@ -239,12 +239,30 @@ LaplaceProblem<dim>::~LaplaceProblem ()
                                 // since it goes to the heart of what
                                 // distinguishes %parallel step-40 from
                                 // sequential step-6.
+                                //
+                                // At the top we do what we always do: tell
+                                // the DoFHandler object to distribute
+                                // degrees of freedom. Since the
+                                // triangulation we use here is distributed,
+                                // the DoFHandler object is smart enough to
+                                // recognize that on each processor it can
+                                // only distribute degrees of freedom on
+                                // cells it owns; this is followed by an
+                                // exchange step in which processors tell
+                                // each other about degrees of freedom on
+                                // ghost cell. The result is a DoFHandler
+                                // that knows about the degrees of freedom on
+                                // locally owned cells and ghost cells
+                                // (i.e. cells adjacent to locally owned
+                                // cells) but nothing about cells that are
+                                // further away, consistent with the basic
+                                // philosophy of distributed computing that
+                                // no processor can know everything.
 template <int dim>
 void LaplaceProblem<dim>::setup_system ()
 {
   dof_handler.distribute_dofs (fe);
 
-                                  // eliminate
   locally_owned_dofs = dof_handler.locally_owned_dofs ();
   DoFTools::extract_locally_relevant_dofs (dof_handler,
                                           locally_relevant_dofs);