How to speed up this problem by MPI

Question

(1). I am wondering how I can speed up the time-consuming computation in the loop of my code below using MPI?

 int main(int argc, char ** argv)   
 {   
 //          


        

Answer 1

Quick edit (because I either can't figure out how to leave comments, or I'm not allowed to leave comments yet) -- 3lectrologos is incorrect about the parallel part of MPI programs. You cannot do serial work before MPI_Init and after MPI_Finalize and expect it to actually be serial -- it will still be executed by all MPI threads.

I think part of the issue is that the "parallel part" of an MPI program is the entire program. MPI will start executing the same program (your main function) on each node you specify at approximately the same time. The MPI_Init call just sets certain things up for the program so it can use the MPI calls correctly.

The correct "template" (in pseudo-code) for what I think you want to do would be:

int main(int argc, char *argv[]) {
    MPI_Init(&argc, &argv);  
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);  
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);

    if (myid == 0) { // Do the serial part on a single MPI thread
        printf("Performing serial computation on cpu %d\n", myid);
        PreParallelWork();
    }

    ParallelWork();  // Every MPI thread will run the parallel work

    if (myid == 0) { // Do the final serial part on a single MPI thread
        printf("Performing the final serial computation on cpu %d\n", myid);
        PostParallelWork();
    }

    MPI_Finalize();  
    return 0;  
}