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:       implicit none
 14: #include <petsc/finclude/petscsys.h>
 15:       PetscErrorCode ierr
 16:       PetscMPIInt    rank, size

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

 21:       call MPI_Init(ierr)

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

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

 29: !     Every PETSc routine should begin with the PetscInitialize()
 30: !     routine.

 32:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)

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

 37:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr)
 38:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)


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

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

 52:       call PetscFinalize(ierr)

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

 56:       call  MPI_Finalize(ierr)
 57:       end