Actual source code: ex6f.F

  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: !
 13: !  The following include statements are required for KSP Fortran programs:
 14: !     petsc.h       - base PETSc routines
 15: !     petscvec.h    - vectors
 16: !     petscmat.h    - matrices
 17: !     petscpc.h     - preconditioners
 18: !     petscksp.h    - Krylov subspace methods
 19: !  Other include statements may be needed if using additional PETSc
 20: !  routines in a Fortran program, e.g.,
 21: !     petscviewer.h - viewers
 22: !     petscis.h     - index sets
 23: !
 24:       program main
 25:  #include include/finclude/petsc.h
 26:  #include include/finclude/petscvec.h
 27:  #include include/finclude/petscmat.h
 28:  #include include/finclude/petscpc.h
 29:  #include include/finclude/petscksp.h

 31: !  Variables:
 32: !
 33: !  A       - matrix that defines linear system
 34: !  ksp    - KSP context
 35: !  ksp     - KSP context
 36: !  x, b, u - approx solution, RHS, exact solution vectors
 37: !
 38:       Vec     x,u,b
 39:       Mat     A
 40:       KSP    ksp
 41:       integer i,j,II,JJ,ierr,m,n
 42:       integer Istart,Iend,flg,nsteps
 43:       PetscScalar  v

 45:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 46:       m      = 3
 47:       n      = 3
 48:       nsteps = 2
 49:       call PetscOptionsGetInt(PETSC_NULL_CHARACTER,'-m',m,flg,ierr)
 50:       call PetscOptionsGetInt(PETSC_NULL_CHARACTER,'-n',n,flg,ierr)
 51:       call PetscOptionsGetInt(PETSC_NULL_CHARACTER,'-nsteps',nsteps,    &
 52:      &     flg,ierr)

 54: !  Create parallel matrix, specifying only its global dimensions.
 55: !  When using MatCreate(), the matrix format can be specified at
 56: !  runtime. Also, the parallel partitioning of the matrix is
 57: !  determined by PETSc at runtime.

 59:       call MatCreate(PETSC_COMM_WORLD,A,ierr)
 60:       call MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,m*n,m*n,ierr)
 61:       call MatSetFromOptions(A,ierr)

 63: !  The matrix is partitioned by contiguous chunks of rows across the
 64: !  processors.  Determine which rows of the matrix are locally owned.

 66:       call MatGetOwnershipRange(A,Istart,Iend,ierr)

 68: !  Set matrix elements.
 69: !   - Each processor needs to insert only elements that it owns
 70: !     locally (but any non-local elements will be sent to the
 71: !     appropriate processor during matrix assembly).
 72: !   - Always specify global rows and columns of matrix entries.

 74:       do 10, II=Istart,Iend-1
 75:         v = -1.0
 76:         i = II/n
 77:         j = II - i*n
 78:         if (i.gt.0) then
 79:           JJ = II - n
 80:           call MatSetValues(A,1,II,1,JJ,v,ADD_VALUES,ierr)
 81:         endif
 82:         if (i.lt.m-1) then
 83:           JJ = II + n
 84:           call MatSetValues(A,1,II,1,JJ,v,ADD_VALUES,ierr)
 85:         endif
 86:         if (j.gt.0) then
 87:           JJ = II - 1
 88:           call MatSetValues(A,1,II,1,JJ,v,ADD_VALUES,ierr)
 89:         endif
 90:         if (j.lt.n-1) then
 91:           JJ = II + 1
 92:           call MatSetValues(A,1,II,1,JJ,v,ADD_VALUES,ierr)
 93:         endif
 94:         v = 4.0
 95:         call  MatSetValues(A,1,II,1,II,v,ADD_VALUES,ierr)
 96:  10   continue

 98: !  Assemble matrix, using the 2-step process:
 99: !       MatAssemblyBegin(), MatAssemblyEnd()
100: !  Computations can be done while messages are in transition
101: !  by placing code between these two statements.

103:       call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
104:       call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)

106: !  Create parallel vectors.
107: !   - When using VecCreate(), the parallel partitioning of the vector
108: !     is determined by PETSc at runtime.
109: !   - Note: We form 1 vector from scratch and then duplicate as needed.

111:       call VecCreate(PETSC_COMM_WORLD,u,ierr)
112:       call VecSetSizes(u,PETSC_DECIDE,m*n,ierr)
113:       call VecSetFromOptions(u,ierr)
114:       call VecDuplicate(u,b,ierr)
115:       call VecDuplicate(b,x,ierr)

117: !  Create linear solver context

119:       call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)

121: !  Set runtime options (e.g., -ksp_type <type> -pc_type <type>)

123:       call KSPSetFromOptions(ksp,ierr)

125: !  Solve several linear systems in succession

127:       do 100 i=1,nsteps
128:          call solve1(ksp,A,x,b,u,i,nsteps,ierr)
129:  100  continue

131: !  Free work space.  All PETSc objects should be destroyed when they
132: !  are no longer needed.

134:       call VecDestroy(u,ierr)
135:       call VecDestroy(x,ierr)
136:       call VecDestroy(b,ierr)
137:       call MatDestroy(A,ierr)
138:       call KSPDestroy(ksp,ierr)

140:       call PetscFinalize(ierr)
141:       end

143: ! -----------------------------------------------------------------------
144: !
145:       subroutine solve1(ksp,A,x,b,u,count,nsteps,ierr)

147:  #include include/finclude/petsc.h
148:  #include include/finclude/petscvec.h
149:  #include include/finclude/petscmat.h
150:  #include include/finclude/petscpc.h
151:  #include include/finclude/petscksp.h

153: !
154: !   solve1 - This routine is used for repeated linear system solves.
155: !   We update the linear system matrix each time, but retain the same
156: !   preconditioning matrix for all linear solves.
157: !
158: !      A - linear system matrix
159: !      A2 - preconditioning matrix
160: !
161:       PetscScalar  v,val
162:       integer II,ierr,Istart,Iend,count,nsteps
163:       Mat     A
164:       KSP     ksp
165:       Vec     x,b,u

167: ! Use common block to retain matrix between successive subroutine calls
168:       Mat              A2
169:       integer          rank,pflag
170:       common /my_data/ A2,pflag,rank

172: ! First time thorough: Create new matrix to define the linear system
173:       if (count .eq. 1) then
174:         call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
175:         pflag = 0
176:         call PetscOptionsHasName(PETSC_NULL_CHARACTER,'-mat_view',       &
177:      &       pflag,ierr)
178:         if (pflag .ne. 0) then
179:           if (rank .eq. 0) write(6,100)
180:         endif
181:         call MatConvert(A,MATSAME,MAT_INITIAL_MATRIX,A2,ierr)
182: ! All other times: Set previous solution as initial guess for next solve.
183:       else
184:         call KSPSetInitialGuessNonzero(ksp,PETSC_TRUE,ierr)
185:       endif

187: ! Alter the matrix A a bit
188:       call MatGetOwnershipRange(A,Istart,Iend,ierr)
189:       do 20, II=Istart,Iend-1
190:         v = 2.0
191:         call MatSetValues(A,1,II,1,II,v,ADD_VALUES,ierr)
192:  20   continue
193:       call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
194:       if (pflag .ne. 0) then
195:         if (rank .eq. 0) write(6,110)
196:       endif
197:       call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)

199: ! Set the exact solution; compute the right-hand-side vector
200:       val = 1.0*count
201:       call VecSet(u,val,ierr)
202:       call MatMult(A,u,b,ierr)

204: ! Set operators, keeping the identical preconditioner matrix for
205: ! all linear solves.  This approach is often effective when the
206: ! linear systems do not change very much between successive steps.
207:       call KSPSetOperators(ksp,A,A2,SAME_PRECONDITIONER,ierr)

209: ! Solve linear system
210:       call KSPSolve(ksp,b,x,ierr)

212: ! Destroy the preconditioner matrix on the last time through
213:       if (count .eq. nsteps) call MatDestroy(A2,ierr)

215:  100  format('previous matrix: preconditioning')
216:  110  format('next matrix: defines linear system')

218:       end