Actual source code: ex5f90.F

  1: !
  2: !  Description: Solves a nonlinear system in parallel with SNES.
  3: !  We solve the  Bratu (SFI - solid fuel ignition) problem in a 2D rectangular
  4: !  domain, using distributed arrays (DAs) to partition the parallel grid.
  5: !  The command line options include:
  6: !    -par <parameter>, where <parameter> indicates the nonlinearity of the problem
  7: !       problem SFI:  <parameter> = Bratu parameter (0 <= par <= 6.81)
  8: !
  9: !/*T
 10: !  Concepts: SNES^parallel Bratu example
 11: !  Concepts: DA^using distributed arrays;
 12: !  Processors: n
 13: !T*/
 14: !
 15: !  --------------------------------------------------------------------------
 16: !
 17: !  Solid Fuel Ignition (SFI) problem.  This problem is modeled by
 18: !  the partial differential equation
 19: !
 20: !          -Laplacian u - lambda*exp(u) = 0,  0 < x,y < 1,
 21: !
 22: !  with boundary conditions
 23: !
 24: !           u = 0  for  x = 0, x = 1, y = 0, y = 1.
 25: !
 26: !  A finite difference approximation with the usual 5-point stencil
 27: !  is used to discretize the boundary value problem to obtain a nonlinear
 28: !  system of equations.
 29: !
 30: !  The uniprocessor version of this code is snes/examples/tutorials/ex4f.F
 31: !
 32: !  --------------------------------------------------------------------------
 33: !  The following define must be used before including any PETSc include files
 34: !  into a module or interface. This is because they can't handle declarations
 35: !  in them
 36: !

 38:       module f90module
 39:       type userctx
 40: #define PETSC_AVOID_DECLARATIONS
 41:  #include include/finclude/petsc.h
 42:  #include include/finclude/petscvec.h
 43:  #include include/finclude/petscda.h
 44: #undef PETSC_AVOID_DECLARATIONS
 45:         DA      da
 46:         integer xs,xe,xm,gxs,gxe,gxm
 47:         integer ys,ye,ym,gys,gye,gym
 48:         integer mx,my,rank
 49:         double precision lambda
 50:       end type userctx
 51:       contains
 52: ! ---------------------------------------------------------------------
 53: !
 54: !  FormFunction - Evaluates nonlinear function, F(x).
 55: !
 56: !  Input Parameters:
 57: !  snes - the SNES context
 58: !  X - input vector
 59: !  dummy - optional user-defined context, as set by SNESSetFunction()
 60: !          (not used here)
 61: !
 62: !  Output Parameter:
 63: !  F - function vector
 64: !
 65: !  Notes:
 66: !  This routine serves as a wrapper for the lower-level routine
 67: !  "FormFunctionLocal", where the actual computations are
 68: !  done using the standard Fortran style of treating the local
 69: !  vector data as a multidimensional array over the local mesh.
 70: !  This routine merely handles ghost point scatters and accesses
 71: !  the local vector data via VecGetArrayF90() and VecRestoreArrayF90().
 72: !
 73:       subroutine FormFunction(snes,X,F,user,ierr)
 74:       implicit none

 76:  #include include/finclude/petsc.h
 77:  #include include/finclude/petscvec.h
 78:  #include include/finclude/petscda.h
 79:  #include include/finclude/petscis.h
 80:  #include include/finclude/petscmat.h
 81:  #include include/finclude/petscksp.h
 82:  #include include/finclude/petscpc.h
 83:  #include include/finclude/petscsnes.h

 85: #include "include/finclude/petscvec.h90"


 88: !  Input/output variables:
 89:       SNES           snes
 90:       Vec            X,F
 91:       integer        ierr
 92:       type (userctx) user

 94: !  Declarations for use with local arrays:
 95:       PetscScalar,pointer :: lx_v(:),lf_v(:)
 96:       Vec            localX

 98: !  Scatter ghost points to local vector, using the 2-step process
 99: !     DAGlobalToLocalBegin(), DAGlobalToLocalEnd().
100: !  By placing code between these two statements, computations can
101: !  be done while messages are in transition.

103:       call DAGetLocalVector(user%da,localX,ierr)
104:       call DAGlobalToLocalBegin(user%da,X,INSERT_VALUES,                &
105:      &     localX,ierr)
106:       call DAGlobalToLocalEnd(user%da,X,INSERT_VALUES,localX,ierr)

108: !  Get a pointer to vector data.
109: !    - For default PETSc vectors, VecGetArray90() returns a pointer to
110: !      the data array. Otherwise, the routine is implementation dependent.
111: !    - You MUST call VecRestoreArrayF90() when you no longer need access to
112: !      the array.
113: !    - Note that the interface to VecGetArrayF90() differs from VecGetArray(),
114: !      and is useable from Fortran-90 Only.

116:       call VecGetArrayF90(localX,lx_v,ierr)
117:       call VecGetArrayF90(F,lf_v,ierr)

119: !  Compute function over the locally owned part of the grid

121:       call FormFunctionLocal(lx_v,lf_v,user,ierr)

123: !  Restore vectors

125:       call VecRestoreArrayF90(localX,lx_v,ierr)
126:       call VecRestoreArrayF90(F,lf_v,ierr)

128: !  Insert values into global vector

130:       call DARestoreLocalVector(user%da,localX,ierr)
131:       call PetscLogFlops(11*user%ym*user%xm,ierr)

133: !      call VecView(X,PETSC_VIEWER_STDOUT_WORLD,ierr)
134: !      call VecView(F,PETSC_VIEWER_STDOUT_WORLD,ierr)

136:       return
137:       end subroutine formfunction
138:       end module f90module



142:       program main
143:       use f90module
144:       implicit none
145: !
146: !
147:  #include include/finclude/petsc.h
148:  #include include/finclude/petscvec.h
149:  #include include/finclude/petscda.h
150:  #include include/finclude/petscis.h
151:  #include include/finclude/petscmat.h
152:  #include include/finclude/petscksp.h
153:  #include include/finclude/petscpc.h
154:  #include include/finclude/petscsnes.h
155: #include "include/finclude/petscvec.h90"
156: #include "include/finclude/petscda.h90"

158: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
159: !                   Variable declarations
160: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
161: !
162: !  Variables:
163: !     snes        - nonlinear solver
164: !     x, r        - solution, residual vectors
165: !     J           - Jacobian matrix
166: !     its         - iterations for convergence
167: !     Nx, Ny      - number of preocessors in x- and y- directions
168: !     matrix_free - flag - 1 indicates matrix-free version
169: !
170: !
171:       SNES                   snes
172:       Vec                    x,r
173:       Mat                    J
174:       integer                its,matrix_free,flg,ierr
175:       double precision       lambda_max,lambda_min
176:       type (userctx)         user

178: !  Note: Any user-defined Fortran routines (such as FormJacobian)
179: !  MUST be declared as external.

181:       external FormInitialGuess,FormJacobian

183: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
184: !  Initialize program
185: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

187:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
188:       call MPI_Comm_rank(PETSC_COMM_WORLD,user%rank,ierr)

190: !  Initialize problem parameters

192:       lambda_max  = 6.81
193:       lambda_min  = 0.0
194:       user%lambda = 6.0
195:       call PetscOptionsGetReal(PETSC_NULL_CHARACTER,'-par',             &
196:      &     user%lambda,flg,ierr)
197:       if (user%lambda .ge. lambda_max .or. user%lambda .le. lambda_min) &
198:      &     then
199:          if (user%rank .eq. 0) write(6,*) 'Lambda is out of range'
200:          SETERRQ(1,' ',ierr)
201:       endif


204: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
205: !  Create nonlinear solver context
206: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

208:       call SNESCreate(PETSC_COMM_WORLD,snes,ierr)

210: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
211: !  Create vector data structures; set function evaluation routine
212: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

214: !  Create distributed array (DA) to manage parallel grid and vectors

216: ! This really needs only the star-type stencil, but we use the box
217: ! stencil temporarily.
218:       call DACreate2d(PETSC_COMM_WORLD,DA_NONPERIODIC,DA_STENCIL_BOX,   &
219:      &     -4,-4,PETSC_DECIDE,PETSC_DECIDE,1,1,                         &
220:      &     PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,user%da,ierr)
221:       call DAGetInfo(user%da,PETSC_NULL_INTEGER,user%mx,user%my,        &
222:      &               PETSC_NULL_INTEGER,                                &
223:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
224:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
225:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
226:      &               PETSC_NULL_INTEGER,ierr)
227: 
228: !
229: !   Visualize the distribution of the array across the processors
230: !
231: !     call DAView(user%da,PETSC_VIEWER_DRAW_WORLD,ierr)

233: !  Extract global and local vectors from DA; then duplicate for remaining
234: !  vectors that are the same types

236:       call DACreateGlobalVector(user%da,x,ierr)
237:       call VecDuplicate(x,r,ierr)

239: !  Get local grid boundaries (for 2-dimensional DA)

241:       call DAGetCorners(user%da,user%xs,user%ys,PETSC_NULL_INTEGER,     &
242:      &     user%xm,user%ym,PETSC_NULL_INTEGER,ierr)
243:       call DAGetGhostCorners(user%da,user%gxs,user%gys,                 &
244:      &     PETSC_NULL_INTEGER,user%gxm,user%gym,                        &
245:      &     PETSC_NULL_INTEGER,ierr)

247: !  Here we shift the starting indices up by one so that we can easily
248: !  use the Fortran convention of 1-based indices (rather 0-based indices).

250:       user%xs  = user%xs+1
251:       user%ys  = user%ys+1
252:       user%gxs = user%gxs+1
253:       user%gys = user%gys+1

255:       user%ye  = user%ys+user%ym-1
256:       user%xe  = user%xs+user%xm-1
257:       user%gye = user%gys+user%gym-1
258:       user%gxe = user%gxs+user%gxm-1

260: !  Set function evaluation routine and vector

262:       call SNESSetFunction(snes,r,FormFunction,user,ierr)

264: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
265: !  Create matrix data structure; set Jacobian evaluation routine
266: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

268: !  Set Jacobian matrix data structure and default Jacobian evaluation
269: !  routine. User can override with:
270: !     -snes_fd : default finite differencing approximation of Jacobian
271: !     -snes_mf : matrix-free Newton-Krylov method with no preconditioning
272: !                (unless user explicitly sets preconditioner)
273: !     -snes_mf_operator : form preconditioning matrix as set by the user,
274: !                         but use matrix-free approx for Jacobian-vector
275: !                         products within Newton-Krylov method
276: !
277: !  Note:  For the parallel case, vectors and matrices MUST be partitioned
278: !     accordingly.  When using distributed arrays (DAs) to create vectors,
279: !     the DAs determine the problem partitioning.  We must explicitly
280: !     specify the local matrix dimensions upon its creation for compatibility
281: !     with the vector distribution.  Thus, the generic MatCreate() routine
282: !     is NOT sufficient when working with distributed arrays.
283: !
284: !     Note: Here we only approximately preallocate storage space for the
285: !     Jacobian.  See the users manual for a discussion of better techniques
286: !     for preallocating matrix memory.

288:       call PetscOptionsHasName(PETSC_NULL_CHARACTER,'-snes_mf',         &
289:      &     matrix_free,ierr)
290:       if (matrix_free .eq. 0) then
291:         call DAGetMatrix(user%da,MATAIJ,J,ierr)
292:         call SNESSetJacobian(snes,J,J,FormJacobian,user,ierr)
293:       endif

295: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
296: !  Customize nonlinear solver; set runtime options
297: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

299: !  Set runtime options (e.g., -snes_monitor -snes_rtol <rtol> -ksp_type <type>)

301:       call SNESSetFromOptions(snes,ierr)

303: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
304: !  Evaluate initial guess; then solve nonlinear system.
305: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

307: !  Note: The user should initialize the vector, x, with the initial guess
308: !  for the nonlinear solver prior to calling SNESSolve().  In particular,
309: !  to employ an initial guess of zero, the user should explicitly set
310: !  this vector to zero by calling VecSet().

312:       call FormInitialGuess(user,x,ierr)
313:       call SNESSolve(snes,PETSC_NULL_OBJECT,x,ierr)
314:       call SNESGetIterationNumber(snes,its,ierr);
315:       if (user%rank .eq. 0) then
316:          write(6,100) its
317:       endif
318:   100 format('Number of Newton iterations = ',i5)

320: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
321: !  Free work space.  All PETSc objects should be destroyed when they
322: !  are no longer needed.
323: ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

325:       if (matrix_free .eq. 0) call MatDestroy(J,ierr)
326:       call VecDestroy(x,ierr)
327:       call VecDestroy(r,ierr)
328:       call SNESDestroy(snes,ierr)
329:       call DADestroy(user%da,ierr)
330:       call PetscFinalize(ierr)
331:       end

333: ! ---------------------------------------------------------------------
334: !
335: !  FormInitialGuess - Forms initial approximation.
336: !
337: !  Input Parameters:
338: !  X - vector
339: !
340: !  Output Parameter:
341: !  X - vector
342: !
343: !  Notes:
344: !  This routine serves as a wrapper for the lower-level routine
345: !  "InitialGuessLocal", where the actual computations are
346: !  done using the standard Fortran style of treating the local
347: !  vector data as a multidimensional array over the local mesh.
348: !  This routine merely handles ghost point scatters and accesses
349: !  the local vector data via VecGetArrayF90() and VecRestoreArrayF90().
350: !
351:       subroutine FormInitialGuess(user,X,ierr)
352:       use f90module
353:       implicit none

355: #include "include/finclude/petscvec.h90"
356:  #include include/finclude/petsc.h
357:  #include include/finclude/petscvec.h
358:  #include include/finclude/petscda.h
359:  #include include/finclude/petscis.h
360:  #include include/finclude/petscmat.h
361:  #include include/finclude/petscksp.h
362:  #include include/finclude/petscpc.h
363:  #include include/finclude/petscsnes.h

365: !  Input/output variables:
366:       type (userctx)         user
367:       Vec      X
368:       integer  ierr
369: 
370: !  Declarations for use with local arrays:
371:       PetscScalar,pointer :: lx_v(:)
372:       Vec               localX

374:       0

376: !  Get a pointer to vector data.
377: !    - For default PETSc vectors, VecGetArray90() returns a pointer to
378: !      the data array. Otherwise, the routine is implementation dependent.
379: !    - You MUST call VecRestoreArrayF90() when you no longer need access to
380: !      the array.
381: !    - Note that the interface to VecGetArrayF90() differs from VecGetArray(),
382: !      and is useable from Fortran-90 Only.

384:       call DAGetLocalVector(user%da,localX,ierr)
385:       call VecGetArrayF90(localX,lx_v,ierr)

387: !  Compute initial guess over the locally owned part of the grid

389:       call InitialGuessLocal(user,lx_v,ierr)

391: !  Restore vector

393:       call VecRestoreArrayF90(localX,lx_v,ierr)

395: !  Insert values into global vector

397:       call DALocalToGlobal(user%da,localX,INSERT_VALUES,X,ierr)
398:       call DARestoreLocalVector(user%da,localX,ierr)

400:       return
401:       end

403: ! ---------------------------------------------------------------------
404: !
405: !  InitialGuessLocal - Computes initial approximation, called by
406: !  the higher level routine FormInitialGuess().
407: !
408: !  Input Parameter:
409: !  x - local vector data
410: !
411: !  Output Parameters:
412: !  x - local vector data
413: !  ierr - error code
414: !
415: !  Notes:
416: !  This routine uses standard Fortran-style computations over a 2-dim array.
417: !
418:       subroutine InitialGuessLocal(user,x,ierr)
419:       use f90module
420:       implicit none

422:  #include include/finclude/petsc.h
423:  #include include/finclude/petscvec.h
424:  #include include/finclude/petscda.h
425:  #include include/finclude/petscis.h
426:  #include include/finclude/petscmat.h
427:  #include include/finclude/petscksp.h
428:  #include include/finclude/petscpc.h
429:  #include include/finclude/petscsnes.h

431: !  Input/output variables:
432:       type (userctx)         user
433:       PetscScalar  x(user%gxs:user%gxe,                                 &
434:      &              user%gys:user%gye)
435:       integer ierr

437: !  Local variables:
438:       integer  i,j
439:       PetscScalar   temp1,temp,hx,hy
440:       PetscScalar   one

442: !  Set parameters

444:       0
445:       one    = 1.0
446:       hx     = one/(dble(user%mx-1))
447:       hy     = one/(dble(user%my-1))
448:       temp1  = user%lambda/(user%lambda + one)

450:       do 20 j=user%ys,user%ye
451:          temp = dble(min(j-1,user%my-j))*hy
452:          do 10 i=user%xs,user%xe
453:             if (i .eq. 1 .or. j .eq. 1                                  &
454:      &             .or. i .eq. user%mx .or. j .eq. user%my) then
455:               x(i,j) = 0.0
456:             else
457:               x(i,j) = temp1 *                                          &
458:      &          sqrt(min(dble(min(i-1,user%mx-i)*hx),dble(temp)))
459:             endif
460:  10      continue
461:  20   continue

463:       return
464:       end

466: ! ---------------------------------------------------------------------
467: !
468: !  FormFunctionLocal - Computes nonlinear function, called by
469: !  the higher level routine FormFunction().
470: !
471: !  Input Parameter:
472: !  x - local vector data
473: !
474: !  Output Parameters:
475: !  f - local vector data, f(x)
476: !  ierr - error code
477: !
478: !  Notes:
479: !  This routine uses standard Fortran-style computations over a 2-dim array.
480: !
481:       subroutine FormFunctionLocal(x,f,user,ierr)
482:       use f90module

484:       implicit none

486: !  Input/output variables:
487:       type (userctx) user
488:       PetscScalar  x(user%gxs:user%gxe,                                 &
489:      &              user%gys:user%gye)
490:       PetscScalar  f(user%xs:user%xe,                                   &
491:      &              user%ys:user%ye)
492:       integer  ierr

494: !  Local variables:
495:       PetscScalar   two,one,hx,hy,hxdhy,hydhx,sc
496:       PetscScalar   u,uxx,uyy
497:       integer  i,j

499:       one    = 1.0
500:       two    = 2.0
501:       hx     = one/dble(user%mx-1)
502:       hy     = one/dble(user%my-1)
503:       sc     = hx*hy*user%lambda
504:       hxdhy  = hx/hy
505:       hydhx  = hy/hx

507: !  Compute function over the locally owned part of the grid

509:       do 20 j=user%ys,user%ye
510:          do 10 i=user%xs,user%xe
511:             if (i .eq. 1 .or. j .eq. 1                                  &
512:      &             .or. i .eq. user%mx .or. j .eq. user%my) then
513:                f(i,j) = x(i,j)
514:             else
515:                u = x(i,j)
516:                uxx = hydhx * (two*u                                     &
517:      &                - x(i-1,j) - x(i+1,j))
518:                uyy = hxdhy * (two*u - x(i,j-1) - x(i,j+1))
519:                f(i,j) = uxx + uyy - sc*exp(u)
520:             endif
521:  10      continue
522:  20   continue

524:       return
525:       end

527: ! ---------------------------------------------------------------------
528: !
529: !  FormJacobian - Evaluates Jacobian matrix.
530: !
531: !  Input Parameters:
532: !  snes     - the SNES context
533: !  x        - input vector
534: !  dummy    - optional user-defined context, as set by SNESSetJacobian()
535: !             (not used here)
536: !
537: !  Output Parameters:
538: !  jac      - Jacobian matrix
539: !  jac_prec - optionally different preconditioning matrix (not used here)
540: !  flag     - flag indicating matrix structure
541: !
542: !  Notes:
543: !  This routine serves as a wrapper for the lower-level routine
544: !  "FormJacobianLocal", where the actual computations are
545: !  done using the standard Fortran style of treating the local
546: !  vector data as a multidimensional array over the local mesh.
547: !  This routine merely accesses the local vector data via
548: !  VecGetArrayF90() and VecRestoreArrayF90().
549: !
550: !  Notes:
551: !  Due to grid point reordering with DAs, we must always work
552: !  with the local grid points, and then transform them to the new
553: !  global numbering with the "ltog" mapping (via DAGetGlobalIndicesF90()).
554: !  We cannot work directly with the global numbers for the original
555: !  uniprocessor grid!
556: !
557: !  Two methods are available for imposing this transformation
558: !  when setting matrix entries:
559: !    (A) MatSetValuesLocal(), using the local ordering (including
560: !        ghost points!)
561: !        - Use DAGetGlobalIndicesF90() to extract the local-to-global map
562: !        - Associate this map with the matrix by calling
563: !          MatSetLocalToGlobalMapping() once
564: !        - Set matrix entries using the local ordering
565: !          by calling MatSetValuesLocal()
566: !    (B) MatSetValues(), using the global ordering
567: !        - Use DAGetGlobalIndicesF90() to extract the local-to-global map
568: !        - Then apply this map explicitly yourself
569: !        - Set matrix entries using the global ordering by calling
570: !          MatSetValues()
571: !  Option (A) seems cleaner/easier in many cases, and is the procedure
572: !  used in this example.
573: !
574:       subroutine FormJacobian(snes,X,jac,jac_prec,flag,user,ierr)
575:       use f90module
576:       implicit none

578:  #include include/finclude/petsc.h
579:  #include include/finclude/petscvec.h
580:  #include include/finclude/petscda.h
581:  #include include/finclude/petscis.h
582:  #include include/finclude/petscmat.h
583:  #include include/finclude/petscksp.h
584:  #include include/finclude/petscpc.h
585:  #include include/finclude/petscsnes.h

587: #include "include/finclude/petscvec.h90"

589: !  Input/output variables:
590:       SNES         snes
591:       Vec          X
592:       Mat          jac,jac_prec
593:       MatStructure flag
594:       type(userctx) user
595:       integer      ierr

597: !  Declarations for use with local arrays:
598:       PetscScalar,pointer :: lx_v(:)
599:       Vec            localX

601: !  Scatter ghost points to local vector, using the 2-step process
602: !     DAGlobalToLocalBegin(), DAGlobalToLocalEnd()
603: !  Computations can be done while messages are in transition,
604: !  by placing code between these two statements.

606:       call DAGetLocalVector(user%da,localX,ierr)
607:       call DAGlobalToLocalBegin(user%da,X,INSERT_VALUES,localX,            &
608:      &     ierr)
609:       call DAGlobalToLocalEnd(user%da,X,INSERT_VALUES,localX,ierr)

611: !  Get a pointer to vector data

613:       call VecGetArrayF90(localX,lx_v,ierr)

615: !  Compute entries for the locally owned part of the Jacobian.

617:       call FormJacobianLocal(lx_v,jac,jac_prec,user,ierr)

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

624:       call MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY,ierr)
625:       call VecRestoreArrayF90(localX,lx_v,ierr)
626:       call DARestoreLocalVector(user%da,localX,ierr)
627:       call MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY,ierr)

629: !  Set flag to indicate that the Jacobian matrix retains an identical
630: !  nonzero structure throughout all nonlinear iterations (although the
631: !  values of the entries change). Thus, we can save some work in setting
632: !  up the preconditioner (e.g., no need to redo symbolic factorization for
633: !  ILU/ICC preconditioners).
634: !   - If the nonzero structure of the matrix is different during
635: !     successive linear solves, then the flag DIFFERENT_NONZERO_PATTERN
636: !     must be used instead.  If you are unsure whether the matrix
637: !     structure has changed or not, use the flag DIFFERENT_NONZERO_PATTERN.
638: !   - Caution:  If you specify SAME_NONZERO_PATTERN, PETSc
639: !     believes your assertion and does not check the structure
640: !     of the matrix.  If you erroneously claim that the structure
641: !     is the same when it actually is not, the new preconditioner
642: !     will not function correctly.  Thus, use this optimization
643: !     feature with caution!

645:       flag = SAME_NONZERO_PATTERN

647: !  Tell the matrix we will never add a new nonzero location to the
648: !  matrix. If we do it will generate an error.

650:        call MatSetOption(jac,MAT_NEW_NONZERO_LOCATION_ERR,ierr)

652:       return
653:       end

655: ! ---------------------------------------------------------------------
656: !
657: !  FormJacobianLocal - Computes Jacobian matrix, called by
658: !  the higher level routine FormJacobian().
659: !
660: !  Input Parameters:
661: !  x        - local vector data
662: !
663: !  Output Parameters:
664: !  jac      - Jacobian matrix
665: !  jac_prec - optionally different preconditioning matrix (not used here)
666: !  ierr     - error code
667: !
668: !  Notes:
669: !  This routine uses standard Fortran-style computations over a 2-dim array.
670: !
671: !  Notes:
672: !  Due to grid point reordering with DAs, we must always work
673: !  with the local grid points, and then transform them to the new
674: !  global numbering with the "ltog" mapping (via DAGetGlobalIndicesF90()).
675: !  We cannot work directly with the global numbers for the original
676: !  uniprocessor grid!
677: !
678: !  Two methods are available for imposing this transformation
679: !  when setting matrix entries:
680: !    (A) MatSetValuesLocal(), using the local ordering (including
681: !        ghost points!)
682: !        - Use DAGetGlobalIndicesF90() to extract the local-to-global map
683: !        - Associate this map with the matrix by calling
684: !          MatSetLocalToGlobalMapping() once
685: !        - Set matrix entries using the local ordering
686: !          by calling MatSetValuesLocal()
687: !    (B) MatSetValues(), using the global ordering
688: !        - Use DAGetGlobalIndicesF90() to extract the local-to-global map
689: !        - Then apply this map explicitly yourself
690: !        - Set matrix entries using the global ordering by calling
691: !          MatSetValues()
692: !  Option (A) seems cleaner/easier in many cases, and is the procedure
693: !  used in this example.
694: !
695:       subroutine FormJacobianLocal(x,jac,jac_prec,user,ierr)
696:       use f90module
697:       implicit none

699:  #include include/finclude/petsc.h
700:  #include include/finclude/petscvec.h
701:  #include include/finclude/petscda.h
702:  #include include/finclude/petscis.h
703:  #include include/finclude/petscmat.h
704:  #include include/finclude/petscksp.h
705:  #include include/finclude/petscpc.h
706:  #include include/finclude/petscsnes.h

708: !  Input/output variables:
709:       type (userctx) user
710:       PetscScalar  x(user%gxs:user%gxe,                                 &
711:      &              user%gys:user%gye)
712:       Mat      jac,jac_prec
713:       integer  ierr

715: !  Local variables:
716:       integer  row,col(5),i,j
717:       PetscScalar   two,one,hx,hy,hxdhy
718:       PetscScalar   hydhx,sc,v(5)

720: !  Set parameters

722:       one    = 1.0
723:       two    = 2.0
724:       hx     = one/dble(user%mx-1)
725:       hy     = one/dble(user%my-1)
726:       sc     = hx*hy
727:       hxdhy  = hx/hy
728:       hydhx  = hy/hx

730: !  Compute entries for the locally owned part of the Jacobian.
731: !   - Currently, all PETSc parallel matrix formats are partitioned by
732: !     contiguous chunks of rows across the processors.
733: !   - Each processor needs to insert only elements that it owns
734: !     locally (but any non-local elements will be sent to the
735: !     appropriate processor during matrix assembly).
736: !   - Here, we set all entries for a particular row at once.
737: !   - We can set matrix entries either using either
738: !     MatSetValuesLocal() or MatSetValues(), as discussed above.
739: !   - Note that MatSetValues() uses 0-based row and column numbers
740: !     in Fortran as well as in C.

742:       do 20 j=user%ys,user%ye
743:          row = (j - user%gys)*user%gxm + user%xs - user%gxs - 1
744:          do 10 i=user%xs,user%xe
745:             row = row + 1
746: !           boundary points
747:             if (i .eq. 1 .or. j .eq. 1                                  &
748:      &             .or. i .eq. user%mx .or. j .eq. user%my) then
749:                col(1) = row
750:                v(1)   = one
751:                call MatSetValuesLocal(jac,1,row,1,col,v,                &
752:      &                           INSERT_VALUES,ierr)
753: !           interior grid points
754:             else
755:                v(1) = -hxdhy
756:                v(2) = -hydhx
757:                v(3) = two*(hydhx + hxdhy)                               &
758:      &                  - sc*user%lambda*exp(x(i,j))
759:                v(4) = -hydhx
760:                v(5) = -hxdhy
761:                col(1) = row - user%gxm
762:                col(2) = row - 1
763:                col(3) = row
764:                col(4) = row + 1
765:                col(5) = row + user%gxm
766:                call MatSetValuesLocal(jac,1,row,5,col,v,                &
767:      &                                INSERT_VALUES,ierr)
768:             endif
769:  10      continue
770:  20   continue

772:       return
773:       end