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

 17:       PetscErrorCode ierr
 18:       PetscMPIInt rank, size, two

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

 23:       call MPI_Init(ierr)
 24:       if (ierr .ne. 0) then
 25:          print*,'Unable to initialize MPI'
 26:          stop
 27:       endif

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

 31:       two = 2
 32:       call MPI_Comm_rank(MPI_COMM_WORLD,rank,ierr)
 33:       call MPI_Comm_split(MPI_COMM_WORLD,mod(rank,two),0,PETSC_COMM_WORLD,ierr)

 35: !     Every PETSc routine should begin with the PetscInitialize()
 36: !     routine.

 38:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 39:       if (ierr .ne. 0) then
 40:         print*,'Unable to initialize PETSc'
 41:         stop
 42:       endif

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

 47:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr);CHKERRA(ierr)
 48:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr);CHKERRA(ierr)


 51: !     Here we would like to print only one message that represents all
 52: !     the processes in the group.
 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)
 63:       call MPI_Comm_free(PETSC_COMM_WORLD,ierr)

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

 67:       call  MPI_Finalize(ierr)
 68:       end

 70: !
 71: !/*TEST
 72: !
 73: !   test:
 74: !
 75: !TEST*/