Master-Slave non-blocking listener - c

Question

Currently I'm trying to create a master-slave program with a "listener" loop where the master waits for a message from slaves to make a decision from there. But, despite using non-blocking MPI routines, I am experiencing an error. Do I need to use some blocking routine?
#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char** argv)
{
// Variable Declarations
int rank, size;
MPI_Request *requestList,requestNull;
MPI_Status status;
// Start MPI
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if( rank == 0 )
{
// Process Zero
int dataOut=13, pr;
float dataIn = -1;
requestList =(MPI_Request*)malloc((size-1)*sizeof(MPI_Request));
while(1){
dataIn = -1;
// We do NOT need to wait for the MPI_ Isend(s), it is the job of the receiver processes.
for(pr=1;pr<size;pr++)
{
MPI_Irecv(&dataIn,1,MPI_FLOAT,pr,1,MPI_COMM_WORLD,&(requestList[pr-1]));
}
if((dataIn > 1.5)){
printf("From the process: %f\n", dataIn);
break;
}
}
}
else
{
// Receiver Process
float message;
int index;
//MPI_Request request;
MPI_Status status;
while(1){
message = random()/(double)1147483648;
// Send the message back to the process zero
MPI_Isend(&message,1,MPI_FLOAT,0,1,MPI_COMM_WORLD, &requestNull);
if(message > 1.5)
break;
}
}
MPI_Finalize();
return 0;
}

Answer 1

The problem seems to be that you're never waiting for your MPI calls to complete.
The way non-blocking calls work in MPI is that when you issue a non-blocking call (like MPI_IRECV), the last input parameter is an MPI_REQUEST object. When the initialization call (MPI_IRECV) is done, that request object contains the information for the non-blocking call. However, that call isn't done yet and you have no guarantee that the call is done until you use a completion call (MPI_WAIT/MPI_TEST/and friends) on the request.
In your case, you probably are using the non-blocking calls unnecessarily since you rely on the information received during the MPI_IRECV call in the next line. You should probably just substitute your non-blocking call for a blocking MPI_RECV call and use MPI_ANY_SOURCE so you don't have to post a separate receive call for each rank in the communicator. Alternatively, you can use MPI_WAITANY to complete your non-blocking calls, but then you'll need to worry about cleaning up all of the extra operations when you finish.