MPI program does nothing - running on linux - linux

Question

I've written this MPI C program on linux. The master is supposed to send tasks to the slaves and receive data from the slaves (and if there're more tasks to give them to the finished slaves).
After all the tasks are completed, it's supposed to print a solution.
It prints nothing and I can't figure out why. It isn't stuck, it just finishes after a second and doesn't print anything.
P.S-
I've tried debugging by placing a printf in different places in the code.
The only place in the code that printed something was before the MPI_Recv in the master section, and it printed a few times (less than the number of processes).
Here's the full code:
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define NUMS_TO_CHECK 2000
#define RANGE_MIN -5
#define RANGE_MAX 5
#define PI 3.1415
#define MAX_ITER 100000
double func(double x);
int main (int argc, char *argv[])
{
int numProcs, procId;
int errorCode= MPI_ERR_COMM;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
MPI_Comm_rank(MPI_COMM_WORLD, &procId);
MPI_Status status;
int i;
double recieve=0;
int countPositives=0;
double arr[NUMS_TO_CHECK];
double difference= (RANGE_MAX - RANGE_MIN) / NUMS_TO_CHECK;
int counter = NUMS_TO_CHECK-1; //from end to start...
//Initiallizing the array.
for(i=0; i<NUMS_TO_CHECK; i++){
arr[i]=RANGE_MIN+i*difference;
}
//master
if(procId==0){
//Send tasks to all procs
for(i=1; i<numProcs; i++){
MPI_Send(&arr[counter], 1, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
counter--;
}
do{
MPI_Recv(&recieve, 1, MPI_DOUBLE, MPI_ANY_SOURCE, 0, MPI_COMM_WORLD, &status);
if(recieve>0)
{
countPositives++;
}
MPI_Send(&arr[counter], 1, MPI_DOUBLE, status.MPI_SOURCE, 0, MPI_COMM_WORLD);
counter--;
}while(counter>0);
printf("Number of positives: %d", countPositives);
MPI_Finalize();
}
//slaves
else{
MPI_Recv(&recieve, 1, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
recieve=func(recieve);
MPI_Send(&recieve, 1, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD);
}
}
double func(double x)
{
int i;
double value = x;
int limit = rand() % 3 + 1;
for(i = 0; i < limit * MAX_ITER; i++)
value = sin(exp(sin(exp(sin(exp(value))))) - PI / 2) - 0.5;
return value;
}

Answer 1

I think your slaves need to read data in a while loop. They only do 1 receive and 1 send. Whereas the master starts at 2000. That may be by design, so I may be wrong.

Answer 2

On the principle, your code looks almost fine. Only two things are missing here:
The most obvious one is a loop of a sort on the slaves' side, to receive their instructions from the master, and then send back their work; and
Less obvious but as essential: a mean for the master to tell when the work is done. It could be a special value send, that is tested by the slaves, and which leads them to exist the recv + work + send loop upon reception, or a different tag that you test. In the latter case, you'd have to use MPI_ANY_TAG for the reception call on the slaves' side.
With this in mind, I'm sure you can make your code to work.