step-55.h

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 *   
 *   namespace LA
 *   {
 *   #if defined(DEAL_II_WITH_PETSC) && !defined(DEAL_II_PETSC_WITH_COMPLEX) && \
 *     !(defined(DEAL_II_WITH_TRILINOS) && defined(FORCE_USE_OF_TRILINOS))
 *     using namespace dealii::LinearAlgebraPETSc;
 *   #  define USE_PETSC_LA
 *   #elif defined(DEAL_II_WITH_TRILINOS)
 *     using namespace dealii::LinearAlgebraTrilinos;
 *   #else
 *   #  error DEAL_II_WITH_PETSC or DEAL_II_WITH_TRILINOS required
 *   #endif
 *   } // namespace LA
 *   
 *   #include <deal.II/lac/vector.h>
 *   #include <deal.II/lac/full_matrix.h>
 *   #include <deal.II/lac/solver_cg.h>
 *   #include <deal.II/lac/solver_gmres.h>
 *   #include <deal.II/lac/solver_minres.h>
 *   #include <deal.II/lac/affine_constraints.h>
 *   #include <deal.II/lac/dynamic_sparsity_pattern.h>
 *   
 *   #include <deal.II/lac/petsc_sparse_matrix.h>
 *   #include <deal.II/lac/petsc_vector.h>
 *   #include <deal.II/lac/petsc_solver.h>
 *   #include <deal.II/lac/petsc_precondition.h>
 *   
 *   #include <deal.II/grid/grid_generator.h>
 *   #include <deal.II/grid/manifold_lib.h>
 *   #include <deal.II/grid/grid_tools.h>
 *   #include <deal.II/dofs/dof_handler.h>
 *   #include <deal.II/dofs/dof_renumbering.h>
 *   #include <deal.II/dofs/dof_tools.h>
 *   #include <deal.II/fe/fe_values.h>
 *   #include <deal.II/fe/fe_q.h>
 *   #include <deal.II/fe/fe_system.h>
 *   #include <deal.II/numerics/vector_tools.h>
 *   #include <deal.II/numerics/data_out.h>
 *   #include <deal.II/numerics/error_estimator.h>
 *   
 *   #include <deal.II/base/utilities.h>
 *   #include <deal.II/base/conditional_ostream.h>
 *   #include <deal.II/base/index_set.h>
 *   #include <deal.II/lac/sparsity_tools.h>
 *   #include <deal.II/distributed/tria.h>
 *   #include <deal.II/distributed/grid_refinement.h>
 *   
 *   #include <cmath>
 *   #include <fstream>
 *   #include <iostream>
 *   
 *   namespace Step55
 *   {
 *     using namespace dealii;
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Linearsolversandpreconditioners"></a> 
 * <h3>Linear solvers and preconditioners</h3>
 * 
 
 * 
 * We need a few helper classes to represent our solver strategy
 * described in the introduction.
 * 
 
 * 
 * 
 * @code
 *     namespace LinearSolvers
 *     {
 * @endcode
 * 
 * This class exposes the action of applying the inverse of a
 * giving matrix via the function
 * InverseMatrix::vmult(). Internally, the inverse is not formed
 * explicitly. Instead, a linear solver with CG is performed. This
 * class extends the InverseMatrix class in @ref step_22 "step-22" with an option
 * to specify a preconditioner, and to allow for different vector
 * types in the vmult function. We use the same mechanism as in
 * @ref step_31 "step-31" to convert a run-time exception into a failed assertion
 * should the inner solver not converge.
 * 
 * @code
 *       template <class Matrix, class Preconditioner>
 *       class InverseMatrix : public EnableObserverPointer
 *       {
 *       public:
 *         InverseMatrix(const Matrix &m, const Preconditioner &preconditioner);
 *   
 *         template <typename VectorType>
 *         void vmult(VectorType &dst, const VectorType &src) const;
 *   
 *       private:
 *         const ObserverPointer<const Matrix> matrix;
 *         const Preconditioner               &preconditioner;
 *       };
 *   
 *   
 *       template <class Matrix, class Preconditioner>
 *       InverseMatrix<Matrix, Preconditioner>::InverseMatrix(
 *         const Matrix         &m,
 *         const Preconditioner &preconditioner)
 *         : matrix(&m)
 *         , preconditioner(preconditioner)
 *       {}
 *   
 *   
 *   
 *       template <class Matrix, class Preconditioner>
 *       template <typename VectorType>
 *       void
 *       InverseMatrix<Matrix, Preconditioner>::vmult(VectorType       &dst,
 *                                                    const VectorType &src) const
 *       {
 *         SolverControl        solver_control(src.size(), 1e-8 * src.l2_norm());
 *         SolverCG<VectorType> cg(solver_control);
 *         dst = 0;
 *   
 *         try
 *           {
 *             cg.solve(*matrix, dst, src, preconditioner);
 *           }
 *         catch (std::exception &e)
 *           {
 *             Assert(false, ExcMessage(e.what()));
 *           }
 *       }
 *   
 *   
 * @endcode
 * 
 * The class A template class for a simple block diagonal preconditioner
 * for 2x2 matrices.
 * 
 * @code
 *       template <class PreconditionerA, class PreconditionerS>
 *       class BlockDiagonalPreconditioner : public EnableObserverPointer
 *       {
 *       public:
 *         BlockDiagonalPreconditioner(const PreconditionerA &preconditioner_A,
 *                                     const PreconditionerS &preconditioner_S);
 *   
 *         void vmult(LA::MPI::BlockVector       &dst,
 *                    const LA::MPI::BlockVector &src) const;
 *   
 *       private:
 *         const PreconditionerA &preconditioner_A;
 *         const PreconditionerS &preconditioner_S;
 *       };
 *   
 *       template <class PreconditionerA, class PreconditionerS>
 *       BlockDiagonalPreconditioner<PreconditionerA, PreconditionerS>::
 *         BlockDiagonalPreconditioner(const PreconditionerA &preconditioner_A,
 *                                     const PreconditionerS &preconditioner_S)
 *         : preconditioner_A(preconditioner_A)
 *         , preconditioner_S(preconditioner_S)
 *       {}
 *   
 *   
 *       template <class PreconditionerA, class PreconditionerS>
 *       void BlockDiagonalPreconditioner<PreconditionerA, PreconditionerS>::vmult(
 *         LA::MPI::BlockVector       &dst,
 *         const LA::MPI::BlockVector &src) const
 *       {
 *         preconditioner_A.vmult(dst.block(0), src.block(0));
 *         preconditioner_S.vmult(dst.block(1), src.block(1));
 *       }
 *   
 *     } // namespace LinearSolvers
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Problemsetup"></a> 
 * <h3>Problem setup</h3>
 * 
 
 * 
 * The following classes represent the right hand side and the exact
 * solution for the test problem.
 * 
 
 * 
 * 
 * @code
 *     template <int dim>
 *     class RightHandSide : public Function<dim>
 *     {
 *     public:
 *       RightHandSide()
 *         : Function<dim>(dim + 1)
 *       {}
 *   
 *       virtual void vector_value(const Point<dim> &p,
 *                                 Vector<double>   &value) const override;
 *     };
 *   
 *   
 *     template <int dim>
 *     void RightHandSide<dim>::vector_value(const Point<dim> &p,
 *                                           Vector<double>   &values) const
 *     {
 *       const double R_x = p[0];
 *       const double R_y = p[1];
 *   
 *       constexpr double pi  = numbers::PI;
 *       constexpr double pi2 = numbers::PI * numbers::PI;
 *   
 * @endcode
 * 
 * velocity
 * 
 * @code
 *       values[0] = -1.0L / 2.0L * (-2 * std::sqrt(25.0 + 4 * pi2) + 10.0) *
 *                     std::exp(R_x * (-2 * std::sqrt(25.0 + 4 * pi2) + 10.0)) -
 *                   0.4 * pi2 * std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                     std::cos(2 * R_y * pi) +
 *                   0.1 *
 *                     Utilities::fixed_power<2>(-std::sqrt(25.0 + 4 * pi2) + 5.0) *
 *                     std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                     std::cos(2 * R_y * pi);
 *       values[1] = 0.2 * pi * (-std::sqrt(25.0 + 4 * pi2) + 5.0) *
 *                     std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                     std::sin(2 * R_y * pi) -
 *                   0.05 *
 *                     Utilities::fixed_power<3>(-std::sqrt(25.0 + 4 * pi2) + 5.0) *
 *                     std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                     std::sin(2 * R_y * pi) / pi;
 *   
 * @endcode
 * 
 * pressure
 * 
 * @code
 *       values[dim] = 0;
 *     }
 *   
 *   
 *     template <int dim>
 *     class ExactSolution : public Function<dim>
 *     {
 *     public:
 *       ExactSolution()
 *         : Function<dim>(dim + 1)
 *       {}
 *   
 *       virtual void vector_value(const Point<dim> &p,
 *                                 Vector<double>   &values) const override;
 *     };
 *   
 *     template <int dim>
 *     void ExactSolution<dim>::vector_value(const Point<dim> &p,
 *                                           Vector<double>   &values) const
 *     {
 *       const double R_x = p[0];
 *       const double R_y = p[1];
 *   
 *       constexpr double pi  = numbers::PI;
 *       constexpr double pi2 = numbers::PI * numbers::PI;
 *   
 * @endcode
 * 
 * velocity
 * 
 * @code
 *       values[0] = -std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                     std::cos(2 * R_y * pi) +
 *                   1;
 *       values[1] = (1.0L / 2.0L) * (-std::sqrt(25.0 + 4 * pi2) + 5.0) *
 *                   std::exp(R_x * (-std::sqrt(25.0 + 4 * pi2) + 5.0)) *
 *                   std::sin(2 * R_y * pi) / pi;
 *   
 * @endcode
 * 
 * pressure
 * 
 * @code
 *       values[dim] =
 *         -1.0L / 2.0L * std::exp(R_x * (-2 * std::sqrt(25.0 + 4 * pi2) + 10.0)) -
 *         2.0 *
 *           (-6538034.74494422 +
 *            0.0134758939981709 * std::exp(4 * std::sqrt(25.0 + 4 * pi2))) /
 *           (-80.0 * std::exp(3 * std::sqrt(25.0 + 4 * pi2)) +
 *            16.0 * std::sqrt(25.0 + 4 * pi2) *
 *              std::exp(3 * std::sqrt(25.0 + 4 * pi2))) -
 *         1634508.68623606 * std::exp(-3.0 * std::sqrt(25.0 + 4 * pi2)) /
 *           (-10.0 + 2.0 * std::sqrt(25.0 + 4 * pi2)) +
 *         (-0.00673794699908547 * std::exp(std::sqrt(25.0 + 4 * pi2)) +
 *          3269017.37247211 * std::exp(-3 * std::sqrt(25.0 + 4 * pi2))) /
 *           (-8 * std::sqrt(25.0 + 4 * pi2) + 40.0) +
 *         0.00336897349954273 * std::exp(1.0 * std::sqrt(25.0 + 4 * pi2)) /
 *           (-10.0 + 2.0 * std::sqrt(25.0 + 4 * pi2));
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Themainprogram"></a> 
 * <h3>The main program</h3>
 *   
 
 * 
 * The main class is very similar to @ref step_40 "step-40", except that matrices and
 * vectors are now block versions, and we store a std::vector<IndexSet>
 * for owned and relevant DoFs instead of a single IndexSet. We have
 * exactly two IndexSets, one for all velocity unknowns and one for all
 * pressure unknowns.
 * 
 * @code
 *     template <int dim>
 *     class StokesProblem
 *     {
 *     public:
 *       StokesProblem(unsigned int velocity_degree);
 *   
 *       void run();
 *   
 *     private:
 *       void make_grid();
 *       void setup_system();
 *       void assemble_system();
 *       void solve();
 *       void refine_grid();
 *       void output_results(const unsigned int cycle);
 *   
 *       const unsigned int velocity_degree;
 *       const double       viscosity;
 *       MPI_Comm           mpi_communicator;
 *   
 *       const FESystem<dim>                       fe;
 *       parallel::distributed::Triangulation<dim> triangulation;
 *       DoFHandler<dim>                           dof_handler;
 *   
 *       std::vector<IndexSet> owned_partitioning;
 *       std::vector<IndexSet> relevant_partitioning;
 *   
 *       AffineConstraints<double> constraints;
 *   
 *       LA::MPI::BlockSparseMatrix system_matrix;
 *       LA::MPI::BlockSparseMatrix preconditioner_matrix;
 *       LA::MPI::BlockVector       locally_relevant_solution;
 *       LA::MPI::BlockVector       system_rhs;
 *   
 *       ConditionalOStream pcout;
 *       TimerOutput        computing_timer;
 *     };
 *   
 *   
 *   
 *     template <int dim>
 *     StokesProblem<dim>::StokesProblem(unsigned int velocity_degree)
 *       : velocity_degree(velocity_degree)
 *       , viscosity(0.1)
 *       , mpi_communicator(MPI_COMM_WORLD)
 *       , fe(FE_Q<dim>(velocity_degree) ^ dim, FE_Q<dim>(velocity_degree - 1))
 *       , triangulation(mpi_communicator,
 *                       typename Triangulation<dim>::MeshSmoothing(
 *                         Triangulation<dim>::smoothing_on_refinement |
 *                         Triangulation<dim>::smoothing_on_coarsening))
 *       , dof_handler(triangulation)
 *       , pcout(std::cout,
 *               (Utilities::MPI::this_mpi_process(mpi_communicator) == 0))
 *       , computing_timer(mpi_communicator,
 *                         pcout,
 *                         TimerOutput::never,
 *                         TimerOutput::wall_times)
 *     {}
 *   
 *   
 * @endcode
 * 
 * The Kovasznay flow is defined on the domain [-0.5, 1.5]^2, which we
 * create by passing the min and max values to GridGenerator::hyper_cube.
 * 
 * @code
 *     template <int dim>
 *     void StokesProblem<dim>::make_grid()
 *     {
 *       GridGenerator::hyper_cube(triangulation, -0.5, 1.5);
 *       triangulation.refine_global(3);
 *     }
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-SystemSetup"></a> 
 * <h3>System Setup</h3>
 *   
 
 * 
 * The construction of the block matrices and vectors is new compared to
 * @ref step_40 "step-40" and is different compared to serial codes like @ref step_22 "step-22", because
 * we need to supply the set of rows that belong to our processor.
 * 
 * @code
 *     template <int dim>
 *     void StokesProblem<dim>::setup_system()
 *     {
 *       TimerOutput::Scope t(computing_timer, "setup");
 *   
 *       dof_handler.distribute_dofs(fe);
 *   
 * @endcode
 * 
 * Put all dim velocities into block 0 and the pressure into block 1,
 * then reorder the unknowns by block. Finally count how many unknowns
 * we have per block.
 * 
 * @code
 *       std::vector<unsigned int> stokes_sub_blocks(dim + 1, 0);
 *       stokes_sub_blocks[dim] = 1;
 *       DoFRenumbering::component_wise(dof_handler, stokes_sub_blocks);
 *   
 *       const std::vector<types::global_dof_index> dofs_per_block =
 *         DoFTools::count_dofs_per_fe_block(dof_handler, stokes_sub_blocks);
 *   
 *       const unsigned int n_u = dofs_per_block[0];
 *       const unsigned int n_p = dofs_per_block[1];
 *   
 *       pcout << "   Number of degrees of freedom: " << dof_handler.n_dofs() << " ("
 *             << n_u << '+' << n_p << ')' << std::endl;
 *   
 * @endcode
 * 
 * We split up the IndexSet for locally owned and locally relevant DoFs
 * into two IndexSets based on how we want to create the block matrices
 * and vectors.
 * 
 * @code
 *       const IndexSet &locally_owned_dofs = dof_handler.locally_owned_dofs();
 *       owned_partitioning                 = {locally_owned_dofs.get_view(0, n_u),
 *                                             locally_owned_dofs.get_view(n_u, n_u + n_p)};
 *   
 *       const IndexSet locally_relevant_dofs =
 *         DoFTools::extract_locally_relevant_dofs(dof_handler);
 *       relevant_partitioning = {locally_relevant_dofs.get_view(0, n_u),
 *                                locally_relevant_dofs.get_view(n_u, n_u + n_p)};
 *   
 * @endcode
 * 
 * Setting up the constraints for boundary conditions and hanging nodes
 * is identical to @ref step_40 "step-40". Even though we don't have any hanging nodes
 * because we only perform global refinement, it is still a good idea
 * to put this function call in, in case adaptive refinement gets
 * introduced later.
 * 
 * @code
 *       {
 *         constraints.reinit(locally_owned_dofs, locally_relevant_dofs);
 *   
 *         const FEValuesExtractors::Vector velocities(0);
 *         DoFTools::make_hanging_node_constraints(dof_handler, constraints);
 *         VectorTools::interpolate_boundary_values(dof_handler,
 *                                                  0,
 *                                                  ExactSolution<dim>(),
 *                                                  constraints,
 *                                                  fe.component_mask(velocities));
 *         constraints.close();
 *       }
 *   
 * @endcode
 * 
 * Now we create the system matrix based on a BlockDynamicSparsityPattern.
 * We know that we won't have coupling between different velocity
 * components (because we use the laplace and not the deformation tensor)
 * and no coupling between pressure with its test functions, so we use
 * a Table to communicate this coupling information to
 * DoFTools::make_sparsity_pattern.
 * 
 * @code
 *       {
 *         system_matrix.clear();
 *   
 *         Table<2, DoFTools::Coupling> coupling(dim + 1, dim + 1);
 *         for (unsigned int c = 0; c < dim + 1; ++c)
 *           for (unsigned int d = 0; d < dim + 1; ++d)
 *             if (c == dim && d == dim)
 *               coupling[c][d] = DoFTools::none;
 *             else if (c == dim || d == dim || c == d)
 *               coupling[c][d] = DoFTools::always;
 *             else
 *               coupling[c][d] = DoFTools::none;
 *   
 *         BlockDynamicSparsityPattern dsp(relevant_partitioning);
 *   
 *         DoFTools::make_sparsity_pattern(
 *           dof_handler, coupling, dsp, constraints, false);
 *   
 *         SparsityTools::distribute_sparsity_pattern(
 *           dsp,
 *           dof_handler.locally_owned_dofs(),
 *           mpi_communicator,
 *           locally_relevant_dofs);
 *   
 *         system_matrix.reinit(owned_partitioning, dsp, mpi_communicator);
 *       }
 *   
 * @endcode
 * 
 * The preconditioner matrix has a different coupling (we only fill in
 * the 1,1 block with the @ref GlossMassMatrix "mass matrix"), otherwise this code is identical
 * to the construction of the system_matrix above.
 * 
 * @code
 *       {
 *         preconditioner_matrix.clear();
 *   
 *         Table<2, DoFTools::Coupling> coupling(dim + 1, dim + 1);
 *         for (unsigned int c = 0; c < dim + 1; ++c)
 *           for (unsigned int d = 0; d < dim + 1; ++d)
 *             if (c == dim && d == dim)
 *               coupling[c][d] = DoFTools::always;
 *             else
 *               coupling[c][d] = DoFTools::none;
 *   
 *         BlockDynamicSparsityPattern dsp(relevant_partitioning);
 *   
 *         DoFTools::make_sparsity_pattern(
 *           dof_handler, coupling, dsp, constraints, false);
 *         SparsityTools::distribute_sparsity_pattern(
 *           dsp,
 *           Utilities::MPI::all_gather(mpi_communicator,
 *                                      dof_handler.locally_owned_dofs()),
 *           mpi_communicator,
 *           locally_relevant_dofs);
 *         preconditioner_matrix.reinit(owned_partitioning, dsp, mpi_communicator);
 *       }
 *   
 * @endcode
 * 
 * Finally, we construct the block vectors with the right sizes. The
 * function call with two std::vector<IndexSet> will create a ghosted
 * vector.
 * 
 * @code
 *       locally_relevant_solution.reinit(owned_partitioning,
 *                                        relevant_partitioning,
 *                                        mpi_communicator);
 *       system_rhs.reinit(owned_partitioning, mpi_communicator);
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Assembly"></a> 
 * <h3>Assembly</h3>
 *   
 
 * 
 * This function assembles the system matrix, the preconditioner matrix,
 * and the right hand side. The code is pretty standard.
 * 
 * @code
 *     template <int dim>
 *     void StokesProblem<dim>::assemble_system()
 *     {
 *       TimerOutput::Scope t(computing_timer, "assembly");
 *   
 *       system_matrix         = 0;
 *       preconditioner_matrix = 0;
 *       system_rhs            = 0;
 *   
 *       const QGauss<dim> quadrature_formula(velocity_degree + 1);
 *   
 *       FEValues<dim> fe_values(fe,
 *                               quadrature_formula,
 *                               update_values | update_gradients |
 *                                 update_quadrature_points | update_JxW_values);
 *   
 *       const unsigned int dofs_per_cell = fe.n_dofs_per_cell();
 *       const unsigned int n_q_points    = quadrature_formula.size();
 *   
 *       FullMatrix<double> system_cell_matrix(dofs_per_cell, dofs_per_cell);
 *       FullMatrix<double> preconditioner_cell_matrix(dofs_per_cell, dofs_per_cell);
 *       Vector<double>     cell_rhs(dofs_per_cell);
 *   
 *       const RightHandSide<dim>    right_hand_side;
 *       std::vector<Vector<double>> rhs_values(n_q_points, Vector<double>(dim + 1));
 *   
 *       std::vector<Tensor<2, dim>> grad_phi_u(dofs_per_cell);
 *       std::vector<double>         div_phi_u(dofs_per_cell);
 *       std::vector<double>         phi_p(dofs_per_cell);
 *   
 *       std::vector<types::global_dof_index> local_dof_indices(dofs_per_cell);
 *       const FEValuesExtractors::Vector     velocities(0);
 *       const FEValuesExtractors::Scalar     pressure(dim);
 *   
 *       for (const auto &cell : dof_handler.active_cell_iterators())
 *         if (cell->is_locally_owned())
 *           {
 *             system_cell_matrix         = 0;
 *             preconditioner_cell_matrix = 0;
 *             cell_rhs                   = 0;
 *   
 *             fe_values.reinit(cell);
 *             right_hand_side.vector_value_list(fe_values.get_quadrature_points(),
 *                                               rhs_values);
 *             for (unsigned int q = 0; q < n_q_points; ++q)
 *               {
 *                 for (unsigned int k = 0; k < dofs_per_cell; ++k)
 *                   {
 *                     grad_phi_u[k] = fe_values[velocities].gradient(k, q);
 *                     div_phi_u[k]  = fe_values[velocities].divergence(k, q);
 *                     phi_p[k]      = fe_values[pressure].value(k, q);
 *                   }
 *   
 *                 for (unsigned int i = 0; i < dofs_per_cell; ++i)
 *                   {
 *                     for (unsigned int j = 0; j < dofs_per_cell; ++j)
 *                       {
 *                         system_cell_matrix(i, j) +=
 *                           (viscosity *
 *                              scalar_product(grad_phi_u[i], grad_phi_u[j]) -
 *                            div_phi_u[i] * phi_p[j] - phi_p[i] * div_phi_u[j]) *
 *                           fe_values.JxW(q);
 *   
 *                         preconditioner_cell_matrix(i, j) += 1.0 / viscosity *
 *                                                             phi_p[i] * phi_p[j] *
 *                                                             fe_values.JxW(q);
 *                       }
 *   
 *                     const unsigned int component_i =
 *                       fe.system_to_component_index(i).first;
 *                     cell_rhs(i) += fe_values.shape_value(i, q) *
 *                                    rhs_values[q](component_i) * fe_values.JxW(q);
 *                   }
 *               }
 *   
 *   
 *             cell->get_dof_indices(local_dof_indices);
 *             constraints.distribute_local_to_global(system_cell_matrix,
 *                                                    cell_rhs,
 *                                                    local_dof_indices,
 *                                                    system_matrix,
 *                                                    system_rhs);
 *   
 *             constraints.distribute_local_to_global(preconditioner_cell_matrix,
 *                                                    local_dof_indices,
 *                                                    preconditioner_matrix);
 *           }
 *   
 *       system_matrix.compress(VectorOperation::add);
 *       preconditioner_matrix.compress(VectorOperation::add);
 *       system_rhs.compress(VectorOperation::add);
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Solving"></a> 
 * <h3>Solving</h3>
 *   
 
 * 
 * This function solves the linear system with MINRES with a block diagonal
 * preconditioner and AMG for the two diagonal blocks as described in the
 * introduction. The preconditioner applies a v cycle to the 0,0 block
 * and a CG with the mass matrix for the 1,1 block (the Schur complement).
 * 
 * @code
 *     template <int dim>
 *     void StokesProblem<dim>::solve()
 *     {
 *       TimerOutput::Scope t(computing_timer, "solve");
 *   
 *       LA::MPI::PreconditionAMG prec_A;
 *       {
 *         LA::MPI::PreconditionAMG::AdditionalData data;
 *   
 *   #ifdef USE_PETSC_LA
 *         data.symmetric_operator = true;
 *   #endif
 *         prec_A.initialize(system_matrix.block(0, 0), data);
 *       }
 *   
 *       LA::MPI::PreconditionAMG prec_S;
 *       {
 *         LA::MPI::PreconditionAMG::AdditionalData data;
 *   
 *   #ifdef USE_PETSC_LA
 *         data.symmetric_operator = true;
 *   #endif
 *         prec_S.initialize(preconditioner_matrix.block(1, 1), data);
 *       }
 *   
 * @endcode
 * 
 * The InverseMatrix is used to solve for the mass matrix:
 * 
 * @code
 *       using mp_inverse_t = LinearSolvers::InverseMatrix<LA::MPI::SparseMatrix,
 *                                                         LA::MPI::PreconditionAMG>;
 *       const mp_inverse_t mp_inverse(preconditioner_matrix.block(1, 1), prec_S);
 *   
 * @endcode
 * 
 * This constructs the block preconditioner based on the preconditioners
 * for the individual blocks defined above.
 * 
 * @code
 *       const LinearSolvers::BlockDiagonalPreconditioner<LA::MPI::PreconditionAMG,
 *                                                        mp_inverse_t>
 *         preconditioner(prec_A, mp_inverse);
 *   
 * @endcode
 * 
 * With that, we can finally set up the linear solver and solve the system:
 * 
 * @code
 *       SolverControl solver_control(system_matrix.m(),
 *                                    1e-10 * system_rhs.l2_norm());
 *   
 *       SolverMinRes<LA::MPI::BlockVector> solver(solver_control);
 *   
 *       LA::MPI::BlockVector distributed_solution(owned_partitioning,
 *                                                 mpi_communicator);
 *   
 *       constraints.set_zero(distributed_solution);
 *   
 *       solver.solve(system_matrix,
 *                    distributed_solution,
 *                    system_rhs,
 *                    preconditioner);
 *   
 *       pcout << "   Solved in " << solver_control.last_step() << " iterations."
 *             << std::endl;
 *   
 *       constraints.distribute(distributed_solution);
 *   
 * @endcode
 * 
 * Like in @ref step_56 "step-56", we subtract the mean pressure to allow error
 * computations against our reference solution, which has a mean value
 * of zero.
 * 
 * @code
 *       locally_relevant_solution = distributed_solution;
 *       const double mean_pressure =
 *         VectorTools::compute_mean_value(dof_handler,
 *                                         QGauss<dim>(velocity_degree + 2),
 *                                         locally_relevant_solution,
 *                                         dim);
 *       distributed_solution.block(1).add(-mean_pressure);
 *       locally_relevant_solution.block(1) = distributed_solution.block(1);
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_55-Therest"></a> 
 * <h3>The rest</h3>
 *   
 
 * 
 * The remainder of the code that deals with mesh refinement, output, and
 * the main loop is pretty standard.
 * 
 * @code
 *     template <int dim>
 *     void StokesProblem<dim>::refine_grid()
 *     {
 *       TimerOutput::Scope t(computing_timer, "refine");
 *   
 *       triangulation.refine_global();
 *     }
 *   
 *   
 *   
 *     template <int dim>
 *     void StokesProblem<dim>::output_results(const unsigned int cycle)
 *     {
 *       TimerOutput::Scope t(computing_timer, "output");
 *   
 *       {
 *         const ComponentSelectFunction<dim> pressure_mask(dim, dim + 1);
 *         const ComponentSelectFunction<dim> velocity_mask(std::make_pair(0, dim),
 *                                                          dim + 1);
 *   
 *         Vector<double>    cellwise_errors(triangulation.n_active_cells());
 *         const QGauss<dim> quadrature(velocity_degree + 2);
 *   
 *         VectorTools::integrate_difference(dof_handler,
 *                                           locally_relevant_solution,
 *                                           ExactSolution<dim>(),
 *                                           cellwise_errors,
 *                                           quadrature,
 *                                           VectorTools::L2_norm,
 *                                           &velocity_mask);
 *   
 *         const double error_u_l2 =
 *           VectorTools::compute_global_error(triangulation,
 *                                             cellwise_errors,
 *                                             VectorTools::L2_norm);
 *   
 *         VectorTools::integrate_difference(dof_handler,
 *                                           locally_relevant_solution,
 *                                           ExactSolution<dim>(),
 *                                           cellwise_errors,
 *                                           quadrature,
 *                                           VectorTools::L2_norm,
 *                                           &pressure_mask);
 *   
 *         const double error_p_l2 =
 *           VectorTools::compute_global_error(triangulation,
 *                                             cellwise_errors,
 *                                             VectorTools::L2_norm);
 *   
 *         pcout << "error: u_0: " << error_u_l2 << " p_0: " << error_p_l2
 *               << std::endl;
 *       }
 *   
 *   
 *       std::vector<std::string> solution_names(dim, "velocity");
 *       solution_names.emplace_back("pressure");
 *       std::vector<DataComponentInterpretation::DataComponentInterpretation>
 *         data_component_interpretation(
 *           dim, DataComponentInterpretation::component_is_part_of_vector);
 *       data_component_interpretation.push_back(
 *         DataComponentInterpretation::component_is_scalar);
 *   
 *       DataOut<dim> data_out;
 *       data_out.attach_dof_handler(dof_handler);
 *       data_out.add_data_vector(locally_relevant_solution,
 *                                solution_names,
 *                                DataOut<dim>::type_dof_data,
 *                                data_component_interpretation);
 *   
 *       LA::MPI::BlockVector interpolated;
 *       interpolated.reinit(owned_partitioning, MPI_COMM_WORLD);
 *       VectorTools::interpolate(dof_handler, ExactSolution<dim>(), interpolated);
 *   
 *       LA::MPI::BlockVector interpolated_relevant(owned_partitioning,
 *                                                  relevant_partitioning,
 *                                                  MPI_COMM_WORLD);
 *       interpolated_relevant = interpolated;
 *       {
 *         std::vector<std::string> solution_names(dim, "ref_u");
 *         solution_names.emplace_back("ref_p");
 *         data_out.add_data_vector(interpolated_relevant,
 *                                  solution_names,
 *                                  DataOut<dim>::type_dof_data,
 *                                  data_component_interpretation);
 *       }
 *   
 *   
 *       Vector<float> subdomain(triangulation.n_active_cells());
 *       for (unsigned int i = 0; i < subdomain.size(); ++i)
 *         subdomain(i) = triangulation.locally_owned_subdomain();
 *       data_out.add_data_vector(subdomain, "subdomain");
 *   
 *       data_out.build_patches();
 *   
 *       data_out.write_vtu_with_pvtu_record(
 *         "./", "solution", cycle, mpi_communicator, 2);
 *     }
 *   
 *   
 *   
 *     template <int dim>
 *     void StokesProblem<dim>::run()
 *     {
 *   #ifdef USE_PETSC_LA
 *       pcout << "Running using PETSc." << std::endl;
 *   #else
 *       pcout << "Running using Trilinos." << std::endl;
 *   #endif
 *       const unsigned int n_cycles = 5;
 *       for (unsigned int cycle = 0; cycle < n_cycles; ++cycle)
 *         {
 *           pcout << "Cycle " << cycle << ':' << std::endl;
 *   
 *           if (cycle == 0)
 *             make_grid();
 *           else
 *             refine_grid();
 *   
 *           setup_system();
 *   
 *           assemble_system();
 *           solve();
 *   
 *           if (Utilities::MPI::n_mpi_processes(mpi_communicator) <= 32)
 *             output_results(cycle);
 *   
 *           computing_timer.print_summary();
 *           computing_timer.reset();
 *   
 *           pcout << std::endl;
 *         }
 *     }
 *   } // namespace Step55
 *   
 *   
 *   
 *   int main(int argc, char *argv[])
 *   {
 *     try
 *       {
 *         using namespace dealii;
 *         using namespace Step55;
 *   
 *         Utilities::MPI::MPI_InitFinalize mpi_initialization(argc, argv, 1);
 *   
 *         StokesProblem<2> problem(2);
 *         problem.run();
 *       }
 *     catch (std::exception &exc)
 *       {
 *         std::cerr << std::endl
 *                   << std::endl
 *                   << "----------------------------------------------------"
 *                   << std::endl;
 *         std::cerr << "Exception on processing: " << std::endl
 *                   << exc.what() << std::endl
 *                   << "Aborting!" << std::endl
 *                   << "----------------------------------------------------"
 *                   << std::endl;
 *   
 *         return 1;
 *       }
 *     catch (...)
 *       {
 *         std::cerr << std::endl
 *                   << std::endl
 *                   << "----------------------------------------------------"
 *                   << std::endl;
 *         std::cerr << "Unknown exception!" << std::endl
 *                   << "Aborting!" << std::endl
 *                   << "----------------------------------------------------"
 *                   << std::endl;
 *         return 1;
 *       }
 *   
 *     return 0;
 *   }
 * @endcode
<a name="step_55-Results"></a><h1>Results</h1>
 
 
As expected from the discussion above, the number of iterations is independent
of the number of processors and only very slightly dependent on @f$h@f$:
 
<table>
<tr>
  <th colspan="2">PETSc</th>
  <th colspan="8">number of processors</th>
</tr>
<tr>
  <th>cycle</th>
  <th>dofs</th>
  <th>1</th>
  <th>2</th>
  <th>4</th>
  <th>8</th>
  <th>16</th>
  <th>32</th>
  <th>64</th>
  <th>128</th>
</tr>
<tr>
  <td>0</td>
  <td>659</td>
  <td>49</td>
  <td>49</td>
  <td>49</td>
  <td>51</td>
  <td>51</td>
  <td>51</td>
  <td>49</td>
  <td>49</td>
</tr>
<tr>
  <td>1</td>
  <td>2467</td>
  <td>52</td>
  <td>52</td>
  <td>52</td>
  <td>52</td>
  <td>52</td>
  <td>54</td>
  <td>54</td>
  <td>53</td>
</tr>
<tr>
  <td>2</td>
  <td>9539</td>
  <td>56</td>
  <td>56</td>
  <td>56</td>
  <td>54</td>
  <td>56</td>
  <td>56</td>
  <td>54</td>
  <td>56</td>
</tr>
<tr>
  <td>3</td>
  <td>37507</td>
  <td>57</td>
  <td>57</td>
  <td>57</td>
  <td>57</td>
  <td>57</td>
  <td>56</td>
  <td>57</td>
  <td>56</td>
</tr>
<tr>
  <td>4</td>
  <td>148739</td>
  <td>58</td>
  <td>59</td>
  <td>57</td>
  <td>59</td>
  <td>57</td>
  <td>57</td>
  <td>57</td>
  <td>57</td>
</tr>
<tr>
  <td>5</td>
  <td>592387</td>
  <td>60</td>
  <td>60</td>
  <td>59</td>
  <td>59</td>
  <td>59</td>
  <td>59</td>
  <td>59</td>
  <td>59</td>
</tr>
<tr>
  <td>6</td>
  <td>2364419</td>
  <td>62</td>
  <td>62</td>
  <td>61</td>
  <td>61</td>
  <td>61</td>
  <td>61</td>
  <td>61</td>
  <td>61</td>
</tr>
</table>
 
<table>
<tr>
  <th colspan="2">Trilinos</th>
  <th colspan="8">number of processors</th>
</tr>
<tr>
  <th>cycle</th>
  <th>dofs</th>
  <th>1</th>
  <th>2</th>
  <th>4</th>
  <th>8</th>
  <th>16</th>
  <th>32</th>
  <th>64</th>
  <th>128</th>
</tr>
<tr>
  <td>0</td>
  <td>659</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
  <td>37</td>
</tr>
<tr>
  <td>1</td>
  <td>2467</td>
  <td>92</td>
  <td>89</td>
  <td>89</td>
  <td>82</td>
  <td>86</td>
  <td>81</td>
  <td>78</td>
  <td>78</td>
</tr>
<tr>
  <td>2</td>
  <td>9539</td>
  <td>102</td>
  <td>99</td>
  <td>96</td>
  <td>95</td>
  <td>95</td>
  <td>88</td>
  <td>83</td>
  <td>95</td>
</tr>
<tr>
  <td>3</td>
  <td>37507</td>
  <td>107</td>
  <td>105</td>
  <td>104</td>
  <td>99</td>
  <td>100</td>
  <td>96</td>
  <td>96</td>
  <td>90</td>
</tr>
<tr>
  <td>4</td>
  <td>148739</td>
  <td>112</td>
  <td>112</td>
  <td>111</td>
  <td>111</td>
  <td>127</td>
  <td>126</td>
  <td>115</td>
  <td>117</td>
</tr>
<tr>
  <td>5</td>
  <td>592387</td>
  <td>116</td>
  <td>115</td>
  <td>114</td>
  <td>112</td>
  <td>118</td>
  <td>120</td>
  <td>131</td>
  <td>130</td>
</tr>
<tr>
  <td>6</td>
  <td>2364419</td>
  <td>130</td>
  <td>126</td>
  <td>120</td>
  <td>120</td>
  <td>121</td>
  <td>122</td>
  <td>121</td>
  <td>123</td>
</tr>
</table>
 
While the PETSc results show a constant number of iterations, the iterations
increase when using Trilinos. This is likely because of the different settings
used for the AMG preconditioner. For performance reasons we do not allow
coarsening below a couple thousand unknowns. As the coarse solver is an exact
solve (we are using LU by default), a change in number of levels will
influence the quality of a V-cycle. Therefore, a V-cycle is closer to an exact
solver for smaller problem sizes.
 
<a name="step-55-extensions"></a>
<a name="step_55-Possibilitiesforextensions"></a><h3>Possibilities for extensions</h3>
 
 
<a name="step_55-InvestigateTrilinositerations"></a><h4>Investigate Trilinos iterations</h4>
 
 
Play with the smoothers, smoothing steps, and other properties for the
Trilinos AMG to achieve an optimal preconditioner.
 
<a name="step_55-SolvetheOseenprobleminsteadoftheStokessystem"></a><h4>Solve the Oseen problem instead of the Stokes system</h4>
 
 
This change requires changing the outer solver to GMRES or BiCGStab, because
the system is no longer symmetric.
 
You can prescribe the exact flow solution as @f$b@f$ in the convective term @f$b
\cdot \nabla u@f$. This should give the same solution as the original problem,
if you set the right hand side to zero.
 
<a name="step_55-Adaptiverefinement"></a><h4>Adaptive refinement</h4>
 
 
So far, this tutorial program refines the mesh globally in each step.
Replacing the code in StokesProblem::refine_grid() by something like
@code
Vector<float> estimated_error_per_cell(triangulation.n_active_cells());
 
FEValuesExtractors::Vector velocities(0);
KellyErrorEstimator<dim>::estimate(
  dof_handler,
  QGauss<dim - 1>(fe.degree + 1),
  std::map<types::boundary_id, const Function<dim> *>(),
  locally_relevant_solution,
  estimated_error_per_cell,
  fe.component_mask(velocities));
parallel::distributed::GridRefinement::refine_and_coarsen_fixed_number(
  triangulation, estimated_error_per_cell, 0.3, 0.0);
triangulation.execute_coarsening_and_refinement();
@endcode
makes it simple to explore adaptive mesh refinement.
 *
 *
<a name="step_55-PlainProg"></a>
<h1> The plain program</h1>
@include "step-55.cc"
*/