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MPI uses an include file to set parameters (in |
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The MPI_COMM_WORLD parameter is a communicator (value) that is defined for all processes that are initialized for MPI. A communicator gives a message an identifying context to distinguish it from messages of other communicators. In many programs only a single communicator is needed but in more complicated situations and library development, multiple contexts may be required. A communicator is analogous to a channel of a citizen's band transceiver: chatter on one channel does not interfere (cannot be heard) on any of the other channels. The MPI_COMM_WORLD parameter might have just as well been named mpi_comm_original because it is a pre-defined communicator assigned to the initial group of all processes.
There are two information subprograms that you will probably want to use at the beginning of a program; and both require the world communicator. MPI_COMM_SIZE and MPI_COMM_RANK return the number of processors and a process number, respectively. In MPI jargon, the unique number assigned to a process is its rank. The world communicator ranks range from 0 to NPES-1, the number of processors less one. The syntax for these two subroutines is:
call mpi_comm_size (icomm, inpes, ierr) call mpi_comm_rank (icomm, irank, ierr)
where:
| Parameter | Description | Status |
|---|---|---|
icomm
| communicator (use parameter MPI_COMM_WORLD as predefined context for initial processes) | [IN] |
inpes
|
number of processes | [OUT] |
irank
|
process number, numbered between 0 and inpes-1
|
[OUT] |
ierr
|
MPI error number (0 = no error) | [OUT] |
In the following code the number of processes and the rank of each process are reported. The first argument of the size and rank subroutines is the communicator; the second is the respective returned value; and the third is a returned error value for the call.
Program param
include 'mpif.h'
call mpi_init(ierr)
call mpi_comm_size(MPI_COMM_WORLD,npes, ierr)
call mpi_comm_rank(MPI_COMM_WORLD,irank,ierr)
print*,npes,irank,' = number of procs and process rank'
call mpi_finalize(ierr)
end
