ex1.cpp

Go to the documentation of this file.

//                                MFEM Example 1
//                              AmgX Modification
//
// Compile with: make ex1
//
// AmgX sample runs:
//               ex1
//               ex1 -d cuda
//               ex1 --amgx-file multi_gs.json --amgx-solver
//               ex1 --amgx-file precon.json --amgx-preconditioner
//               ex1 --amgx-file multi_gs.json --amgx-solver -d cuda
//               ex1 --amgx-file precon.json --amgx-preconditioner -d cuda
//
// Description:  This example code demonstrates the use of MFEM to define a
//               simple finite element discretization of the Laplace problem
//               -Delta u = 1 with homogeneous Dirichlet boundary conditions.
//               Specifically, we discretize using a FE space of the specified
//               order, or if order < 1 using an isoparametric/isogeometric
//               space (i.e. quadratic for quadratic curvilinear mesh, NURBS for
//               NURBS mesh, etc.)
//
//               The example highlights the use of mesh refinement, finite
//               element grid functions, as well as linear and bilinear forms
//               corresponding to the left-hand side and right-hand side of the
//               discrete linear system. We also cover the explicit elimination
//               of essential boundary conditions, static condensation, and the
//               optional connection to the GLVis tool for visualization.
 
#include "mfem.hpp"
#include <fstream>
#include <iostream>
 
#ifndef MFEM_USE_AMGX
#error This example requires that MFEM is built with MFEM_USE_AMGX=YES
#endif
 
using namespace std;
using namespace mfem;
 
int main(int argc, char *argv[])
{
   // 1. Parse command-line options.
   const char *mesh_file = "../../data/star.mesh";
   int order = 1;
   bool static_cond = false;
   bool pa = false;
   const char *device_config = "cpu";
   bool visualization = true;
   bool amgx_lib = true;
   bool amgx_solver = true;
   const char* amgx_json_file = ""; // JSON file for AmgX
 
   OptionsParser args(argc, argv);
   args.AddOption(&mesh_file, "-m", "--mesh",
                  "Mesh file to use.");
   args.AddOption(&order, "-o", "--order",
                  "Finite element order (polynomial degree) or -1 for"
                  " isoparametric space.");
   args.AddOption(&static_cond, "-sc", "--static-condensation", "-no-sc",
                  "--no-static-condensation", "Enable static condensation.");
   args.AddOption(&pa, "-pa", "--partial-assembly", "-no-pa",
                  "--no-partial-assembly", "Enable Partial Assembly.");
   args.AddOption(&amgx_lib, "-amgx", "--amgx-lib", "-no-amgx",
                  "--no-amgx-lib", "Use AmgX in example.");
   args.AddOption(&amgx_json_file, "--amgx-file", "--amgx-file",
                  "AMGX solver config file (overrides --amgx-solver, --amgx-verbose)");
   args.AddOption(&amgx_solver, "--amgx-solver", "--amgx-solver",
                  "--amgx-preconditioner", "--amgx-preconditioner",
                  "Configure AMGX as solver or preconditioner.");
   args.AddOption(&device_config, "-d", "--device",
                  "Device configuration string, see Device::Configure().");
   args.AddOption(&visualization, "-vis", "--visualization", "-no-vis",
                  "--no-visualization",
                  "Enable or disable GLVis visualization.");
   args.Parse();
   if (!args.Good())
   {
      args.PrintUsage(cout);
      return 1;
   }
   args.PrintOptions(cout);
 
   // 2. Enable hardware devices such as GPUs, and programming models such as
   //    CUDA, OCCA, RAJA and OpenMP based on command line options.
   Device device(device_config);
   device.Print();
 
   // 3. Read the mesh from the given mesh file. We can handle triangular,
   //    quadrilateral, tetrahedral, hexahedral, surface and volume meshes with
   //    the same code.
   Mesh mesh(mesh_file, 1, 1);
   int dim = mesh.Dimension();
 
   // 4. Refine the mesh to increase the resolution. In this example we do
   //    'ref_levels' of uniform refinement. We choose 'ref_levels' to be the
   //    largest number that gives a final mesh with no more than 50,000
   //    elements.
   {
      int ref_levels =
         (int)floor(log(50000./mesh.GetNE())/log(2.)/dim);
      for (int l = 0; l < ref_levels; l++)
      {
         mesh.UniformRefinement();
      }
   }
 
   // 5. Define a finite element space on the mesh. Here we use continuous
   //    Lagrange finite elements of the specified order. If order < 1, we
   //    instead use an isoparametric/isogeometric space.
   FiniteElementCollection *fec;
   bool delete_fec;
   if (order > 0)
   {
      fec = new H1_FECollection(order, dim);
      delete_fec = true;
   }
   else if (mesh.GetNodes())
   {
      fec = mesh.GetNodes()->OwnFEC();
      delete_fec = false;
      cout << "Using isoparametric FEs: " << fec->Name() << endl;
   }
   else
   {
      fec = new H1_FECollection(order = 1, dim);
      delete_fec = true;
   }
   FiniteElementSpace fespace(&mesh, fec);
   cout << "Number of finite element unknowns: "
        << fespace.GetTrueVSize() << endl;
 
   // 6. Determine the list of true (i.e. conforming) essential boundary dofs.
   //    In this example, the boundary conditions are defined by marking all
   //    the boundary attributes from the mesh as essential (Dirichlet) and
   //    converting them to a list of true dofs.
   Array<int> ess_tdof_list;
   if (mesh.bdr_attributes.Size())
   {
      Array<int> ess_bdr(mesh.bdr_attributes.Max());
      ess_bdr = 1;
      fespace.GetEssentialTrueDofs(ess_bdr, ess_tdof_list);
   }
 
   // 7. Set up the linear form b(.) which corresponds to the right-hand side of
   //    the FEM linear system, which in this case is (1,phi_i) where phi_i are
   //    the basis functions in the finite element fespace.
   LinearForm b(&fespace);
   ConstantCoefficient one(1.0);
   b.AddDomainIntegrator(new DomainLFIntegrator(one));
   b.Assemble();
 
   // 8. Define the solution vector x as a finite element grid function
   //    corresponding to fespace. Initialize x with initial guess of zero,
   //    which satisfies the boundary conditions.
   GridFunction x(&fespace);
   x = 0.0;
 
   // 9. Set up the bilinear form a(.,.) on the finite element space
   //    corresponding to the Laplacian operator -Delta, by adding the Diffusion
   //    domain integrator.
   BilinearForm a(&fespace);
   if (pa) { a.SetAssemblyLevel(AssemblyLevel::PARTIAL); }
   a.AddDomainIntegrator(new DiffusionIntegrator(one));
 
   // 10. Assemble the bilinear form and the corresponding linear system,
   //     applying any necessary transformations such as: eliminating boundary
   //     conditions, applying conforming constraints for non-conforming AMR,
   //     static condensation, etc.
   if (static_cond) { a.EnableStaticCondensation(); }
   a.Assemble();
 
   OperatorPtr A;
   Vector B, X;
   a.FormLinearSystem(ess_tdof_list, x, b, A, X, B);
 
   cout << "Size of linear system: " << A->Height() << endl;
 
   // 11. Solve the linear system A X = B.
   if (pa)
   {
      // Jacobi preconditioning in partial assembly mode
      if (UsesTensorBasis(fespace))
      {
         OperatorJacobiSmoother M(a, ess_tdof_list);
         PCG(*A, M, B, X, 1, 400, 1e-12, 0.0);
      }
      else
      {
         CG(*A, B, X, 1, 400, 1e-12, 0.0);
      }
   }
   else if (amgx_lib && strcmp(amgx_json_file,"") == 0)
   {
      bool amgx_verbose = false;
      AmgXSolver amgx(AmgXSolver::PRECONDITIONER, amgx_verbose);
      amgx.SetOperator(*A.As<SparseMatrix>());
      PCG(*A, amgx, B, X, 1, 200, 1e-12, 0.0);
   }
   else if (amgx_lib && strcmp(amgx_json_file,"") != 0)
   {
      AmgXSolver amgx;
      amgx.ReadParameters(amgx_json_file, AmgXSolver::EXTERNAL);
      amgx.InitSerial();
      amgx.SetOperator(*A.As<SparseMatrix>());
 
      if (amgx_solver)
      {
         amgx.SetConvergenceCheck(true);
         amgx.Mult(B,X);
      }
      else
      {
         // Omit convergence check at the AmgX level when using as a
         // preconditioner.
         amgx.SetConvergenceCheck(false);
         PCG(*A.As<SparseMatrix>(), amgx, B, X, 3, 40, 1e-12, 0.0);
      }
   }
   else
   {
#ifndef MFEM_USE_SUITESPARSE
      // Use a simple symmetric Gauss-Seidel preconditioner with PCG.
      GSSmoother M((SparseMatrix&)(*A));
      PCG(*A, M, B, X, 1, 200, 1e-12, 0.0);
#else
      // If MFEM was compiled with SuiteSparse, use UMFPACK to solve the system.
      UMFPackSolver umf_solver;
      umf_solver.Control[UMFPACK_ORDERING] = UMFPACK_ORDERING_METIS;
      umf_solver.SetOperator(*A);
      umf_solver.Mult(B, X);
#endif
   }
 
   // 12. Recover the solution as a finite element grid function.
   a.RecoverFEMSolution(X, b, x);
 
   // 13. Save the refined mesh and the solution. This output can be viewed later
   //     using GLVis: "glvis -m refined.mesh -g sol.gf".
   ofstream mesh_ofs("refined.mesh");
   mesh_ofs.precision(8);
   mesh.Print(mesh_ofs);
   ofstream sol_ofs("sol.gf");
   sol_ofs.precision(8);
   x.Save(sol_ofs);
 
   // 14. Send the solution by socket to a GLVis server.
   if (visualization)
   {
      char vishost[] = "localhost";
      int  visport   = 19916;
      socketstream sol_sock(vishost, visport);
      sol_sock.precision(8);
      sol_sock << "solution\n" << mesh << x << flush;
   }
 
   // 15. Free the used memory.
   if (delete_fec)
   {
      delete fec;
   }
 
   return 0;
}