Not quite understanding MPI - c

Question

I am attempting to make a program using MPI that will find the value of PI using MPI.
Currently I can find the sum this way:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define NUMSTEPS 1000000
int main() {
int i;
double x, pi, sum = 0.0;
struct timespec start, end;
clock_gettime(CLOCK_MONOTONIC, &start);
double step = 1.0/(double) NUMSTEPS;
x = 0.5 * step;
for (i=0;i<= NUMSTEPS; i++){
x+=step;
sum += 4.0/(1.0+x*x);
}
pi = step * sum;
clock_gettime(CLOCK_MONOTONIC, &end);
u_int64_t diff = 1000000000L * (end.tv_sec - start.tv_sec) + end.tv_nsec - start.tv_nsec;
printf("PI is %.20f\n",pi);
printf("elapsed time = %llu nanoseconds\n", (long long unsigned int) diff);
return 0;
}
But this does not use MPI.
So I have tried to make my own in MPI. My logic is:
Split the 1000000 into equal parts based on how many processors I have
Calculate the values for each range
Send the calculated value back to the master and then divide by the number of processors. I would like to keep the main thread free and not do any work. Similar to a master-slave system.
Here's what I have currently. This doesn't seem to be working and the send/receive gives errors about incompatible variables for receive and send.
#include <mpi.h>
#include <stdio.h>
#include <string.h>
#define NUMSTEPS 1000000
int main(int argc, char** argv) {
int comm_sz; //number of processes
int my_rank; //my process rank
// Initialize the MPI environment
MPI_Init(NULL, NULL);
MPI_Comm_size(MPI_COMM_WORLD, &comm_sz);
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
// Get the name of the processor
char processor_name[MPI_MAX_PROCESSOR_NAME];
int name_len;
MPI_Get_processor_name(processor_name, &name_len);
// Slaves
if (my_rank != 0) {
// Process math then send
int i;
double x, pi, sum = 0.0;
double step = 1.0/(double) NUMSTEPS;
x = 0.5 * step;
// Find the start and end for the number
int processors = comm_sz - 1;
int thread_multi = NUMSTEPS / processors;
int start = my_rank * thread_multi;
if((my_rank - 1) != 0){
start += 1;
}
int end = start + thread_multi ;
for (i=start; i <= end; i++){
x+=step;
sum += 4.0 / (1.0 + x * x);
}
pi = step * sum;
MPI_Send(pi, 1.0, MPI_DOUBLE 1, 0, MPI_COMM_WORLD);
// Master
} else {
// Things in here only get called once.
double pi = 0.0;
double total = 0.0;
for (int q = 1; q < comm_sz; q++) {
MPI_Recv(pi, 1, MPI_DOUBLE, q, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
total += pi;
pi = 0.0;
}
// Take the added totals and divide by amount of processors that processed, to get the average
double finished = total / (comm_sz - 1);
// Print sum here
printf("Pi Is: %d", finished);
}
// Finalize the MPI environment.
MPI_Finalize();
}
I've currently spent around 3 hours working on this. Never used MPI. Any help would be greatly appreciated.

Answer 1

Try compiling with more compiler warnings and try to fix them, for instance -Wall -Wextra should give you excellent clues about what the issues are.
According to MPI_Send documentation the first argument is a pointer, so you seem to be ignoring an automatic "conversion to pointer" error. You have the same issue in the MPI_Recv() call.
You can try to pass pi as &pi in MPI_Recv and MPI_Send and check if that fixes the error.
As a comment, you can declare dummy variables as pi as a local variables inside the master loop to avoid side-effects:
for (int q = 1; q < comm_sz; q++) {
double pi = 0;
MPI_Recv(&pi, 1, MPI_DOUBLE, q, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
total += pi;
}