Add geometric twogrid demo

cpp/demo/CMakeLists.txt

@@ -19,6 +19,7 @@ endmacro(add_demo_subdirectory)
 add_demo_subdirectory(biharmonic)
 add_demo_subdirectory(codim_0_assembly)
 add_demo_subdirectory(custom_kernel)
+add_demo_subdirectory(geometric-multigrid)
 add_demo_subdirectory(poisson)
 add_demo_subdirectory(poisson_matrix_free)
 add_demo_subdirectory(hyperelasticity)

cpp/demo/geometric-twogrid/CMakeLists.txt

@@ -0,0 +1,67 @@
+# This file was generated by running
+#
+# python cmake/scripts/generate-cmakefiles from dolfinx/cpp
+#
+cmake_minimum_required(VERSION 3.19)
+
+set(PROJECT_NAME demo_geometric-twogrid)
+project(${PROJECT_NAME} LANGUAGES C CXX)
+
+# Set C++20 standard
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+if(NOT TARGET dolfinx)
+  find_package(DOLFINX REQUIRED)
+endif()
+
+include(CheckSymbolExists)
+set(CMAKE_REQUIRED_INCLUDES ${PETSC_INCLUDE_DIRS})
+check_symbol_exists(PETSC_USE_COMPLEX petscsystypes.h PETSC_SCALAR_COMPLEX)
+check_symbol_exists(PETSC_USE_REAL_DOUBLE petscsystypes.h PETSC_REAL_DOUBLE)
+
+# Add target to compile UFL files
+if(PETSC_SCALAR_COMPLEX EQUAL 1)
+  if(PETSC_REAL_DOUBLE EQUAL 1)
+    set(SCALAR_TYPE "--scalar_type=complex128")
+  else()
+    set(SCALAR_TYPE "--scalar_type=complex64")
+  endif()
+else()
+  if(PETSC_REAL_DOUBLE EQUAL 1)
+    set(SCALAR_TYPE "--scalar_type=float64")
+  else()
+    set(SCALAR_TYPE "--scalar_type=float32")
+  endif()
+endif()
+add_custom_command(
+  OUTPUT poisson.c
+  COMMAND ffcx ${CMAKE_CURRENT_SOURCE_DIR}/poisson.py ${SCALAR_TYPE}
+  VERBATIM
+  DEPENDS poisson.py
+  COMMENT "Compile poisson.py using FFCx"
+)
+
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
+add_executable(${PROJECT_NAME} main.cpp ${CMAKE_CURRENT_BINARY_DIR}/poisson.c)
+target_link_libraries(${PROJECT_NAME} dolfinx)
+
+# Do not throw error for 'multi-line comments' (these are typical in rst which
+# includes LaTeX)
+include(CheckCXXCompilerFlag)
+check_cxx_compiler_flag("-Wno-comment" HAVE_NO_MULTLINE)
+set_source_files_properties(
+  main.cpp
+  PROPERTIES
+    COMPILE_FLAGS
+    "$<$<BOOL:${HAVE_NO_MULTLINE}>:-Wno-comment -Wall -Wextra -pedantic -Werror>"
+)
+
+# Test targets (used by DOLFINx testing system)
+set(TEST_PARAMETERS2 -np 2 ${MPIEXEC_PARAMS} "./${PROJECT_NAME}")
+set(TEST_PARAMETERS3 -np 3 ${MPIEXEC_PARAMS} "./${PROJECT_NAME}")
+add_test(NAME ${PROJECT_NAME}_mpi_2 COMMAND "mpirun" ${TEST_PARAMETERS2})
+add_test(NAME ${PROJECT_NAME}_mpi_3 COMMAND "mpirun" ${TEST_PARAMETERS3})
+add_test(NAME ${PROJECT_NAME}_serial COMMAND ${PROJECT_NAME})

cpp/demo/geometric-twogrid/main.cpp

@@ -0,0 +1,242 @@
+// Copyright (C) 2024 Paul Kühner
+//
+// This file is part of DOLFINX (https://www.fenicsproject.org)
+//
+// SPDX-License-Identifier:    LGPL-3.0-or-later
+
+/// @cond TODO
+
+#include <cassert>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <numbers>
+
+#include <petscksp.h>
+#include <petsclog.h>
+#include <petscmat.h>
+#include <petscpc.h>
+#include <petscpctypes.h>
+#include <petscsys.h>
+#include <petscsystypes.h>
+#include <petscvec.h>
+#include <petscviewer.h>
+
+#include <mpi.h>
+
+#include <basix/finite-element.h>
+
+#include <dolfinx/common/MPI.h>
+#include <dolfinx/fem/DirichletBC.h>
+#include <dolfinx/fem/FunctionSpace.h>
+#include <dolfinx/fem/petsc.h>
+#include <dolfinx/fem/utils.h>
+#include <dolfinx/io/ADIOS2Writers.h>
+#include <dolfinx/io/VTKFile.h>
+#include <dolfinx/io/XDMFFile.h>
+#include <dolfinx/la/SparsityPattern.h>
+#include <dolfinx/la/petsc.h>
+#include <dolfinx/mesh/cell_types.h>
+#include <dolfinx/mesh/generation.h>
+#include <dolfinx/mesh/utils.h>
+#include <dolfinx/refinement/refine.h>
+#include <dolfinx/transfer/transfer_matrix.h>
+
+#include "poisson.h"
+
+using namespace dolfinx;
+using T = PetscScalar;
+using U = typename dolfinx::scalar_value_type_t<T>;
+
+struct PetscEnv
+{
+  PetscEnv(int argc, char** argv) { PetscInitialize(&argc, &argv, NULL, NULL); }
+
+  ~PetscEnv() { PetscFinalize(); }
+};
+
+// recommended to run with
+// ./demo_geometric-multigrid -pc_mg_log -all_ksp_monitor -ksp_converged_reason
+// -ksp_view
+int main(int argc, char** argv)
+{
+  PetscEnv petscEnv(argc, argv);
+  // PetscLogDefaultBegin();
+
+  auto element = basix::create_element<U>(
+      basix::element::family::P, basix::cell::type::tetrahedron, 1,
+      basix::element::lagrange_variant::unset,
+      basix::element::dpc_variant::unset, false);
+  auto part = mesh::create_cell_partitioner(mesh::GhostMode::shared_vertex);
+
+  auto mesh_coarse = std::make_shared<mesh::Mesh<U>>(dolfinx::mesh::create_box(
+      MPI_COMM_WORLD, {{{0, 0, 0}, {1, 1, 1}}}, {10, 10, 10},
+      mesh::CellType::tetrahedron, part));
+
+  auto V_coarse = std::make_shared<fem::FunctionSpace<U>>(
+      fem::create_functionspace<U>(mesh_coarse, element, {}));
+
+  // refinement routine requires edges to be initialized
+  mesh_coarse->topology()->create_entities(1);
+  auto [mesh, parent_cells, parent_facets]
+      = dolfinx::refinement::refine(*mesh_coarse, std::nullopt);
+
+  auto V = std::make_shared<fem::FunctionSpace<U>>(fem::create_functionspace<U>(
+      std::make_shared<mesh::Mesh<U>>(mesh), element, {}));
+
+  auto f_ana = [](auto x) -> std::pair<std::vector<T>, std::vector<std::size_t>>
+  {
+    std::vector<T> f;
+    for (std::size_t p = 0; p < x.extent(1); ++p)
+    {
+      auto x0 = x(0, p);
+      auto x1 = x(1, p);
+      auto x2 = x(2, p);
+      auto pi = std::numbers::pi;
+      f.push_back(2 * pi * pi * std::sin(pi * x0) * std::sin(pi * x1)
+                  * std::sin(pi * x2));
+    }
+    return {f, {f.size()}};
+  };
+  auto f = std::make_shared<fem::Function<T>>(V);
+  f->interpolate(f_ana);
+
+  {
+    io::VTKFile file(MPI_COMM_WORLD, "f.pvd", "w");
+    file.write<T>({*f}, 0.0);
+  }
+
+  auto a = std::make_shared<fem::Form<T>>(
+      fem::create_form<T>(*form_poisson_a, {V, V}, {}, {}, {}));
+  auto a_coarse = std::make_shared<fem::Form<T>>(
+      fem::create_form<T>(*form_poisson_a, {V_coarse, V_coarse}, {}, {}, {}));
+  auto L = std::make_shared<fem::Form<T>>(
+      fem::create_form<T>(*form_poisson_L, {V}, {{"f", f}}, {}, {}));
+  la::petsc::Matrix A(fem::petsc::create_matrix(*a), true);
+  la::petsc::Matrix A_coarse(fem::petsc::create_matrix(*a_coarse), true);
+
+  la::Vector<T> b(L->function_spaces()[0]->dofmap()->index_map,
+                  L->function_spaces()[0]->dofmap()->index_map_bs());
+
+  // TOOD: this somehow breaks?!?
+  // V->mesh()->topology_mutable()->create_connectivity(2, 3);
+  // mesh::exterior_facet_indices(*V->mesh()->topology())
+  auto bc = std::make_shared<const fem::DirichletBC<T>>(
+      0.0,
+      mesh::locate_entities_boundary(
+          *V->mesh(), 0,
+          [](auto x) { return std::vector<std::int8_t>(x.extent(1), true); }),
+      V);
+
+  // V_coarse->mesh()->topology_mutable()->create_connectivity(2, 3);
+  auto bc_coarse = std::make_shared<const fem::DirichletBC<T>>(
+      0.0,
+      mesh::locate_entities_boundary(
+          *V_coarse->mesh(), 0,
+          [](auto x) { return std::vector<std::int8_t>(x.extent(1), true); }),
+      V_coarse);
+
+  // assemble A
+  MatZeroEntries(A.mat());
+  fem::assemble_matrix(la::petsc::Matrix::set_block_fn(A.mat(), ADD_VALUES), *a,
+                       {bc});
+  MatAssemblyBegin(A.mat(), MAT_FLUSH_ASSEMBLY);
+  MatAssemblyEnd(A.mat(), MAT_FLUSH_ASSEMBLY);
+  fem::set_diagonal<T>(la::petsc::Matrix::set_fn(A.mat(), INSERT_VALUES), *V,
+                       {bc});
+  MatAssemblyBegin(A.mat(), MAT_FINAL_ASSEMBLY);
+  MatAssemblyEnd(A.mat(), MAT_FINAL_ASSEMBLY);
+
+  // assemble b
+  b.set(0.0);
+  fem::assemble_vector(b.mutable_array(), *L);
+  fem::apply_lifting<T, U>(b.mutable_array(), {a}, {{bc}}, {}, T(1));
+  b.scatter_rev(std::plus<T>());
+  fem::set_bc<T, U>(b.mutable_array(), {bc});
+
+  // assemble A_coarse
+  MatZeroEntries(A_coarse.mat());
+  fem::assemble_matrix(
+      la::petsc::Matrix::set_block_fn(A_coarse.mat(), ADD_VALUES), *a_coarse,
+      {bc_coarse});
+  MatAssemblyBegin(A_coarse.mat(), MAT_FLUSH_ASSEMBLY);
+  MatAssemblyEnd(A_coarse.mat(), MAT_FLUSH_ASSEMBLY);
+  fem::set_diagonal<T>(la::petsc::Matrix::set_fn(A_coarse.mat(), INSERT_VALUES),
+                       *V_coarse, {bc_coarse});
+  MatAssemblyBegin(A_coarse.mat(), MAT_FINAL_ASSEMBLY);
+  MatAssemblyEnd(A_coarse.mat(), MAT_FINAL_ASSEMBLY);
+
+  KSP ksp;
+  KSPCreate(MPI_COMM_WORLD, &ksp);
+  KSPSetType(ksp, "cg");
+
+  PC pc;
+  KSPGetPC(ksp, &pc);
+  KSPSetFromOptions(ksp);
+  PCSetType(pc, "mg");
+
+  PCMGSetLevels(pc, 2, NULL);
+  PCMGSetType(pc, PC_MG_MULTIPLICATIVE);
+  PCMGSetCycleType(pc, PC_MG_CYCLE_V);
+
+  // do not rely on coarse grid operators to be generated by
+  // restriction/prolongation
+  PCMGSetGalerkin(pc, PC_MG_GALERKIN_NONE);
+  PCMGSetOperators(pc, 0, A_coarse.mat(), A_coarse.mat());
+
+  // PCMGSetNumberSmooth(pc, 1);
+  PCSetFromOptions(pc);
+
+  mesh.topology_mutable()->create_connectivity(0, 1);
+  mesh.topology_mutable()->create_connectivity(1, 0);
+  auto inclusion_map = transfer::inclusion_mapping(*mesh_coarse, mesh);
+  la::SparsityPattern sp
+      = transfer::create_sparsity_pattern(*V_coarse, *V, inclusion_map);
+  la::petsc::Matrix restriction(MPI_COMM_WORLD, sp);
+  transfer::assemble_transfer_matrix<double>(
+      la::petsc::Matrix::set_block_fn(restriction.mat(), INSERT_VALUES),
+      *V_coarse, *V, inclusion_map,
+      [](std::int32_t d) -> double { return d == 0 ? 1. : .5; });
+
+  MatAssemblyBegin(restriction.mat(), MAT_FINAL_ASSEMBLY);
+  MatAssemblyEnd(restriction.mat(), MAT_FINAL_ASSEMBLY);
+
+  // PETSc figures out to use transpose by dimensions!
+  PCMGSetInterpolation(pc, 1, restriction.mat());
+  PCMGSetRestriction(pc, 1, restriction.mat());
+
+  // MatView(A.mat(), PETSC_VIEWER_STDOUT_SELF);
+  KSPSetOperators(ksp, A.mat(), A.mat());
+  KSPSetUp(ksp);
+
+  auto u = std::make_shared<fem::Function<T>>(V);
+
+  la::petsc::Vector _u(la::petsc::create_vector_wrap(*u->x()), false);
+  la::petsc::Vector _b(la::petsc::create_vector_wrap(b), false);
+
+  KSPSolve(ksp, _b.vec(), _u.vec());
+  u->x()->scatter_fwd();
+
+  {
+    io::VTKFile file(MPI_COMM_WORLD, "u.pvd", "w");
+    file.write<T>({*u}, 0.0);
+
+    io::XDMFFile file_xdmf(mesh.comm(), "u_xdmf.xdmf", "w");
+    file_xdmf.write_mesh(mesh);
+    file_xdmf.write_function(*u, 0.0);
+    file_xdmf.close();
+
+#ifdef HAS_ADIOS2
+    io::VTXWriter<U> vtx_writer(mesh.comm(), std::filesystem::path("u_vtx.bp"),
+                                {u}, io::VTXMeshPolicy::reuse);
+    vtx_writer.write(0);
+#endif
+  }
+
+  KSPDestroy(&ksp);
+
+  // PetscLogView(PETSC_VIEWER_STDOUT_SELF);
+}
+
+/// @endcond

cpp/demo/geometric-twogrid/poisson.py

@@ -0,0 +1,44 @@
+# The first step is to define the variational problem at hand. We define
+# the variational problem in UFL terms in a separate form file
+# {download}`demo_poisson/poisson.py`.  We begin by defining the finite
+# element:
+
+from basix.ufl import element
+from ufl import (
+    Coefficient,
+    FunctionSpace,
+    Mesh,
+    TestFunction,
+    TrialFunction,
+    dx,
+    grad,
+    inner,
+)
+
+e = element("Lagrange", "tetrahedron", 1)
+
+# The first argument to :py:class:`FiniteElement` is the finite element
+# family, the second argument specifies the domain, while the third
+# argument specifies the polynomial degree. Thus, in this case, our
+# element `element` consists of first-order, continuous Lagrange basis
+# functions on triangles (or in order words, continuous piecewise linear
+# polynomials on triangles).
+#
+# Next, we use this element to initialize the trial and test functions
+# ($u$ and $v$) and the coefficient functions ($f$ and $g$):
+
+coord_element = element("Lagrange", "tetrahedron", 1, shape=(3,))
+mesh = Mesh(coord_element)
+
+V = FunctionSpace(mesh, e)
+
+u = TrialFunction(V)
+v = TestFunction(V)
+f = Coefficient(V)
+g = Coefficient(V)
+
+# Finally, we define the bilinear and linear forms according to the
+# variational formulation of the equations:
+
+a = inner(grad(u), grad(v)) * dx
+L = inner(f, v) * dx