step-65.h

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 = 0) const override
 *       {
 *         if (p.norm_square() < 0.25)
 *           return p.norm_square();
 *         else
 *           return 0.1 * p.norm_square() + (0.25 - 0.025);
 *       }
 *   
 *       virtual Tensor<1, dim>
 *       gradient(const Point<dim> &p,
 *                const unsigned int /*component*/ = 0) const override
 *       {
 *         if (p.norm_square() < 0.25)
 *           return 2. * p;
 *         else
 *           return 0.2 * p;
 *       }
 *     };
 *   
 *   
 *     template <int dim>
 *     double coefficient(const Point<dim> &p)
 *     {
 *       if (p.norm_square() < 0.25)
 *         return 0.5;
 *       else
 *         return 5.0;
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-ThePoissonProblemclass"></a> 
 * <h3>The PoissonProblem class</h3>
 *   
 
 * 
 * The implementation of the Poisson problem is very similar to what
 * we used in the @ref step_5 "step-5" tutorial program. The two main differences
 * are that we pass a mapping object to the various steps in the
 * program in order to switch between two mapping representations as
 * explained in the introduction, and the `timer` object (of type
 * TimerOutput) that will be used for measuring the run times in the
 * various cases. (The concept of mapping objects was first
 * introduced in @ref step_10 "step-10" and @ref step_11 "step-11", in case you want to look up
 * the use of these classes.)
 * 
 * @code
 *     template <int dim>
 *     class PoissonProblem
 *     {
 *     public:
 *       PoissonProblem();
 *       void run();
 *   
 *     private:
 *       void create_grid();
 *       void setup_system(const Mapping<dim> &mapping);
 *       void assemble_system(const Mapping<dim> &mapping);
 *       void solve();
 *       void postprocess(const Mapping<dim> &mapping);
 *   
 *       Triangulation<dim> triangulation;
 *       const FE_Q<dim>    fe;
 *       DoFHandler<dim>    dof_handler;
 *   
 *       AffineConstraints<double> constraints;
 *       SparsityPattern           sparsity_pattern;
 *       SparseMatrix<double>      system_matrix;
 *       Vector<double>            solution;
 *       Vector<double>            system_rhs;
 *   
 *       TimerOutput timer;
 *     };
 *   
 *   
 *   
 * @endcode
 * 
 * In the constructor, we set up the timer object to record wall times but
 * be quiet during the normal execution. We will query it for timing details
 * in the `PoissonProblem::run()` function. Furthermore, we select a
 * relatively high polynomial degree of three for the finite element in use.
 * 
 * @code
 *     template <int dim>
 *     PoissonProblem<dim>::PoissonProblem()
 *       : fe(3)
 *       , dof_handler(triangulation)
 *       , timer(std::cout, TimerOutput::never, TimerOutput::wall_times)
 *     {}
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-Gridcreationandinitializationofthemanifolds"></a> 
 * <h3>Grid creation and initialization of the manifolds</h3>
 *   
 
 * 
 * The next function presents the typical usage of
 * TransfiniteInterpolationManifold. The first step is to create the desired
 * grid, which can be done by composition of two grids from
 * GridGenerator. The inner ball mesh is simple enough: We run
 * GridGenerator::hyper_cube() centered at the origin with radius 0.5 (third
 * function argument). The second mesh is more interesting and constructed
 * as follows: We want to have a mesh that is spherical in the interior but
 * flat on the outer surface. Furthermore, the mesh topology of the inner
 * ball should be compatible with the outer grid in the sense that their
 * vertices coincide so as to allow the two grid to be merged. The grid coming
 * out of GridGenerator::hyper_shell fulfills the requirements on the inner
 * side in case it is created with @f$2d@f$ coarse cells (6 coarse cells in 3d
 * which we are going to use) &ndash; this is the same number of cells as
 * there are boundary faces for the ball. For the outer surface, we use the
 * fact that the 6 faces on the surface of the shell without a manifold
 * attached would degenerate to the surface of a cube. What we are still
 * missing is the radius of the outer shell boundary. Since we desire a cube
 * of extent
 * @f$[-1, 1]@f$ and the 6-cell shell puts its 8 outer vertices at the 8
 * opposing diagonals, we must translate the points @f$(\pm 1, \pm 1, \pm 1)@f$
 * into a radius: Clearly, the radius must be @f$\sqrt{d}@f$ in @f$d@f$ dimensions,
 * i.e., @f$\sqrt{3}@f$ for the three-dimensional case we want to consider.
 *   
 
 * 
 * Thus, we have a plan: After creating the inner triangulation for
 * the ball and the one for the outer shell, we merge those two
 * grids but remove all manifolds that the functions in
 * GridGenerator may have set from the resulting triangulation, to
 * ensure that we have full control over manifolds. In particular,
 * we want additional points added on the boundary during refinement
 * to follow a flat manifold description. To start the process of
 * adding more appropriate manifold ids, we assign the manifold id 0
 * to all mesh entities (cells, faces, lines), which will later be
 * associated with the TransfiniteInterpolationManifold. Then, we
 * must identify the faces and lines that are along the sphere of
 * radius 0.5 and mark them with a different manifold id, so as to then
 * assign a SphericalManifold to those. We will choose the manifold
 * id of 1. Since we have thrown away all manifolds that pre-existed
 * after calling GridGenerator::hyper_ball(), we manually go through
 * the cells of the mesh and all their faces. We have found a face
 * on the sphere if all four vertices have a radius of 0.5, or, as
 * we write in the program, have @f$r^2-0.25 \approx 0@f$. Note that we call
 * `cell->face(f)->set_all_manifold_ids(1)` to set the manifold id
 * both on the faces and the surrounding lines. Furthermore, we want
 * to distinguish the cells inside the ball and outside the ball by
 * a material id for visualization, corresponding to the picture in the
 * introduction.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::create_grid()
 *     {
 *       Triangulation<dim> tria_inner;
 *       GridGenerator::hyper_ball(tria_inner, Point<dim>(), 0.5);
 *   
 *       Triangulation<dim> tria_outer;
 *       GridGenerator::hyper_shell(
 *         tria_outer, Point<dim>(), 0.5, std::sqrt(dim), 2 * dim);
 *   
 *       GridGenerator::merge_triangulations(tria_inner, tria_outer, triangulation);
 *   
 *       triangulation.reset_all_manifolds();
 *       triangulation.set_all_manifold_ids(0);
 *   
 *       for (const auto &cell : triangulation.cell_iterators())
 *         {
 *           for (const auto &face : cell->face_iterators())
 *             {
 *               bool face_at_sphere_boundary = true;
 *               for (const auto v : face->vertex_indices())
 *                 {
 *                   if (std::abs(face->vertex(v).norm_square() - 0.25) > 1e-12)
 *                     face_at_sphere_boundary = false;
 *                 }
 *               if (face_at_sphere_boundary)
 *                 face->set_all_manifold_ids(1);
 *             }
 *           if (cell->center().norm_square() < 0.25)
 *             cell->set_material_id(1);
 *           else
 *             cell->set_material_id(0);
 *         }
 *   
 * @endcode
 * 
 * With all cells, faces and lines marked appropriately, we can
 * attach the Manifold objects to those numbers. The entities with
 * manifold id 1 will get a spherical manifold, whereas the other
 * entities, which have the manifold id 0, will be assigned the
 * TransfiniteInterpolationManifold. As mentioned in the
 * introduction, we must explicitly initialize the manifold with
 * the current mesh using a call to
 * TransfiniteInterpolationManifold::initialize() in order to pick
 * up the coarse mesh cells and the manifolds attached to the
 * boundaries of those cells. We also note that the manifold
 * objects we create locally in this function are allowed to go
 * out of scope (as they do at the end of the function scope),
 * because the Triangulation object internally copies them.
 *     
 
 * 
 * With all manifolds attached, we will finally go about and refine the
 * mesh a few times to create a sufficiently large test case.
 * 
 * @code
 *       triangulation.set_manifold(1, SphericalManifold<dim>());
 *   
 *       TransfiniteInterpolationManifold<dim> transfinite_manifold;
 *       transfinite_manifold.initialize(triangulation);
 *       triangulation.set_manifold(0, transfinite_manifold);
 *   
 *       triangulation.refine_global(9 - 2 * dim);
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-Setupofdatastructures"></a> 
 * <h3>Setup of data structures</h3>
 *   
 
 * 
 * The following function is well-known from other tutorials in that
 * it enumerates the degrees of freedom, creates a constraint object
 * and sets up a sparse matrix for the linear system. The only thing
 * worth mentioning is the fact that the function receives a
 * reference to a mapping object that we then pass to the
 * VectorTools::interpolate_boundary_values() function to ensure
 * that our boundary values are evaluated on the high-order mesh
 * used for assembly. In the present example, it does not really
 * matter because the outer surfaces are flat, but for curved outer
 * cells this leads to more accurate approximation of the boundary
 * values.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::setup_system(const Mapping<dim> &mapping)
 *     {
 *       dof_handler.distribute_dofs(fe);
 *       std::cout << "   Number of active cells:       "
 *                 << triangulation.n_global_active_cells() << std::endl;
 *       std::cout << "   Number of degrees of freedom: " << dof_handler.n_dofs()
 *                 << std::endl;
 *   
 *       {
 *         TimerOutput::Scope scope(timer, "Compute constraints");
 *   
 *         constraints.clear();
 *   
 *         DoFTools::make_hanging_node_constraints(dof_handler, constraints);
 *         VectorTools::interpolate_boundary_values(
 *           mapping, dof_handler, 0, ExactSolution<dim>(), constraints);
 *   
 *         constraints.close();
 *       }
 *   
 *       DynamicSparsityPattern dsp(dof_handler.n_dofs());
 *       DoFTools::make_sparsity_pattern(dof_handler, dsp, constraints, false);
 *   
 *       sparsity_pattern.copy_from(dsp);
 *       system_matrix.reinit(sparsity_pattern);
 *   
 *       solution.reinit(dof_handler.n_dofs());
 *       system_rhs.reinit(dof_handler.n_dofs());
 *     }
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-Assemblyofthesystemmatrixandrighthandside"></a> 
 * <h3>Assembly of the system matrix and right hand side</h3>
 *   
 
 * 
 * The function that assembles the linear system is also well known
 * from the previous tutorial programs. One thing to note is that we
 * set the number of quadrature points to the polynomial degree plus
 * two, not the degree plus one as in most other tutorials. This is
 * because we expect some extra accuracy as the mapping also
 * involves a degree one more than the polynomials for the solution.
 *   
 
 * 
 * The only somewhat unusual code in the assembly is the way we compute the
 * cell matrix. Rather than using three nested loop over the quadrature
 * point index, the row, and the column of the matrix, we first collect the
 * derivatives of the shape function, multiplied by the square root of the
 * product of the coefficient and the integration factor `JxW` in a separate
 * matrix `partial_matrix`. To compute the cell matrix, we then execute
 * `cell_matrix = partial_matrix * transpose(partial_matrix)` in the line
 * `partial_matrix.mTmult(cell_matrix, partial_matrix);`. To understand why
 * this works, we realize that the matrix-matrix multiplication performs a
 * summation over the columns of `partial_matrix`. If we denote the
 * coefficient by @f$a(\mathbf{x}_q)@f$, the entries in the temporary matrix are
 * @f$\sqrt{\text{det}(J) w_q a(x)} \frac{\partial \varphi_i(\boldsymbol
 * \xi_q)}{\partial x_k}@f$. If we take the product of the <i>i</i>th row with
 * the <i>j</i>th column of that matrix, we compute a nested sum involving
 * @f$\sum_q \sum_{k=1}^d \sqrt{\text{det}(J) w_q a(x)} \frac{\partial
 * \varphi_i(\boldsymbol \xi_q)}{\partial x_k} \sqrt{\text{det}(J) w_q a(x)}
 * \frac{\partial \varphi_j(\boldsymbol \xi_q)}{\partial x_k} = \sum_q
 * \sum_{k=1}^d\text{det}(J) w_q a(x)\frac{\partial \varphi_i(\boldsymbol
 * \xi_q)}{\partial x_k} \frac{\partial \varphi_j(\boldsymbol
 * \xi_q)}{\partial x_k}@f$, which is exactly the terms needed for the
 * bilinear form of the Laplace equation.
 *   
 
 * 
 * The reason for choosing this somewhat unusual scheme is due to the heavy
 * work involved in computing the cell matrix for a relatively high
 * polynomial degree in 3d. As we want to highlight the cost of the mapping
 * in this tutorial program, we better do the assembly in an optimized way
 * in order to not chase bottlenecks that have been solved by the community
 * already. Matrix-matrix multiplication is one of the best optimized
 * kernels in the HPC context, and the FullMatrix::mTmult() function will
 * call into those optimized BLAS functions. If the user has provided a good
 * BLAS library when configuring deal.II (like OpenBLAS or Intel's MKL), the
 * computation of the cell matrix will execute close to the processor's peak
 * arithmetic performance. As a side note, we mention that despite an
 * optimized matrix-matrix multiplication, the current strategy is
 * sub-optimal in terms of complexity as the work to be done is proportional
 * to @f$(p+1)^9@f$ operations for degree @f$p@f$ (this also applies to the usual
 * evaluation with FEValues). One could compute the cell matrix with
 * @f$\mathcal O((p+1)^7)@f$ operations by utilizing the tensor product
 * structure of the shape functions, as is done by the matrix-free framework
 * in deal.II. We refer to @ref step_37 "step-37" and the documentation of the
 * tensor-product-aware evaluators FEEvaluation for details on how an even
 * more efficient cell matrix computation could be realized.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::assemble_system(const Mapping<dim> &mapping)
 *     {
 *       TimerOutput::Scope scope(timer, "Assemble linear system");
 *   
 *       const QGauss<dim> quadrature_formula(fe.degree + 2);
 *       FEValues<dim>     fe_values(mapping,
 *                               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> cell_matrix(dofs_per_cell, dofs_per_cell);
 *       Vector<double>     cell_rhs(dofs_per_cell);
 *       FullMatrix<double> partial_matrix(dofs_per_cell, dim * n_q_points);
 *       std::vector<types::global_dof_index> local_dof_indices(dofs_per_cell);
 *   
 *       for (const auto &cell : dof_handler.active_cell_iterators())
 *         {
 *           cell_rhs = 0.;
 *           fe_values.reinit(cell);
 *   
 *           for (unsigned int q_index = 0; q_index < n_q_points; ++q_index)
 *             {
 *               const double current_coefficient =
 *                 coefficient(fe_values.quadrature_point(q_index));
 *               for (unsigned int i = 0; i < dofs_per_cell; ++i)
 *                 {
 *                   for (unsigned int d = 0; d < dim; ++d)
 *                     partial_matrix(i, q_index * dim + d) =
 *                       std::sqrt(fe_values.JxW(q_index) * current_coefficient) *
 *                       fe_values.shape_grad(i, q_index)[d];
 *                   cell_rhs(i) +=
 *                     (fe_values.shape_value(i, q_index) * // phi_i(x_q)
 *                      (-dim) *                            // f(x_q)
 *                      fe_values.JxW(q_index));            // dx
 *                 }
 *             }
 *   
 *           partial_matrix.mTmult(cell_matrix, partial_matrix);
 *   
 *           cell->get_dof_indices(local_dof_indices);
 *           constraints.distribute_local_to_global(
 *             cell_matrix, cell_rhs, local_dof_indices, system_matrix, system_rhs);
 *         }
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-Solutionofthelinearsystem"></a> 
 * <h3>Solution of the linear system</h3>
 *   
 
 * 
 * For solving the linear system, we pick a simple Jacobi-preconditioned
 * conjugate gradient solver, similar to the settings in the early tutorials.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::solve()
 *     {
 *       TimerOutput::Scope scope(timer, "Solve linear system");
 *   
 *       SolverControl            solver_control(1000, 1e-12);
 *       SolverCG<Vector<double>> solver(solver_control);
 *   
 *       PreconditionJacobi<SparseMatrix<double>> preconditioner;
 *       preconditioner.initialize(system_matrix);
 *   
 *       solver.solve(system_matrix, solution, system_rhs, preconditioner);
 *       constraints.distribute(solution);
 *   
 *       std::cout << "   Number of solver iterations:  "
 *                 << solver_control.last_step() << std::endl;
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-Outputofthesolutionandcomputationoferrors"></a> 
 * <h3>Output of the solution and computation of errors</h3>
 *   
 
 * 
 * In the next function we do various post-processing steps with the
 * solution, all of which involve the mapping in one way or the other.
 *   
 
 * 
 * The first operation we do is to write the solution as well as the
 * material ids to a VTU file. This is similar to what was done in many
 * other tutorial programs. The new ingredient presented in this tutorial
 * program is that we want to ensure that the data written to the file
 * used for visualization is actually a faithful representation of what
 * is used internally by deal.II. That is because most of the visualization
 * data formats only represent cells by their vertex coordinates, but
 * have no way of representing the curved boundaries that are used
 * in deal.II when using higher order mappings -- in other words, what
 * you see in the visualization tool is not actually what you are computing
 * on. (The same, incidentally, is true when using higher order shape
 * functions: Most visualization tools only render bilinear/trilinear
 * representations. This is discussed in detail in DataOut::build_patches().)
 *   
 
 * 
 * So we need to ensure that a high-order representation is written
 * to the file. We need to consider two particular topics. Firstly, we tell
 * the DataOut object via the DataOutBase::VtkFlags that we intend to
 * interpret the subdivisions of the elements as a high-order Lagrange
 * polynomial rather than a collection of bilinear patches.
 * Recent visualization programs, like ParaView version 5.5
 * or newer, can then render a high-order solution (see a <a
 * href="https://github.com/dealii/dealii/wiki/Notes-on-visualizing-high-order-output">wiki
 * page</a> for more details).
 * Secondly, we need to make sure that the mapping is passed to the
 * DataOut::build_patches() method. Finally, the DataOut class only prints
 * curved faces for <i>boundary</i> cells by default, so we need to ensure
 * that also inner cells are printed in a curved representation via the
 * mapping.
 *   
 
 * 
 * @note As of 2023, Visit 3.3.3 can still not deal with higher-order cells.
 * Rather, it simply reports that there is no data to show. To view the
 * results of this program with Visit, you will want to comment out the
 * line that sets `flags.write_higher_order_cells = true;`. On the other
 * hand, Paraview is able to understand VTU files with higher order cells
 * just fine.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::postprocess(const Mapping<dim> &mapping)
 *     {
 *       {
 *         TimerOutput::Scope scope(timer, "Write output");
 *   
 *         DataOut<dim> data_out;
 *   
 *         DataOutBase::VtkFlags flags;
 *         flags.write_higher_order_cells = true;
 *         data_out.set_flags(flags);
 *   
 *         data_out.attach_dof_handler(dof_handler);
 *         data_out.add_data_vector(solution, "solution");
 *   
 *         Vector<double> material_ids(triangulation.n_active_cells());
 *         for (const auto &cell : triangulation.active_cell_iterators())
 *           material_ids[cell->active_cell_index()] = cell->material_id();
 *         data_out.add_data_vector(material_ids, "material_ids");
 *   
 *         data_out.build_patches(mapping,
 *                                fe.degree,
 *                                DataOut<dim>::curved_inner_cells);
 *   
 *         std::ofstream file(
 *           ("solution-" +
 *            std::to_string(triangulation.n_global_levels() - 10 + 2 * dim) +
 *            ".vtu")
 *             .c_str());
 *   
 *         data_out.write_vtu(file);
 *       }
 *   
 * @endcode
 * 
 * The next operation in the postprocessing function is to compute the @f$L_2@f$
 * and @f$H^1@f$ errors against the analytical solution. As the analytical
 * solution is a quadratic polynomial, we expect a very accurate result at
 * this point. If we were solving on a simple mesh with planar faces and a
 * coefficient whose jumps are aligned with the faces between cells, then
 * we would expect the numerical result to coincide with the
 * analytical solution up to roundoff accuracy. However, since we are using
 * deformed cells following a sphere, which are only tracked by
 * polynomials of degree 4 (one more than the degree for the finite
 * elements), we will see that there is an error around @f$10^{-7}@f$. We could
 * get more accuracy by increasing the polynomial degree or refining the
 * mesh.
 * 
 * @code
 *       {
 *         TimerOutput::Scope scope(timer, "Compute error norms");
 *   
 *         Vector<double> norm_per_cell_p(triangulation.n_active_cells());
 *   
 *         VectorTools::integrate_difference(mapping,
 *                                           dof_handler,
 *                                           solution,
 *                                           ExactSolution<dim>(),
 *                                           norm_per_cell_p,
 *                                           QGauss<dim>(fe.degree + 2),
 *                                           VectorTools::L2_norm);
 *         std::cout << "   L2 error vs exact solution:   "
 *                   << norm_per_cell_p.l2_norm() << std::endl;
 *   
 *         VectorTools::integrate_difference(mapping,
 *                                           dof_handler,
 *                                           solution,
 *                                           ExactSolution<dim>(),
 *                                           norm_per_cell_p,
 *                                           QGauss<dim>(fe.degree + 2),
 *                                           VectorTools::H1_norm);
 *         std::cout << "   H1 error vs exact solution:   "
 *                   << norm_per_cell_p.l2_norm() << std::endl;
 *       }
 *   
 * @endcode
 * 
 * The final post-processing operation we do here is to compute an error
 * estimate with the KellyErrorEstimator. We use the exact same settings
 * as in the @ref step_6 "step-6" tutorial program, except for the fact that we also
 * hand in the mapping to ensure that errors are evaluated along the
 * curved element, consistent with the remainder of the program. However,
 * we do not really use the result here to drive a mesh adaptation step
 * (that would refine the mesh around the material interface along the
 * sphere), as the focus here is on the cost of this operation.
 * 
 * @code
 *       {
 *         TimerOutput::Scope scope(timer, "Compute error estimator");
 *   
 *         Vector<float> estimated_error_per_cell(triangulation.n_active_cells());
 *         KellyErrorEstimator<dim>::estimate(
 *           mapping,
 *           dof_handler,
 *           QGauss<dim - 1>(fe.degree + 1),
 *           std::map<types::boundary_id, const Function<dim> *>(),
 *           solution,
 *           estimated_error_per_cell);
 *         std::cout << "   Max cell-wise error estimate: "
 *                   << estimated_error_per_cell.linfty_norm() << std::endl;
 *       }
 *     }
 *   
 *   
 *   
 * @endcode
 * 
 * 
 * <a name="step_65-ThePoissonProblemrunfunction"></a> 
 * <h3>The PoissonProblem::run() function</h3>
 *   
 
 * 
 * Finally, we define the `run()` function that controls how we want to
 * execute this program (which is called by the main() function in the usual
 * way). We start by calling the `create_grid()` function that sets up our
 * geometry with the appropriate manifolds. We then run two instances of a
 * solver chain, starting from the setup of the equations, the assembly of
 * the linear system, its solution with a simple iterative solver, and the
 * postprocessing discussed above. The two instances differ in the way they
 * use the mapping. The first uses a conventional MappingQ mapping
 * object which we initialize to a degree one more than we use for the
 * finite element &ndash; after all, we expect the geometry representation
 * to be the bottleneck as the analytic solution is only a quadratic
 * polynomial. (In reality, things are interlinked to quite some extent
 * because the evaluation of the polynomials in real coordinates involves
 * the mapping of a higher-degree polynomials, which represent some smooth
 * rational functions. As a consequence, higher-degree polynomials still pay
 * off, so it does not make sense to increase the degree of the mapping
 * further.) Once the first pass is completed, we let the timer print a
 * summary of the compute times of the individual stages.
 * 
 * @code
 *     template <int dim>
 *     void PoissonProblem<dim>::run()
 *     {
 *       create_grid();
 *   
 *       {
 *         std::cout << std::endl
 *                   << "====== Running with the basic MappingQ class ====== "
 *                   << std::endl
 *                   << std::endl;
 *   
 *         const MappingQ<dim> mapping(fe.degree + 1);
 *         setup_system(mapping);
 *         assemble_system(mapping);
 *         solve();
 *         postprocess(mapping);
 *   
 *         timer.print_summary();
 *         timer.reset();
 *       }
 *   
 * @endcode
 * 
 * For the second instance, we instead set up the MappingQCache class. Its
 * use is very simple: After constructing it (with the degree, given that
 * we want it to show the correct degree functionality in other contexts),
 * we fill the cache via the MappingQCache::initialize() function. At this
 * stage, we specify which mapping we want to use (obviously, the same
 * MappingQ as previously in order to repeat the same computations)
 * for the cache, and then run through the same functions again, now
 * handing in the modified mapping. In the end, we again print the
 * accumulated wall times since the reset to see how the times compare to
 * the original setting.
 * 
 * @code
 *       {
 *         std::cout
 *           << "====== Running with the optimized MappingQCache class ====== "
 *           << std::endl
 *           << std::endl;
 *   
 *         MappingQCache<dim> mapping(fe.degree + 1);
 *         {
 *           TimerOutput::Scope scope(timer, "Initialize mapping cache");
 *           mapping.initialize(MappingQ<dim>(fe.degree + 1), triangulation);
 *         }
 *         std::cout << "   Memory consumption cache:     "
 *                   << 1e-6 * mapping.memory_consumption() << " MB" << std::endl;
 *   
 *         setup_system(mapping);
 *         assemble_system(mapping);
 *         solve();
 *         postprocess(mapping);
 *   
 *         timer.print_summary();
 *       }
 *     }
 *   } // namespace Step65
 *   
 *   
 *   
 *   int main()
 *   {
 *     Step65::PoissonProblem<3> test_program;
 *     test_program.run();
 *     return 0;
 *   }
 * @endcode
 
<a name="step_65-Results"></a><h1>Results</h1>
 
 
<a name="step_65-Programoutput"></a><h3>Program output</h3>
 
 
If we run the three-dimensional version of this program with polynomials of
degree three, we get the following program output:
 
@code
> make run
Scanning dependencies of target step-65
[ 33%] Building CXX object CMakeFiles/step-65.dir/step-65.cc.o
[ 66%] Linking CXX executable step-65
[ 66%] Built target step-65
[100%] Run step-65 with Release configuration
 
====== Running with the basic MappingQ class ======
 
   Number of active cells:       6656
   Number of degrees of freedom: 181609
   Number of solver iterations:  285
   L2 error vs exact solution:   8.99339e-08
   H1 error vs exact solution:   6.45341e-06
   Max cell-wise error estimate: 0.00743406
 
 
+---------------------------------------------+------------+------------+
| Total wallclock time elapsed since start    |      49.4s |            |
|                                             |            |            |
| Section                         | no. calls |  wall time | % of total |
+---------------------------------+-----------+------------+------------+
| Assemble linear system          |         1 |       5.8s |        12% |
| Compute constraints             |         1 |     0.109s |      0.22% |
| Compute error estimator         |         1 |      16.5s |        33% |
| Compute error norms             |         1 |      9.11s |        18% |
| Solve linear system             |         1 |      9.92s |        20% |
| Write output                    |         1 |      4.85s |       9.8% |
+---------------------------------+-----------+------------+------------+
 
====== Running with the optimized MappingQCache class ======
 
   Memory consumption cache:     22.9981 MB
   Number of active cells:       6656
   Number of degrees of freedom: 181609
   Number of solver iterations:  285
   L2 error vs exact solution:   8.99339e-08
   H1 error vs exact solution:   6.45341e-06
   Max cell-wise error estimate: 0.00743406
 
 
+---------------------------------------------+------------+------------+
| Total wallclock time elapsed since start    |      18.4s |            |
|                                             |            |            |
| Section                         | no. calls |  wall time | % of total |
+---------------------------------+-----------+------------+------------+
| Assemble linear system          |         1 |      1.44s |       7.8% |
| Compute constraints             |         1 |   0.00336s |         0% |
| Compute error estimator         |         1 |     0.476s |       2.6% |
| Compute error norms             |         1 |     0.505s |       2.7% |
| Initialize mapping cache        |         1 |      4.96s |        27% |
| Solve linear system             |         1 |      9.95s |        54% |
| Write output                    |         1 |     0.875s |       4.8% |
+---------------------------------+-----------+------------+------------+
 
[100%] Built target run
@endcode
 
Before discussing the timings, we look at the memory consumption for the
MappingQCache object: Our program prints that it utilizes 23 MB of
memory. If we relate this number to the memory consumption of a single
(solution or right hand side) vector,
which is 1.5 MB (namely, 181,609 elements times 8 bytes per entry in
double precision), or to the memory consumed by the
system matrix and the sparsity pattern (which is 274 MB), we realize that it is
not an overly heavy data structure, given its benefits.
 
With respect to the timers, we see a clear improvement in the overall run time
of the program by a factor of 2.7. If we disregard the iterative solver, which
is the same in both cases (and not optimal, given the simple preconditioner we
use, and the fact that sparse matrix-vector products waste operations for
cubic polynomials), the advantage is a factor of almost 5. This is pretty
impressive for a linear stationary problem, and cost savings would indeed be
much more prominent for time-dependent and nonlinear problems where assembly
is called several times. If we look into the individual components, we get a
clearer picture of what is going on and why the cache is so efficient: In the
MappingQ case, essentially every operation that involves a mapping take
at least 5 seconds to run. The norm computation runs two
VectorTools::integrate_difference() functions, which each take almost 5
seconds. (The computation of constraints is cheaper because it only evaluates
the mapping in cells at the boundary for the interpolation of boundary
conditions.) If we compare these 5 seconds to the time it takes to fill the
MappingQCache, which is 5.2 seconds (for all cells, not just the active ones),
it becomes obvious that the computation of the mapping support points
dominates over everything else in the MappingQ case. Perhaps the most
striking result is the time for the error estimator, labeled "Compute error
estimator", where the MappingQ implementation takes 17.3 seconds and
the MappingQCache variant less than 0.5 seconds. The reason why the former is
so expensive (three times more expensive than the assembly, for instance) is
that the error estimation involves evaluation of quantities over faces, where
each face in the mesh requests additional points of the mapping that in turn
go through the very expensive TransfiniteInterpolationManifold class. As there
are six faces per cell, this happens much more often than in assembly. Again,
MappingQCache nicely eliminates the repeated evaluation, aggregating all the
expensive steps involving the manifold in a single initialization call that
gets repeatedly used.
 *
 *
<a name="step_65-PlainProg"></a>
<h1> The plain program</h1>
@include "step-65.cc"
*/