MPI - Parallel dot product calculation - c

Question

I'm struggling to modify a program that takes two files as input (each representing a vector) and calculates the dot product between them. It's supposed to be done in parallel, but I was told that the number of points in each file might not be evenly divisible by the number of available processors and each process might read from incorrect positions within the files. What I mean is that, if there are four processors, the first 250 points might be correctly read and calculated but the second processor might read over those same 250 points and provide an incorrect result. This is what I've done so far. Any modifications I've made are noted.
#include "fstream"
#include "stdlib.h"
#include "stdio.h"
#include "iostream"
#include "mpi.h"
int main(int argc, char *argv[]){
MPI_Init(&argc, argv);
//parse command line arguments
if( argc < 3 || argc > 3){
std::cout << "*** syntax: " << argv[0] << " vecFile1.txt vecFile2.txt" << std::endl;
return(0);
}
//get input file names
std::string vecFile1(argv[1]);
std::string vecFile2(argv[2]);
//open file streams
std::ifstream vecStream1(vecFile1.c_str());
std::ifstream vecStream2(vecFile2.c_str());
//check that streams opened properly
if(!vecStream1.is_open() || !vecStream2.is_open()){
std::cout << "*** Could not open Files ***" << std::endl;
return(0);
}
//if files are open read their lengths and make sure they are compatible
long vecLength1 = 0; vecStream1 >> vecLength1;
long vecLength2 = 0; vecStream2 >> vecLength2;
if( vecLength1 != vecLength2){
std::cout << "*** Vectors are no the same length ***" << std::endl;
return(0);
}
int numProc; //New variable for managing number of processors
MPI_Comm_size(&numProc,MPI_COMM_WORLD); //Added line to obtain number of processors
int subDomainSize = (vecLength1+numProc-1)/numProc; //Not sure if this is correct calculation; meant to account for divisibility with remainders
//read in the vector components and perform dot product
double dotSum = 0.;
for(long i = 0; i < subDomainSize; i++){ //Original parameter used was vecLength1; subDomainSize used instead for each process
double ind1 = 0.; vecStream1 >> ind1;
double ind2 = 0.; vecStream2 >> ind2;
dotSum += ind1*ind2;
}
std::cout << "VECTOR DOT PRODUCT: " << dotSum << std::endl;
MPI_Finalize();
}
Aside from those changes, I don't know where to go from here. What can I do to make this program properly calculate a dot product of two vectors using paralleling processing with two text files as input? Each contains 100000 points so it's impractical to manually modify the files.

Answer 1

I wont write the code here as it seems to be an assignment problem but I would try to give you some tips to go into right direction.
Each processor has an assigned rank that can be found out using the MPI_Comm_rank API. So for parallel processing you can divide the vectors of the files among the processors such that processor with rank r processes the vectors r*subdomainsize to (r+1)*subdomainsize - 1.
You need to make sure that the vector from correct position is read from the file by a particular processor. Use seek api to go to the right offset and then call the read(>>) operator of your filestream.
For calculating subdomainsize I am not sure whether the equation you mentioned works or not. There can be several approaches. The simplest is to use vectorlength/numProc as subdomainsize. Each processor can handle subdomainsize elements, however the last processor (rank == numProc) will handle the remaining elements.
After the for loop, you should use a reduction operation to collect the individual sums from the processors and sum it up globally for the final result. See MPI_Reduce.
Use Barrier for synchronization between the processors. A barrier must be placed after the for loop and before calling reduction.