Actual source code: ex20.c
2: static char help[] = "This example solves a linear system in parallel with KSP. The matrix\n\
3: uses simple bilinear elements on the unit square. To test the parallel\n\
4: matrix assembly,the matrix is intentionally laid out across processors\n\
5: differently from the way it is assembled. Input arguments are:\n\
6: -m <size> : problem size\n\n";
8: #include petscksp.h
12: int FormElementStiffness(PetscReal H,PetscScalar *Ke)
13: {
14: Ke[0] = H/6.0; Ke[1] = -.125*H; Ke[2] = H/12.0; Ke[3] = -.125*H;
15: Ke[4] = -.125*H; Ke[5] = H/6.0; Ke[6] = -.125*H; Ke[7] = H/12.0;
16: Ke[8] = H/12.0; Ke[9] = -.125*H; Ke[10] = H/6.0; Ke[11] = -.125*H;
17: Ke[12] = -.125*H; Ke[13] = H/12.0; Ke[14] = -.125*H; Ke[15] = H/6.0;
18: return 0;
19: }
23: int main(int argc,char **args)
24: {
25: Mat C;
26: int i,m = 5,rank,size,N,start,end,M;
27: int ierr,idx[4];
28: PetscTruth flg;
29: PetscScalar Ke[16];
30: PetscReal h;
31: Vec u,b;
32: KSP ksp;
33: MatNullSpace nullsp;
35: PetscInitialize(&argc,&args,(char *)0,help);
36: PetscOptionsGetInt(PETSC_NULL,"-m",&m,PETSC_NULL);
37: N = (m+1)*(m+1); /* dimension of matrix */
38: M = m*m; /* number of elements */
39: h = 1.0/m; /* mesh width */
40: MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
41: MPI_Comm_size(PETSC_COMM_WORLD,&size);
43: /* Create stiffness matrix */
44: MatCreate(PETSC_COMM_WORLD,&C);
45: MatSetSizes(C,PETSC_DECIDE,PETSC_DECIDE,N,N);
46: MatSetFromOptions(C);
47: start = rank*(M/size) + ((M%size) < rank ? (M%size) : rank);
48: end = start + M/size + ((M%size) > rank);
50: /* Assemble matrix */
51: FormElementStiffness(h*h,Ke); /* element stiffness for Laplacian */
52: for (i=start; i<end; i++) {
53: /* location of lower left corner of element */
54: /* node numbers for the four corners of element */
55: idx[0] = (m+1)*(i/m) + (i % m);
56: idx[1] = idx[0]+1; idx[2] = idx[1] + m + 1; idx[3] = idx[2] - 1;
57: MatSetValues(C,4,idx,4,idx,Ke,ADD_VALUES);
58: }
59: MatAssemblyBegin(C,MAT_FINAL_ASSEMBLY);
60: MatAssemblyEnd(C,MAT_FINAL_ASSEMBLY);
62: /* Create right-hand-side and solution vectors */
63: VecCreate(PETSC_COMM_WORLD,&u);
64: VecSetSizes(u,PETSC_DECIDE,N);
65: VecSetFromOptions(u);
66: PetscObjectSetName((PetscObject)u,"Approx. Solution");
67: VecDuplicate(u,&b);
68: PetscObjectSetName((PetscObject)b,"Right hand side");
70: VecSet(u,1.0);
71: MatMult(C,u,b);
72: VecSet(u,0.0);
74: /* Solve linear system */
75: KSPCreate(PETSC_COMM_WORLD,&ksp);
76: KSPSetOperators(ksp,C,C,DIFFERENT_NONZERO_PATTERN);
77: KSPSetFromOptions(ksp);
78: KSPSetInitialGuessNonzero(ksp,PETSC_TRUE);
80: flg = PETSC_FALSE;
81: PetscOptionsGetTruth(PETSC_NULL,"-fixnullspace",&flg,PETSC_NULL);
82: if (flg) {
83: MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,PETSC_NULL,&nullsp);
84: KSPSetNullSpace(ksp,nullsp);
85: MatNullSpaceDestroy(nullsp);
86: }
87: KSPSolve(ksp,b,u);
90: /* Free work space */
91: KSPDestroy(ksp);
92: VecDestroy(u);
93: VecDestroy(b);
94: MatDestroy(C);
95: PetscFinalize();
96: return 0;
97: }