1: !
  2: !     This introductory example illustrates running PETSc on a subset
  3: !     of processes
  4: !
  5: ! -----------------------------------------------------------------------

  7:       program main
  8: #include <petsc/finclude/petscsys.h>
  9:       use petscmpi  ! or mpi or mpi_f08
 10:       use petscsys
 11:       implicit none

 13:       PetscErrorCode ierr
 14:       PetscMPIInt rank, size, zero, two

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

 19:       PetscCallMPIA(MPI_Init(ierr))

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

 23:       zero = 0
 24:       two = 2
 25:       PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD,rank,ierr))
 26:       PetscCallMPIA(MPI_Comm_split(MPI_COMM_WORLD,mod(rank,two),zero,PETSC_COMM_WORLD,ierr))

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

 31:       PetscCallA(PetscInitialize(ierr))

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

 36:       PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD,size,ierr))
 37:       PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr))

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

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

 50:       PetscCallA(PetscFinalize(ierr))
 51:       PetscCallMPIA(MPI_Comm_free(PETSC_COMM_WORLD,ierr))

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

 55:       PetscCallMPIA(MPI_Finalize(ierr))
 56:       end

 58: !
 59: !/*TEST
 60: !
 61: !   test:
 62: !
 63: !TEST*/