Actual source code: ex4f.F
petsc-3.12.4 2020-02-04
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      1: !
      2: !     This introductory example illustrates running PETSc on a subset
      3: !     of processes
      4: !
      5: !/*T
      6: !   Concepts: introduction to PETSc;
      7: !   Concepts: process^subset set PETSC_COMM_WORLD
      8: !   Processors: 2
      9: !T*/
     10: ! -----------------------------------------------------------------------

     12:       program main
     13:  #include <petsc/finclude/petscsys.h>
     14:       use petscsys
     15:       implicit none
     16:       PetscErrorCode ierr
     17:       PetscMPIInt    rank, size,grank
     18:       PetscReal globalrank

     20: !     We must call MPI_Init() first, making us, not PETSc, responsible
     21: !     for MPI

     23:       call MPI_Init(ierr)

     25: !     We can now change the communicator universe for PETSc

     27:       call MPI_Comm_rank(MPI_COMM_WORLD,rank,ierr)
     28:       call MPI_Comm_split(MPI_COMM_WORLD,mod(rank,2),0,                 &
     29:      &     PETSC_COMM_WORLD,ierr)

     31: !     Every PETSc routine should begin with the PetscInitialize()
     32: !     routine.

     34:       call PetscInitializeNoArguments(ierr)
     35:       if (ierr .ne. 0) then
     36:          print*,'Unable to initialize PETSc'
     37:          stop
     38:       endif

     40: !     The following MPI calls return the number of processes being used
     41: !     and the rank of this process in the group.

     43:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr)
     44:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)

     47: !     Here we would like to print only one message that represents all
     48: !     the processes in the group. Sleep so that IO from different ranks
     49: !     don't get mixed up. Note this is not an ideal solution
     50:       call MPI_Comm_rank(MPI_COMM_WORLD,grank,ierr)
     51:       globalrank = grank
     52:       call PetscSleep(globalrank,ierr)
     53:       if (rank .eq. 0) write(6,100) size,rank
     54:  100  format('No of Procs = ',i4,' rank = ',i4)

     56: !     Always call PetscFinalize() before exiting a program.  This
     57: !     routine - finalizes the PETSc libraries as well as MPI - provides
     58: !     summary and diagnostic information if certain runtime options are
     59: !     chosen (e.g., -log_view).  See PetscFinalize() manpage for more
     60: !     information.

     62:       call PetscFinalize(ierr)

     64:       call MPI_Comm_free(PETSC_COMM_WORLD,ierr)

     67: !     Since we initialized MPI, we must call MPI_Finalize()

     69:       call  MPI_Finalize(ierr)
     70:       end

     72: !/*TEST
     73: !
     74: !   test:
     75: !      nsize: 5
     76: !      filter: sort -b
     77: !      filter_output: sort -b
     78: !
     79: !TEST*/