dof_handler.n_locally_owned_dofs());
system_rhs = 0;
- // xxx
+ // The next step is to compute hanging node
+ // and boundary value constraints, which we
+ // combine into a single object storing all
+ // constraints.
+ //
+ // As with all other things in %parallel,
+ // the mantra must be that no processor can
+ // store all information about the entire
+ // universe. As a consequence, we need to
+ // tell the constraints object for which
+ // degrees of freedom it can store
+ // constraints and for which it may not
+ // expect any information to store. In our
+ // case, as explained in the @ref
+ // distributed module, the degrees of
+ // freedom we need to care about on each
+ // processor are the locally relevant ones,
+ // so we pass this to the
+ // ConstraintMatrix::reinit function. As a
+ // side note, if you forget to pass this
+ // argument, the ConstraintMatrix class
+ // will allocate an array with length equal
+ // to the largest DoF index it has seen so
+ // far. For processors with high MPI
+ // process number, this may be very large
+ // -- maybe on the order of billions. The
+ // program would then allocate more memory
+ // than for likely all other operations
+ // combined for this single array.
constraints.clear ();
constraints.reinit (locally_relevant_dofs);
DoFTools::make_hanging_node_constraints (dof_handler, constraints);
constraints);
constraints.close ();
-
+ // xxx
CompressedSimpleSparsityPattern csp (dof_handler.n_dofs(),
dof_handler.n_dofs(),
locally_relevant_dofs);
// PETSc functionality. It is
// relatively well known that the
// primary bottleneck of massively
- // parallel linear solvers is not
+ // %parallel linear solvers is not
// actually the communication
// between processors, but the fact
// that it is difficult to produce
// finer mesh (5 global refinement
// cycles) -- there just isn't much
// of a point showing a massively
- // parallel program starting on 4
+ // %parallel program starting on 4
// cells (although admittedly the
// point is only slightly stronger
// starting on 1024).