How to convert MPI_Reduce into MPI_Send and MPI_Recv? - c

Question

I am working on a parallel processing program that uses MPI_Send() and MPI_Recv() instead of using MPI_Reduce(). I understand that MPI_Send() will need to send a value from each processor to the root processor aka 0 and MPI_Recv() will need to receive all of the values from each processor.
I keep getting the error where the value in Send will not be sent to the Receiving side thus making the final value 0. The MPI_Reduce() function is still in the code but commented out to see what needs to be replaced. Can anyone help?
#include "mpi.h"
#include <stdio.h>
#include <math.h>
int main( int argc, char *argv[])
{
int n, i;
double PI25DT = 3.141592653589793238462643;
double pi, h, sum, x;
int numprocs, myid;
double startTime, endTime;
/* Initialize MPI and get number of processes and my number or rank*/
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
/* Processor zero sets the number of intervals and starts its clock*/
if (myid==0) {
n=600000000;
startTime=MPI_Wtime();
for (int i = 0; i < numprocs; i++) {
if (i != myid) {
MPI_Send(&n, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
}
}
}
else {
MPI_Recv(&n, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
}
/* Calculate the width of intervals */
h = 1.0 / (double) n;
/* Initialize sum */
sum = 0.0;
/* Step over each inteval I own */
for (i = myid+1; i <= n; i += numprocs) {
/* Calculate midpoint of interval */
x = h * ((double)i - 0.5);
/* Add rectangle's area = height*width = f(x)*h */
sum += (4.0/(1.0+x*x))*h;
}
/* Get sum total on processor zero */
//MPI_Reduce(&sum,&pi,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);
double value = 0;
if (myid != 0) {
MPI_Send(&sum, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
else {
for (int i = 1; i < numprocs; i++) {
MPI_Recv(&value, 1, MPI_DOUBLE, i, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
pi += value;
}
}
/* Print approximate value of pi and runtime*/
if (myid==0) {
printf("pi is approximately %.16f, Error is %e\n",
pi, fabs(pi - PI25DT));
endTime=MPI_Wtime();
printf("runtime is=%.16f",endTime-startTime);
}
MPI_Finalize();
return 0;
}

Answer 1

You are using MPI_INT to send a value of type double:
if (myid != 0) {
MPI_Send(&sum, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
// ^^^^^^^
}
int is 4 bytes long; double is 8 bytes long. Although the receive operation succeeds, it cannot construct a value of type MPI_DOUBLE given only 4 bytes from the message, so it doesn't write anything into value and it remains 0.0. Indeed, if you replace:
MPI_Recv(&value, 1, MPI_DOUBLE, i, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
with
MPI_Status status;
int count;
MPI_Recv(&value, 1, MPI_DOUBLE, i, 0, MPI_COMM_WORLD, &status);
MPI_Get_count(&status, MPI_DOUBLE, &count);
if (count == MPI_UNDEFINED) {
printf("Short message received\n");
MPI_Abort(MPI_COMM_WORLD, 0);
}
your program will abort, indicating that the body of the conditional statement was executed due to MPI_Get_count() returning MPI_UNDEFINED in count, which signals that the length of the received message was not an integer multiple of the size of MPI_DOUBLE.
Also, pi must be explicitly initialised to sum before the receive loop, otherwise you will get the wrong value of pi due to either of the following errors:
pi is left uninitialised and has arbitrary initial value, and
the contribution of rank 0 is not added to the final result.