Actual source code: ex64.c
petsc-3.13.6 2020-09-29
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      1: /*
      2:  * ex64.c
      3:  *
      4:  *  Created on: Aug 10, 2015
      5:  *      Author: Fande Kong  <fdkong.jd@gmail.com>
      6:  */

      8: static char help[] = "Illustrates use of the preconditioner GASM.\n \
      9:    using hierarchical partitioning and MatIncreaseOverlapSplit \
     10:         -pc_gasm_total_subdomains\n \
     11:         -pc_gasm_print_subdomains\n \n";

     13: /*
     14:    Note:  This example focuses on setting the subdomains for the GASM
     15:    preconditioner for a problem on a 2D rectangular grid.  See ex1.c
     16:    and ex2.c for more detailed comments on the basic usage of KSP
     17:    (including working with matrices and vectors).

     19:    The GASM preconditioner is fully parallel.  The user-space routine
     20:    CreateSubdomains2D that computes the domain decomposition is also parallel
     21:    and attempts to generate both subdomains straddling processors and multiple
     22:    domains per processor.

     25:    This matrix in this linear system arises from the discretized Laplacian,
     26:    and thus is not very interesting in terms of experimenting with variants
     27:    of the GASM preconditioner.
     28: */

     30: /*T
     31:    Concepts: KSP^Additive Schwarz Method (GASM) with user-defined subdomains
     32:    Processors: n
     33: T*/

     35: /*
     36:   Include "petscksp.h" so that we can use KSP solvers.  Note that this file
     37:   automatically includes:
     38:      petscsys.h    - base PETSc routines   petscvec.h - vectors
     39:      petscmat.h    - matrices
     40:      petscis.h     - index sets            petscksp.h - Krylov subspace methods
     41:      petscviewer.h - viewers               petscpc.h  - preconditioners
     42: */
     43:  #include <petscksp.h>
     44:  #include <petscmat.h>

     47: int main(int argc,char **args)
     48: {
     49:   Vec            x,b,u;                  /* approx solution, RHS, exact solution */
     50:   Mat            A,perA;                      /* linear system matrix */
     51:   KSP            ksp;                    /* linear solver context */
     52:   PC             pc;                     /* PC context */
     53:   PetscInt       overlap;                /* width of subdomain overlap */
     54:   PetscInt       m,n;                    /* mesh dimensions in x- and y- directions */
     55:   PetscInt       M,N;                    /* number of subdomains in x- and y- directions */
     56:   PetscInt       i,j,Ii,J,Istart,Iend;
     58:   PetscMPIInt    size;
     59:   PetscBool      flg;
     60:   PetscBool       user_set_subdomains;
     61:   PetscScalar     v, one = 1.0;
     62:   MatPartitioning part;
     63:   IS              coarseparts,fineparts;
     64:   IS              is,isn,isrows;
     65:   MPI_Comm        comm;
     66:   PetscReal       e;

     68:   PetscInitialize(&argc,&args,(char*)0,help);if (ierr) return ierr;
     69:   comm = PETSC_COMM_WORLD;
     70:   MPI_Comm_size(comm,&size);
     71:   PetscOptionsBegin(PETSC_COMM_WORLD,NULL,"ex62","PC");
     72:   m = 15;
     73:   PetscOptionsInt("-M", "Number of mesh points in the x-direction","PCGASMCreateSubdomains2D",m,&m,NULL);
     74:   n = 17;
     75:   PetscOptionsInt("-N","Number of mesh points in the y-direction","PCGASMCreateSubdomains2D",n,&n,NULL);
     76:   user_set_subdomains = PETSC_FALSE;
     77:   PetscOptionsBool("-user_set_subdomains","Use the user-specified 2D tiling of mesh by subdomains","PCGASMCreateSubdomains2D",user_set_subdomains,&user_set_subdomains,NULL);
     78:   M = 2;
     79:   PetscOptionsInt("-Mdomains","Number of subdomain tiles in the x-direction","PCGASMSetSubdomains2D",M,&M,NULL);
     80:   N = 1;
     81:   PetscOptionsInt("-Ndomains","Number of subdomain tiles in the y-direction","PCGASMSetSubdomains2D",N,&N,NULL);
     82:   overlap = 1;
     83:   PetscOptionsInt("-overlap","Size of tile overlap.","PCGASMSetSubdomains2D",overlap,&overlap,NULL);
     84:   PetscOptionsEnd();

     86:   /* -------------------------------------------------------------------
     87:          Compute the matrix and right-hand-side vector that define
     88:          the linear system, Ax = b.
     89:      ------------------------------------------------------------------- */

     91:   /*
     92:      Assemble the matrix for the five point stencil, YET AGAIN
     93:   */
     94:   MatCreate(comm,&A);
     95:   MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n);
     96:   MatSetFromOptions(A);
     97:   MatSetUp(A);
     98:   MatGetOwnershipRange(A,&Istart,&Iend);
     99:   for (Ii=Istart; Ii<Iend; Ii++) {
    100:     v = -1.0; i = Ii/n; j = Ii - i*n;
    101:     if (i>0)   {J = Ii - n; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
    102:     if (i<m-1) {J = Ii + n; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
    103:     if (j>0)   {J = Ii - 1; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
    104:     if (j<n-1) {J = Ii + 1; MatSetValues(A,1,&Ii,1,&J,&v,INSERT_VALUES);}
    105:     v = 4.0; MatSetValues(A,1,&Ii,1,&Ii,&v,INSERT_VALUES);
    106:   }
    107:   MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY);
    108:   MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY);

    110:   /*
    111:     Partition the graph of the matrix
    112:   */
    113:   MatPartitioningCreate(comm,&part);
    114:   MatPartitioningSetAdjacency(part,A);
    115:   MatPartitioningSetType(part,MATPARTITIONINGHIERARCH);
    116:   MatPartitioningHierarchicalSetNcoarseparts(part,2);
    117:   MatPartitioningHierarchicalSetNfineparts(part,2);
    118:   MatPartitioningSetFromOptions(part);
    119:   /* get new processor owner number of each vertex */
    120:   MatPartitioningApply(part,&is);
    121:   /* get coarse parts according to which we rearrange the matrix */
    122:   MatPartitioningHierarchicalGetCoarseparts(part,&coarseparts);
    123:   /* fine parts are used with coarse parts */
    124:   MatPartitioningHierarchicalGetFineparts(part,&fineparts);
    125:   /* get new global number of each old global number */
    126:   ISPartitioningToNumbering(is,&isn);
    127:   ISBuildTwoSided(is,NULL,&isrows);
    128:   MatCreateSubMatrix(A,isrows,isrows,MAT_INITIAL_MATRIX,&perA);
    129:   MatCreateVecs(perA,&b,NULL);
    130:   VecSetFromOptions(b);
    131:   VecDuplicate(b,&u);
    132:   VecDuplicate(b,&x);
    133:   VecSet(u,one);
    134:   MatMult(perA,u,b);
    135:   KSPCreate(comm,&ksp);
    136:   /*
    137:      Set operators. Here the matrix that defines the linear system
    138:      also serves as the preconditioning matrix.
    139:   */
    140:   KSPSetOperators(ksp,perA,perA);

    142:   /*
    143:      Set the default preconditioner for this program to be GASM
    144:   */
    145:   KSPGetPC(ksp,&pc);
    146:   PCSetType(pc,PCGASM);
    147:   KSPSetFromOptions(ksp);
    148:   /* -------------------------------------------------------------------
    149:                       Solve the linear system
    150:      ------------------------------------------------------------------- */

    152:   KSPSolve(ksp,b,x);
    153:   /* -------------------------------------------------------------------
    154:                       Compare result to the exact solution
    155:      ------------------------------------------------------------------- */
    156:   VecAXPY(x,-1.0,u);
    157:   VecNorm(x,NORM_INFINITY, &e);

    159:   flg  = PETSC_FALSE;
    160:   PetscOptionsGetBool(NULL,NULL,"-print_error",&flg,NULL);
    161:   if (flg) {
    162:     PetscPrintf(PETSC_COMM_WORLD, "Infinity norm of the error: %g\n", (double)e);
    163:   }
    164:   /*
    165:      Free work space.  All PETSc objects should be destroyed when they
    166:      are no longer needed.
    167:   */
    168:   KSPDestroy(&ksp);
    169:   VecDestroy(&u);
    170:   VecDestroy(&x);
    171:   VecDestroy(&b);
    172:   MatDestroy(&A);
    173:   MatDestroy(&perA);
    174:   ISDestroy(&is);
    175:   ISDestroy(&coarseparts);
    176:   ISDestroy(&fineparts);
    177:   ISDestroy(&isrows);
    178:   ISDestroy(&isn);
    179:   MatPartitioningDestroy(&part);
    180:   PetscFinalize();
    181:   return ierr;
    182: }

    185: /*TEST

    187:    test:
    188:       nsize: 4
    189:       requires: superlu_dist parmetis
    190:       args: -ksp_monitor -pc_gasm_overlap 1 -sub_pc_type lu -sub_pc_factor_mat_solver_type superlu_dist -pc_gasm_total_subdomains 2

    192: TEST*/