Actual source code: ex20.c
2: static char help[] = "Bilinear elements on the unit square for Laplacian. To test the parallel\n\
3: matrix assembly,the matrix is intentionally laid out across processors\n\
4: differently from the way it is assembled. Input arguments are:\n\
5: -m <size> : problem size\n\n";
7: #include <petscksp.h>
9: int FormElementStiffness(PetscReal H,PetscScalar *Ke)
10: {
11: Ke[0] = H/6.0; Ke[1] = -.125*H; Ke[2] = H/12.0; Ke[3] = -.125*H;
12: Ke[4] = -.125*H; Ke[5] = H/6.0; Ke[6] = -.125*H; Ke[7] = H/12.0;
13: Ke[8] = H/12.0; Ke[9] = -.125*H; Ke[10] = H/6.0; Ke[11] = -.125*H;
14: Ke[12] = -.125*H; Ke[13] = H/12.0; Ke[14] = -.125*H; Ke[15] = H/6.0;
15: return 0;
16: }
18: int main(int argc,char **args)
19: {
20: Mat C;
21: PetscMPIInt rank,size;
22: PetscInt i,m = 5,N,start,end,M;
23: PetscInt idx[4];
24: PetscScalar Ke[16];
25: PetscReal h;
26: Vec u,b;
27: KSP ksp;
28: MatNullSpace nullsp;
30: PetscInitialize(&argc,&args,(char*)0,help);
31: PetscOptionsGetInt(NULL,NULL,"-m",&m,NULL);
32: N = (m+1)*(m+1); /* dimension of matrix */
33: M = m*m; /* number of elements */
34: h = 1.0/m; /* mesh width */
35: MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
36: MPI_Comm_size(PETSC_COMM_WORLD,&size);
38: /* Create stiffness matrix */
39: MatCreate(PETSC_COMM_WORLD,&C);
40: MatSetSizes(C,PETSC_DECIDE,PETSC_DECIDE,N,N);
41: MatSetFromOptions(C);
42: MatSetUp(C);
43: start = rank*(M/size) + ((M%size) < rank ? (M%size) : rank);
44: end = start + M/size + ((M%size) > rank);
46: /* Assemble matrix */
47: FormElementStiffness(h*h,Ke); /* element stiffness for Laplacian */
48: for (i=start; i<end; i++) {
49: /* location of lower left corner of element */
50: /* node numbers for the four corners of element */
51: idx[0] = (m+1)*(i/m) + (i % m);
52: idx[1] = idx[0]+1; idx[2] = idx[1] + m + 1; idx[3] = idx[2] - 1;
53: MatSetValues(C,4,idx,4,idx,Ke,ADD_VALUES);
54: }
55: MatAssemblyBegin(C,MAT_FINAL_ASSEMBLY);
56: MatAssemblyEnd(C,MAT_FINAL_ASSEMBLY);
58: /* Create right-hand-side and solution vectors */
59: VecCreate(PETSC_COMM_WORLD,&u);
60: VecSetSizes(u,PETSC_DECIDE,N);
61: VecSetFromOptions(u);
62: PetscObjectSetName((PetscObject)u,"Approx. Solution");
63: VecDuplicate(u,&b);
64: PetscObjectSetName((PetscObject)b,"Right hand side");
66: VecSet(b,1.0);
67: VecSetValue(b,0,1.2,ADD_VALUES);
68: VecSet(u,0.0);
70: /* Solve linear system */
71: KSPCreate(PETSC_COMM_WORLD,&ksp);
72: KSPSetOperators(ksp,C,C);
73: KSPSetFromOptions(ksp);
74: KSPSetInitialGuessNonzero(ksp,PETSC_TRUE);
76: MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,NULL,&nullsp);
77: /*
78: The KSP solver will remove this nullspace from the solution at each iteration
79: */
80: MatSetNullSpace(C,nullsp);
81: /*
82: The KSP solver will remove from the right hand side any portion in this nullspace, thus making the linear system consistent.
83: */
84: MatSetTransposeNullSpace(C,nullsp);
85: MatNullSpaceDestroy(&nullsp);
87: KSPSolve(ksp,b,u);
89: /* Free work space */
90: KSPDestroy(&ksp);
91: VecDestroy(&u);
92: VecDestroy(&b);
93: MatDestroy(&C);
94: PetscFinalize();
95: return 0;
96: }
98: /*TEST
100: test:
101: args: -ksp_monitor_short
103: TEST*/