Have to Send MPI Message Twice for Some Reason - c++

Question

I've been working on an MPI project in which the slaves all send data back to the master. For some reason the master will only receive the data if I do 2 consecutive Sends in a row. This is very odd and I think it is causing some other weird problems I am getting. Any idea what would cause this? I think the first send is sending some kind of junk data or something. The sends are the exact same line of code though.
EDIT: Code below...
if (numProcs > 0)
MPI_Barrier( MPI_COMM_WORLD ) ; //only wait if there are other processes to wait for
if (rank != 0)
{
MPI_Send(handArray, 10, MPI_DOUBLE, 0, TAG_HAND, MPI_COMM_WORLD);
MPI_Send(handArray, 10, MPI_DOUBLE, 0, TAG_HAND, MPI_COMM_WORLD);
}
//8. After the main loop the master process receives and sums together the hand counts array
// from each slave process.
else
{
int activeProcs = numProcs - 1;
getHandsFromSlaves( activeProcs, handArray );
then the master proceeds to print some data...
Here is the getHands FromSlaves method. Please note I have also tried using blocking calls for this as well with the same problems.
void getHandsFromSlaves( int& activeCount, double handTotals[10] ){
static MPI_Request request;
static int msgBuff, recvFlag;
static double handBuff[10];
MPI_Status status;
while (activeCount > 0)
{
if( request )
{
// Already listening for a message
// Test to see if message has been received
MPI_Test( &request, &recvFlag, &status );
//cout << "TAG: " << status.MPI_TAG << " SOURCE: "<< status.MPI_SOURCE << " ERROR: " << status.MPI_ERROR << endl;
if( recvFlag )
{
// Message received
if( status.MPI_TAG == TAG_HAND )
{
cout << "Hand Received!" << endl;
for(int m = 0; m < 10; ++m)
{
handTotals[m] += handBuff[m];
}
activeCount--;
}
else
{
//error report... what happened?
cout << "TAG: " << status.MPI_TAG << " SOURCE: "<< status.MPI_SOURCE << " ERROR: " << status.MPI_ERROR << endl;
}
// Reset the request handle
request = 0;
}
}
if( !request && activeCount > 0 )
// Start listening again
MPI_Irecv(&handBuff, 10, MPI_DOUBLE, MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &request);
}
}

Answer 1

Well, you're probably trying to process one too many messages because your request variable is undefined on entering your getHandsFromSlaves() routine. Since on enter, request is almost certainly non-zero, you immediately try to MPI_Test for a message something even though you haven't posted an Irecv.
In fact, there's a lot of really strange things about the code excerpt posted here. Why are the local variables static? Why would you implement your own busywait on MPI_Test() instead of using MPI_Wait()? Why are you using non-blocking receives at all if you're not doing anything useful between receives? And indeed, if you're just summing up all of the arrays anyway, why are you doing individual point-to-point receives at all instead of doing an MPI_Reduce()?
The following much shorter code seeems to do what you're trying to do above:
#include <stdio.h>
#include <mpi.h>
int main (int argc, char **argv) {
int rank, numProcs;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
double handArray[10];
double handTotal[10];
for (int i=0; i<10; i++)
handArray[i] = rank + i;
if (rank == 0) // Since apparently rank 0 doesn't do anything
{
for (int i=0; i<10; i++)
handArray[i] = 0;
}
MPI_Reduce(handArray, handTotal, 10, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
if (rank == 0) {
printf("Hand Totals= \n");
for (int i=0; i<10; i++)
printf(" %lf ", handTotal[i]);
printf("\n");
}
MPI_Finalize();
}