parallel for loop in c++ using MPI

Question

I am trying to make my for loop parallel in c++. The iterations are completely independent. Below is a similar program that captures the idea of the task.

class A{

    // create experiment 
    // perform experiment
    // append results to file 
    // reset the experiment 

};

main {

    // open a file 

    // instance class
    A a;
    int N = 10000;

    for ( int i = 0; i <= N; i++ ){
        a.do_something()
    }

    // close file
    // return
}

Each iteration will simply print its data to an output file, the order of this is unimportant too. Since a.do_something() is lengthy, I would like to make it parallel. I have installed MPI and am now somewhat familiar with its basic use.

My logic is to split the range N into partitions depending on the number of processors available. I am looking for some assistance on how to take my serial version into parallel with MPI. My attempt is:

class A{

    // create experiment 
    // perform experiment
    // append results to file 
    // reset the experiment 

};

main {

    // open a file 

    // instance class
    A a;


    // initialise the MPI 
    int ierr = MPI_Init(&argc, &argv);
    int procid, numprocs;

    ierr = MPI_Comm_rank(MPI_COMM_WORLD, &procid);
    ierr = MPI_Comm_size(MPI_COMM_WORLD, &numprocs);

    // partition = (job size) over (processors). 
    unsigned int partition = N / numprocs;


    int N = 10000;

    for ( int i = 0; i <= N; i++ ){
        a.do_something()
    }



    ierr = MPI_Finalize();
    // close file
    // return
}

But I am really struggling to split the for loop and don't know how to proceed.

This will just run the serial code twice (on my 2-core machine). I want to split the for loop into N/2 chunks and have each thread tackle a different chunk.

Would I need to keep a core back to broadcast the jobs to the other cores? Could I iterate over the partitions? I have search online and haven't had much luck. Any suggestions?

for ( 0 --> N) cant you post real code? Such weirdness just distracts from the code that is important — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 25, 2018 at 11:11

john · Accepted Answer · 2018-09-25 11:49:06Z

5

when the MPI part of the code starts, think of it as independent programms running on processor. This means that the loop you wrote is run independently on both processors. A way to split it would for example be

for ( int i = rank*partition; i <= rank*partition+partition; i++ )

{
    a.do_something()
}

Also, declare N before you use it :-)

edited Sep 25, 2018 at 11:49

answered Sep 25, 2018 at 11:23

john

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Kafka · Accepted Answer · 2020-09-09 09:28:53Z

2

A simple way to do that is :

for ( int i = 0; i <= N; i++ )
{
   if (i% numprocs != procid) continue;

   a.do_something()
}

answered Sep 9, 2020 at 9:28

Kafka

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