1: !
  2: !     Program usage: mpiexec -n 2 ex4f [-help] [all PETSc options]
  3: !
  4: !     This introductory example illustrates running PETSc on a subset
  5: !     of processes
  6: !
  7: !/*T
  8: !   Concepts: introduction to PETSc;
  9: !   Concepts: process^subset set PETSC_COMM_WORLD
 10: !   Processors: 2
 11: !T*/
 12: ! -----------------------------------------------------------------------

 14:       program main
 15:       use petscsys
 16:       implicit none

 18:       integer ierr
 19:       integer rank, size

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

 24:       call MPI_Init(ierr)

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

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

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

 35:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)

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

 40:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr)
 41:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)


 44: !     Here we would like to print only one message that represents all
 45: !     the processes in the group.
 46:       if (rank .eq. 0) write(6,100) size,rank
 47:  100  format("No of Procs = ",i4," rank = ",i4)

 49: !     Always call PetscFinalize() before exiting a program.  This
 50: !     routine - finalizes the PETSc libraries as well as MPI - provides
 51: !     summary and diagnostic information if certain runtime options are
 52: !     chosen (e.g., -log_summary).  See PetscFinalize() manpage for more
 53: !     information.

 55:       call PetscFinalize(ierr)

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

 59:       call  MPI_Finalize(ierr)
 60:       end