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ring.c
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Fri, Feb 14, 13:27

ring.c
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#include <stdio.h>
#include <mpi.h>
// Rotate values clockwise on a ring with MPI
// ------------------------------------------
int main (int argc, char * argv [])
{
// Each task owns its private set of the following variables
// ---------------------------------------------------------
int my_rank, tasks_number;
char machine_name [512];
int name_length;
int my_var, left_neighbour, right_neighbour;
MPI_Status status;
// We should start with...
// -----------------------
MPI_Init (& argc, & argv);
// Getting information about the MPI environnement
// -----------------------------------------------
MPI_Comm_size (MPI_COMM_WORLD, & tasks_number);
MPI_Comm_rank (MPI_COMM_WORLD, & my_rank);
MPI_Get_processor_name (machine_name, & name_length);
// Initialization
// --------------
my_var = my_rank * 10;
right_neighbour = (my_rank + 1) % tasks_number;
left_neighbour =
(my_rank-1 >= 0)
? my_rank-1
: tasks_number-1; /* rank of the sender */
// Show initial status
// -------------------
printf (
"Hello, my rank is %d among %d tasks on machine %s, with my_var = %d initially\n",
my_rank, tasks_number, machine_name, my_var
);
fflush (stdout);
// Let's wait until all the tasks have reached this point
// ------------------------------------------------------
MPI_Barrier (MPI_COMM_WORLD);
if (my_rank == 0){
printf ("--- BARRIER! ---\n");
fflush (stdout);
}
// Exchange values with the neighbours
// -----------------------------------
printf (
"%s: rank %d sends to %d and receives from %d, my_var = %d\n",
machine_name, my_rank, right_neighbour, left_neighbour, my_var );
MPI_Sendrecv_replace (
& my_var, 1, MPI_INT,
right_neighbour, 0, left_neighbour, 0,
MPI_COMM_WORLD, & status );
// Let's wait until all the tasks have reached this point
// ------------------------------------------------------
MPI_Barrier (MPI_COMM_WORLD);
if (my_rank == 0){
printf ("--- BARRIER! ---\n");
fflush (stdout);
}
// Show final status
// -------------------
printf (
"Hello, my rank is %d among %d tasks on machine %s, with my_var = %d finally\n",
my_rank, tasks_number, machine_name, my_var
);
fflush (stdout);
// ...and we should terminate by...
// --------------------------------
MPI_Finalize ();
return 0;
}

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