Actual source code: ex20.c
1: /*$Id: ex20.c,v 1.19 2001/08/07 21:30:50 bsmith Exp $*/
3: static char help[] = "This example solves a linear system in parallel with SLES. The matrixn
4: uses simple bilinear elements on the unit square. To test the paralleln
5: matrix assembly,the matrix is intentionally laid out across processorsn
6: differently from the way it is assembled. Input arguments are:n
7: -m <size> : problem sizenn";
9: #include petscsles.h
11: int FormElementStiffness(PetscReal H,PetscScalar *Ke)
12: {
13: Ke[0] = H/6.0; Ke[1] = -.125*H; Ke[2] = H/12.0; Ke[3] = -.125*H;
14: Ke[4] = -.125*H; Ke[5] = H/6.0; Ke[6] = -.125*H; Ke[7] = H/12.0;
15: Ke[8] = H/12.0; Ke[9] = -.125*H; Ke[10] = H/6.0; Ke[11] = -.125*H;
16: Ke[12] = -.125*H; Ke[13] = H/12.0; Ke[14] = -.125*H; Ke[15] = H/6.0;
17: return 0;
18: }
20: int main(int argc,char **args)
21: {
22: Mat C;
23: int i,m = 5,rank,size,N,start,end,M,its;
24: int ierr,idx[4];
25: PetscTruth flg;
26: PetscScalar zero = 0.0,Ke[16], one = 1.0;
27: PetscReal h;
28: Vec u,b;
29: SLES sles;
30: KSP ksp;
31: MatNullSpace nullsp;
32: PC pc;
34: PetscInitialize(&argc,&args,(char *)0,help);
35: PetscOptionsGetInt(PETSC_NULL,"-m",&m,PETSC_NULL);
36: N = (m+1)*(m+1); /* dimension of matrix */
37: M = m*m; /* number of elements */
38: h = 1.0/m; /* mesh width */
39: MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
40: MPI_Comm_size(PETSC_COMM_WORLD,&size);
42: /* Create stiffness matrix */
43: MatCreate(PETSC_COMM_WORLD,PETSC_DECIDE,PETSC_DECIDE,N,N,&C);
44: MatSetFromOptions(C);
45: start = rank*(M/size) + ((M%size) < rank ? (M%size) : rank);
46: end = start + M/size + ((M%size) > rank);
48: /* Assemble matrix */
49: FormElementStiffness(h*h,Ke); /* element stiffness for Laplacian */
50: for (i=start; i<end; i++) {
51: /* location of lower left corner of element */
52: /* node numbers for the four corners of element */
53: idx[0] = (m+1)*(i/m) + (i % m);
54: idx[1] = idx[0]+1; idx[2] = idx[1] + m + 1; idx[3] = idx[2] - 1;
55: MatSetValues(C,4,idx,4,idx,Ke,ADD_VALUES);
56: }
57: MatAssemblyBegin(C,MAT_FINAL_ASSEMBLY);
58: MatAssemblyEnd(C,MAT_FINAL_ASSEMBLY);
60: /* Create right-hand-side and solution vectors */
61: VecCreate(PETSC_COMM_WORLD,&u);
62: VecSetSizes(u,PETSC_DECIDE,N);
63: VecSetFromOptions(u);
64: PetscObjectSetName((PetscObject)u,"Approx. Solution");
65: VecDuplicate(u,&b);
66: PetscObjectSetName((PetscObject)b,"Right hand side");
68: VecSet(&one,u);
69: MatMult(C,u,b);
70: VecSet(&zero,u);
72: /* Solve linear system */
73: SLESCreate(PETSC_COMM_WORLD,&sles);
74: SLESSetOperators(sles,C,C,DIFFERENT_NONZERO_PATTERN);
75: SLESSetFromOptions(sles);
76: SLESGetKSP(sles,&ksp);
77: KSPSetInitialGuessNonzero(ksp,PETSC_TRUE);
79: PetscOptionsHasName(PETSC_NULL,"-fixnullspace",&flg);
80: if (flg) {
81: SLESGetPC(sles,&pc);
82: MatNullSpaceCreate(PETSC_COMM_WORLD,1,0,PETSC_NULL,&nullsp);
83: PCNullSpaceAttach(pc,nullsp);
84: MatNullSpaceDestroy(nullsp);
85: }
87: SLESSolve(sles,b,u,&its);
90: /* Free work space */
91: SLESDestroy(sles);
92: VecDestroy(u);
93: VecDestroy(b);
94: MatDestroy(C);
95: PetscFinalize();
96: return 0;
97: }