Actual source code: ex16.c
petsc-3.13.6 2020-09-29
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      2: /* Usage:  mpiexec ex16 [-help] [all PETSc options] */

      4: static char help[] = "Solves a sequence of linear systems with different right-hand-side vectors.\n\
      5: Input parameters include:\n\
      6:   -ntimes <ntimes>  : number of linear systems to solve\n\
      7:   -view_exact_sol   : write exact solution vector to stdout\n\
      8:   -m <mesh_x>       : number of mesh points in x-direction\n\
      9:   -n <mesh_y>       : number of mesh points in y-direction\n\n";

     11: /*T
     12:    Concepts: KSP^repeatedly solving linear systems;
     13:    Concepts: KSP^Laplacian, 2d
     14:    Concepts: Laplacian, 2d
     15:    Processors: n
     16: T*/

     18: /*
     19:   Include "petscksp.h" so that we can use KSP solvers.  Note that this file
     20:   automatically includes:
     21:      petscsys.h       - base PETSc routines   petscvec.h - vectors
     22:      petscmat.h - matrices
     23:      petscis.h     - index sets            petscksp.h - Krylov subspace methods
     24:      petscviewer.h - viewers               petscpc.h  - preconditioners
     25: */
     26:  #include <petscksp.h>

     28: int main(int argc,char **args)
     29: {
     30:   Vec            x,b,u;  /* approx solution, RHS, exact solution */
     31:   Mat            A;        /* linear system matrix */
     32:   KSP            ksp;     /* linear solver context */
     33:   PetscReal      norm;     /* norm of solution error */
     35:   PetscInt       ntimes,i,j,k,Ii,J,Istart,Iend;
     36:   PetscInt       m   = 8,n = 7,its;
     37:   PetscBool      flg = PETSC_FALSE;
     38:   PetscScalar    v,one = 1.0,rhs;

     40:   PetscInitialize(&argc,&args,(char*)0,help);if (ierr) return ierr;
     41:   PetscOptionsGetInt(NULL,NULL,"-m",&m,NULL);
     42:   PetscOptionsGetInt(NULL,NULL,"-n",&n,NULL);

     44:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     45:          Compute the matrix for use in solving a series of
     46:          linear systems of the form, A x_i = b_i, for i=1,2,...
     47:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
     48:   /*
     49:      Create parallel matrix, specifying only its global dimensions.
     50:      When using MatCreate(), the matrix format can be specified at
     51:      runtime. Also, the parallel partitioning of the matrix is
     52:      determined by PETSc at runtime.
     53:   */
     54:   MatCreate(PETSC_COMM_WORLD,&A);
     55:   MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n);
     56:   MatSetFromOptions(A);
     57:   MatSetUp(A);

     59:   /*
     60:      Currently, all PETSc parallel matrix formats are partitioned by
     61:      contiguous chunks of rows across the processors.  Determine which
     62:      rows of the matrix are locally owned.
     63:   */
     64:   MatGetOwnershipRange(A,&Istart,&Iend);

     66:   /*
     67:      Set matrix elements for the 2-D, five-point stencil in parallel.
     68:       - Each processor needs to insert only elements that it owns
     69:         locally (but any non-local elements will be sent to the
     70:         appropriate processor during matrix assembly).
     71:       - Always specify global rows and columns of matrix entries.
     72:    */
     73:   for (Ii=Istart; Ii<Iend; Ii++) {
     74:     v = -1.0; i = Ii/n; j = Ii - i*n;
     75:     if (i>0)   {J = Ii - n; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
     76:     if (i<m-1) {J = Ii + n; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
     77:     if (j>0)   {J = Ii - 1; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
     78:     if (j<n-1) {J = Ii + 1; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
     79:     v = 4.0; MatSetValues(A,1,&Ii,1,&Ii,&v,INSERT_VALUES);
     80:   }

     82:   /*
     83:      Assemble matrix, using the 2-step process:
     84:        MatAssemblyBegin(), MatAssemblyEnd()
     85:      Computations can be done while messages are in transition
     86:      by placing code between these two statements.
     87:   */
     88:   MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY);
     89:   MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY);

     91:   /*
     92:      Create parallel vectors.
     93:       - When using VecCreate(), VecSetSizes() and VecSetFromOptions(),
     94:         we specify only the vector's global
     95:         dimension; the parallel partitioning is determined at runtime.
     96:       - When solving a linear system, the vectors and matrices MUST
     97:         be partitioned accordingly.  PETSc automatically generates
     98:         appropriately partitioned matrices and vectors when MatCreate()
     99:         and VecCreate() are used with the same communicator.
    100:       - Note: We form 1 vector from scratch and then duplicate as needed.
    101:   */
    102:   VecCreate(PETSC_COMM_WORLD,&u);
    103:   VecSetSizes(u,PETSC_DECIDE,m*n);
    104:   VecSetFromOptions(u);
    105:   VecDuplicate(u,&b);
    106:   VecDuplicate(b,&x);

    108:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    109:                 Create the linear solver and set various options
    110:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

    112:   /*
    113:      Create linear solver context
    114:   */
    115:   KSPCreate(PETSC_COMM_WORLD,&ksp);

    117:   /*
    118:      Set operators. Here the matrix that defines the linear system
    119:      also serves as the preconditioning matrix.
    120:   */
    121:   KSPSetOperators(ksp,A,A);

    123:   /*
    124:     Set runtime options, e.g.,
    125:         -ksp_type <type> -pc_type <type> -ksp_monitor -ksp_rtol <rtol>
    126:     These options will override those specified above as long as
    127:     KSPSetFromOptions() is called _after_ any other customization
    128:     routines.
    129:   */
    130:   KSPSetFromOptions(ksp);

    132:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    133:        Solve several linear systems of the form  A x_i = b_i
    134:        I.e., we retain the same matrix (A) for all systems, but
    135:        change the right-hand-side vector (b_i) at each step.

    137:        In this case, we simply call KSPSolve() multiple times.  The
    138:        preconditioner setup operations (e.g., factorization for ILU)
    139:        be done during the first call to KSPSolve() only; such operations
    140:        will NOT be repeated for successive solves.
    141:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

    143:   ntimes = 2;
    144:   PetscOptionsGetInt(NULL,NULL,"-ntimes",&ntimes,NULL);
    145:   for (k=1; k<ntimes+1; k++) {

    147:     /*
    148:        Set exact solution; then compute right-hand-side vector.  We use
    149:        an exact solution of a vector with all elements equal to 1.0*k.
    150:     */
    151:     rhs  = one * (PetscReal)k;
    152:     VecSet(u,rhs);
    153:     MatMult(A,u,b);

    155:     /*
    156:        View the exact solution vector if desired
    157:     */
    158:     PetscOptionsGetBool(NULL,NULL,"-view_exact_sol",&flg,NULL);
    159:     if (flg) {VecView(u,PETSC_VIEWER_STDOUT_WORLD);}

    161:     KSPSolve(ksp,b,x);

    163:     /*
    164:        Check the error
    165:     */
    166:     VecAXPY(x,-1.0,u);
    167:     VecNorm(x,NORM_2,&norm);
    168:     KSPGetIterationNumber(ksp,&its);
    169:     /*
    170:        Print convergence information.  PetscPrintf() produces a single
    171:        print statement from all processes that share a communicator.
    172:     */
    173:     PetscPrintf(PETSC_COMM_WORLD,"Norm of error %g System %D: iterations %D\n",(double)norm,k,its);
    174:   }

    176:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    177:                       Clean up
    178:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
    179:   /*
    180:      Free work space.  All PETSc objects should be destroyed when they
    181:      are no longer needed.
    182:   */
    183:   KSPDestroy(&ksp);
    184:   VecDestroy(&u);  VecDestroy(&x);
    185:   VecDestroy(&b);  MatDestroy(&A);

    187:   /*
    188:      Always call PetscFinalize() before exiting a program.  This routine
    189:        - finalizes the PETSc libraries as well as MPI
    190:        - provides summary and diagnostic information if certain runtime
    191:          options are chosen (e.g., -log_view).
    192:   */
    193:   PetscFinalize();
    194:   return ierr;
    195: }

    198: /*TEST

    200:    test:
    201:       nsize: 2
    202:       args: -ntimes 4 -ksp_gmres_cgs_refinement_type refine_always

    204: TEST*/