[MPI] Point to point communication

- MPI_Send : send data to another process
- MPI_Send(buf, count, data_type, dest, tag, comm)

Arguments	Meanings
buf	Starting address of send buffer
count	# of elements
data_type	Data type of each send buffer element
dest	Processor ID (rank) destination
tag	Message tag
comm	communicator

-Examples:
C/C++: MPI_Send(&x,1,MPI_INT,5,0,MPI_COMM_WORLD);
Fortran:MPI_Send(x,1,MPI_INTEGER,5,0,MPI_COMM_WORLD,ierr)

- MPI_Recv : receive data from another process
- MPI_Recv(buf, count, data_type, src, tag, comm, status)

Arguments	Meanings
buf	Starting address of send buffer
count	# of elements
data_type	Data type of each send buffer element
src	Processor ID (rank) destination
tag	Message tag
comm	communicator
status	Status object (an integer array in Fortran)

- Examples
C/C++: MPI_Recv(&x,1,MPI_INT,5,0,MPI_COMM_WORLD,&stat);
Fortran: MPI_Recv(x,1,MPI_INTEGER,5,0,MPI_COMM_WORLD,stat,ierr)

* Notes on MPI_Recv
- A message is received when the followings are matched:
   - Source (sending process ID/rank)
   - Tag
   - Communicator (e.g. MPI_COMM_WORLD)
- Wildcard values may be used:
   - MPI_ANY_TAG : don't care what the tag value is
   - MPI_ANY_SOURCE : don't care where it comes from; always receive

- Send/Recv Fortran90 Example Source Code
program hello
    use mpi
    integer :: f(10)
    integer :: status(MPI_STATUS_SIZE)
    integer :: rank, src=0, dest=1, ierr, i
   
    call mpi_init(ierr) 
    call MPI_Comm_rank( MPI_COMM_WORLD, rank,ierr);
   
    if (rank == src) then  !process “dest” ignores this
        call MPI_Send(f, 10, MPI_INT, dest, 0, MPI_COMM_WORLD,ierr)
        do i = 1, 10
            f(i) = f(i) - i
        end do
    end if
    if (rank == dest) then !process “src” ignores this
        call MPI_Recv(f, 10, MPI_INT, src, 0, MPI_COMM_WORLD, status,ierr)
        do i = 1, 10
            f(i) = f(i) + i
        end do
    end if
    print *, f
    call mpi_finalize(ierr)
end program hello

-Compile & Out put
>>mpiifort mpicomm.f90
>>mpiexec -n 2 mpicomm.exe
          -1          -2          -3          -4          -5          -6          -7          -8          -9         -10
           1           2           3           4           5           6           7           8           9          10