Actual source code: ex6f.F90
1: !
2: ! Description: This example demonstrates repeated linear solves as
3: ! well as the use of different preconditioner and linear system
4: ! matrices. This example also illustrates how to save PETSc objects
5: ! in common blocks.
6: !
7: !/*T
8: ! Concepts: KSP^repeatedly solving linear systems;
9: ! Concepts: KSP^different matrices for linear system and preconditioner;
10: ! Processors: n
11: !T*/
12: !
14: program main
15: #include <petsc/finclude/petscksp.h>
16: use petscksp
17: implicit none
19: ! Variables:
20: !
21: ! A - matrix that defines linear system
22: ! ksp - KSP context
23: ! ksp - KSP context
24: ! x, b, u - approx solution, RHS, exact solution vectors
25: !
26: Vec x,u,b
27: Mat A,A2
28: KSP ksp
29: PetscInt i,j,II,JJ,m,n
30: PetscInt Istart,Iend
31: PetscInt nsteps,one
32: PetscErrorCode ierr
33: PetscBool flg
34: PetscScalar v
36: call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
37: if (ierr .ne. 0) then
38: print*,'Unable to initialize PETSc'
39: stop
40: endif
41: m = 3
42: n = 3
43: nsteps = 2
44: one = 1
45: call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-m',m,flg,ierr)
46: call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-n',n,flg,ierr)
47: call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-nsteps',nsteps,flg,ierr)
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.
54: call MatCreate(PETSC_COMM_WORLD,A,ierr)
55: call MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n,ierr)
56: call MatSetFromOptions(A,ierr)
57: call MatSetUp(A,ierr)
59: ! The matrix is partitioned by contiguous chunks of rows across the
60: ! processors. Determine which rows of the matrix are locally owned.
62: call MatGetOwnershipRange(A,Istart,Iend,ierr)
64: ! Set matrix elements.
65: ! - Each processor needs to insert only elements that it owns
66: ! locally (but any non-local elements will be sent to the
67: ! appropriate processor during matrix assembly).
68: ! - Always specify global rows and columns of matrix entries.
70: do 10, II=Istart,Iend-1
71: v = -1.0
72: i = II/n
73: j = II - i*n
74: if (i.gt.0) then
75: JJ = II - n
76: call MatSetValues(A,one,II,one,JJ,v,ADD_VALUES,ierr)
77: endif
78: if (i.lt.m-1) then
79: JJ = II + n
80: call MatSetValues(A,one,II,one,JJ,v,ADD_VALUES,ierr)
81: endif
82: if (j.gt.0) then
83: JJ = II - 1
84: call MatSetValues(A,one,II,one,JJ,v,ADD_VALUES,ierr)
85: endif
86: if (j.lt.n-1) then
87: JJ = II + 1
88: call MatSetValues(A,one,II,one,JJ,v,ADD_VALUES,ierr)
89: endif
90: v = 4.0
91: call MatSetValues(A,one,II,one,II,v,ADD_VALUES,ierr)
92: 10 continue
94: ! Assemble matrix, using the 2-step process:
95: ! MatAssemblyBegin(), MatAssemblyEnd()
96: ! Computations can be done while messages are in transition
97: ! by placing code between these two statements.
99: call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
100: call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)
102: ! Create parallel vectors.
103: ! - When using VecCreate(), the parallel partitioning of the vector
104: ! is determined by PETSc at runtime.
105: ! - Note: We form 1 vector from scratch and then duplicate as needed.
107: call VecCreate(PETSC_COMM_WORLD,u,ierr)
108: call VecSetSizes(u,PETSC_DECIDE,m*n,ierr)
109: call VecSetFromOptions(u,ierr)
110: call VecDuplicate(u,b,ierr)
111: call VecDuplicate(b,x,ierr)
113: ! Create linear solver context
115: call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)
117: ! Set runtime options (e.g., -ksp_type <type> -pc_type <type>)
119: call KSPSetFromOptions(ksp,ierr)
121: ! Solve several linear systems in succession
123: do 100 i=1,nsteps
124: call solve1(ksp,A,x,b,u,i,nsteps,A2,ierr)
125: 100 continue
127: ! Free work space. All PETSc objects should be destroyed when they
128: ! are no longer needed.
130: call VecDestroy(u,ierr)
131: call VecDestroy(x,ierr)
132: call VecDestroy(b,ierr)
133: call MatDestroy(A,ierr)
134: call KSPDestroy(ksp,ierr)
136: call PetscFinalize(ierr)
137: end
139: ! -----------------------------------------------------------------------
140: !
141: subroutine solve1(ksp,A,x,b,u,count,nsteps,A2,ierr)
142: use petscksp
143: implicit none
145: !
146: ! solve1 - This routine is used for repeated linear system solves.
147: ! We update the linear system matrix each time, but retain the same
148: ! preconditioning matrix for all linear solves.
149: !
150: ! A - linear system matrix
151: ! A2 - preconditioning matrix
152: !
153: PetscScalar v,val
154: PetscInt II,Istart,Iend
155: PetscInt count,nsteps,one
156: PetscErrorCode ierr
157: Mat A
158: KSP ksp
159: Vec x,b,u
161: ! Use common block to retain matrix between successive subroutine calls
162: Mat A2
163: PetscMPIInt rank
164: PetscBool pflag
165: common /my_data/ pflag,rank
167: one = 1
168: ! First time thorough: Create new matrix to define the linear system
169: if (count .eq. 1) then
170: call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
171: pflag = .false.
172: call PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-mat_view',pflag,ierr)
173: if (pflag) then
174: if (rank .eq. 0) write(6,100)
175: call PetscFlush(6)
176: endif
177: call MatConvert(A,MATSAME,MAT_INITIAL_MATRIX,A2,ierr)
178: ! All other times: Set previous solution as initial guess for next solve.
179: else
180: call KSPSetInitialGuessNonzero(ksp,PETSC_TRUE,ierr)
181: endif
183: ! Alter the matrix A a bit
184: call MatGetOwnershipRange(A,Istart,Iend,ierr)
185: do 20, II=Istart,Iend-1
186: v = 2.0
187: call MatSetValues(A,one,II,one,II,v,ADD_VALUES,ierr)
188: 20 continue
189: call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
190: if (pflag) then
191: if (rank .eq. 0) write(6,110)
192: call PetscFlush(6)
193: endif
194: call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)
196: ! Set the exact solution; compute the right-hand-side vector
197: val = 1.0*real(count)
198: call VecSet(u,val,ierr)
199: call MatMult(A,u,b,ierr)
201: ! Set operators, keeping the identical preconditioner matrix for
202: ! all linear solves. This approach is often effective when the
203: ! linear systems do not change very much between successive steps.
204: call KSPSetReusePreconditioner(ksp,PETSC_TRUE,ierr)
205: call KSPSetOperators(ksp,A,A2,ierr)
207: ! Solve linear system
208: call KSPSolve(ksp,b,x,ierr)
210: ! Destroy the preconditioner matrix on the last time through
211: if (count .eq. nsteps) call MatDestroy(A2,ierr)
213: 100 format('previous matrix: preconditioning')
214: 110 format('next matrix: defines linear system')
216: end
218: !/*TEST
219: !
220: ! test:
221: ! args: -pc_type jacobi -mat_view -ksp_monitor_short -ksp_gmres_cgs_refinement_type refine_always
222: !
223: !TEST*/