MPI: It's Easy to Get Started

For basic applications, MPI is as easy to use as any other message-passing system. The sample code below contains the complete communications skeleton for a dynamically load balanced master/slave application. Following the code is a description of the few functions necessary to write typical parallel applications.

#include <mpi.h>

#define WORKTAG		1
#define DIETAG		2

main(argc, argv)

int			argc;
char			*argv[];

{
	int		myrank;

	MPI_Init(&argc, &argv);		/* initialize MPI */
	MPI_Comm_rank(MPI_COMM_WORLD,	/* always use this */
			&myrank);	/* process rank, 0 thru N-1 */

	if (myrank == 0) {
		master();
	} else {
		slave();
	}

	MPI_Finalize();			/* cleanup MPI */
}

master()

{
	int		ntasks, rank, work;
	double		result;
	MPI_Status	status;

	MPI_Comm_size(MPI_COMM_WORLD,	/* always use this */
			&ntasks);	/* #processes in application */
/*
 * Seed the slaves.
 */
	for (rank = 1; rank < ntasks; ++rank) {

		work = /* get_next_work_request */;

		MPI_Send(&work,		/* message buffer */
			1,		/* one data item */
			MPI_INT,	/* data item is an integer */
			rank,		/* destination process rank */
			WORKTAG,	/* user chosen message tag */
			MPI_COMM_WORLD);/* always use this */
	}
/*
 * Receive a result from any slave and dispatch a new work request
 * work requests have been exhausted.
 */
	work = /* get_next_work_request */;

	while (/* valid new work request */) {

		MPI_Recv(&result,	/* message buffer */
			1,		/* one data item */
			MPI_DOUBLE,	/* data item is a double real */
			MPI_ANY_SOURCE,	/* receive from any sender */
			MPI_ANY_TAG,	/* receive any type of message */
			MPI_COMM_WORLD,	/* always use this */
			&status);	/* info about received message */

		MPI_Send(&work, 1, MPI_INT, status.MPI_SOURCE,
				WORKTAG, MPI_COMM_WORLD);

		work = /* get_next_work_request */;
	}
/*
 * Receive results for outstanding work requests.
 */
	for (rank = 1; rank < ntasks; ++rank) {
		MPI_Recv(&result, 1, MPI_DOUBLE, MPI_ANY_SOURCE,
				MPI_ANY_TAG, MPI_COMM_WORLD, &status);
	}
/*
 * Tell all the slaves to exit.
 */
	for (rank = 1; rank < ntasks; ++rank) {
		MPI_Send(0, 0, MPI_INT, rank, DIETAG, MPI_COMM_WORLD);
	}
}

slave()

{
	double		result;
	int		work;
	MPI_Status	status;

	for (;;) {
		MPI_Recv(&work, 1, MPI_INT, 0, MPI_ANY_TAG,
				MPI_COMM_WORLD, &status);
/*
 * Check the tag of the received message.
 */
		if (status.MPI_TAG == DIETAG) {
			return;
		}

		result = /* do the work */;

		MPI_Send(&result, 1, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD);
	}
}

The World of MPI

Enter and Exit MPI

	MPI_Init(&argc, &argv);
	MPI_Finalize();

Who Am I? Who Are They?

	int		myrank;

	MPI_Comm_rank(MPI_COMM_WORLD, &myrank);

	int		nprocs;

	MPI_Comm_size(MPI_COMM_WORLD, &nprocs);

Sending Messages

A message is sent to a specific process and is marked by a tag (integer value) specified by the user. Tags are used to distinguish between different message types a process might send/receive. In the sample code above, the tag is used to distinguish between work and termination messages.

	MPI_Send(buffer, count, datatype, destination, tag, MPI_COMM_WORLD);

Receiving Messages

	MPI_Recv(buffer, maxcount, datatype,
	         source, tag, MPI_COMM_WORLD, &status);

Another function, not used in the sample code, returns the number of datatype elements received. It is used when the number of elements received might be smaller than `maxcount'.

	MPI_Get_count(&status, datatype, &nelements);