Question

In my parallel programming book, I came across this code that says the slaves generate the data set, however, I think the master acutally generates the data set.

This line in particular is why I believe that master generates the data set.

for (i=0; i < ARRAY_SIZE; i++) 
        numbers[i] = i; 

Can someone confirm if master or slaves generate the data set?

#include "mpi.h" 
#include <stdio.h> 
#include <math.h> 
#include <stdlib.h> 

#define TRIALS 20 
#define ARRAY_SIZE 1000000 

int main(int argc, char *argv[]) 
{ 
    int myid, numprocs; 
    double startwtime, endwtime; 
    int namelen; 
    int* numbers = new int[ARRAY_SIZE]; 
    int i, j, sum, part_sum; 
    int s, s0, startIndex, endIndex; 
    double totalTime; 

    char processor_name[MPI_MAX_PROCESSOR_NAME]; 

    MPI_Init(&argc,&argv); 
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs); 
    MPI_Comm_rank(MPI_COMM_WORLD,&myid); 
    MPI_Get_processor_name(processor_name,&namelen); 

    fprintf(stderr,"Process %d on %s\n", myid, processor_name); 
    fflush(stderr); 

    for (i=0; i < ARRAY_SIZE; i++) 
        numbers[i] = i; 

    if (myid == 0) 
    { 
        s = (int) floor(ARRAY_SIZE/numprocs); 
        s0 = s + ARRAY_SIZE%numprocs; 
        //printf("s=%d , s0= %d\n", s, s0); 
    } 

    MPI_Bcast(&s, 1, MPI_INT, 0, MPI_COMM_WORLD); 
    MPI_Bcast(&s0, 1, MPI_INT, 0, MPI_COMM_WORLD); 

    startIndex = s0 + (myid - 1)*s; 
    endIndex = startIndex + s; 

    totalTime = 0; 

    for (j = 1; j <= TRIALS; j++) 
    { 
        if (myid == 0) 
        { 
            startwtime = MPI_Wtime(); 
        } 

        sum = 0; 
        part_sum = 0; 

        if (myid == 0) // master 
        { 
            // compute sum of master's numbers 
            for (i = 0; i < s0; i++) 
            { 
                part_sum += numbers[i]; 
            } 
        } 
        else 
        { 
            for (i = startIndex; i < endIndex; i++) 
            { 
                part_sum += numbers[i]; 
            } 
        } 
        MPI_Reduce(&part_sum, &sum, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD); 
        if (myid == 0) 
        { 
            double runTime; 
            endwtime = MPI_Wtime(); 
            runTime = endwtime - startwtime; 

            printf("Trial %d : Execution time (sec) = %f\n", j, runTime); 
            printf("Sum = %d \n", sum); 
            totalTime += runTime; 
        } 
    } // end for 
    if (myid == 0) 
        printf("Average time for %d trials = %f", TRIALS, totalTime/TRIALS); 

    MPI_Finalize(); 
} 

Answer 1

Both the master and the slaves generate the entire array. You have to remember that your program runs on all nodes and the part of the code in question doesn’t distinguish between master/slave. So the wording of your book isn’t wrong, but it could be clarified. 🙂