Embaressingly parallel processing using Open MPI and OpenMP - openmp

Question

It may be a silly question, anyway I am working on an embarrassingly parallel problem. I can divide the work into independent tasks (no communication) that can be performed in parallel.
In a shell script.sh one can use the following:
#!/bin/bash
let MY_ID=${OMPI_COMM_WORLD_RANK}
./a.out $MY_ID
In prog.c we have a simple independent program:
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char *argv[]){
int myid = atoi(argv[1]);
if(myid%2==0)
printf("\nI am %d and I am an even process\n", myid);
else
printf("\nI am %d and I am an odd process\n", myid);
return 0;
}
And finally, to execute the program with 12 different processors:
mpirun -np 12 script.sh
My question is that is it possible to do the same thing using OpenMP and perhaps environment variables like OMP_NUM_THREADS ?

Answer 1

There is no OpenMP environment variable which, when passed to a program in the way you indicate, will be given a value different for each invocation of the program. If you think about how OpenMP works you might think, as I do, that this doesn't make much sense - since the OpenMP program is implemented as a collection of threads. This is in marked contrast to the model of operations of MPI in which each process is a separate process, bolstered with a library for inter-process communications - in this case it makes sense that each process have a unique identifier for facilitating communication. When an OpenMP program executes communication between threads is effected by operations on shared memory locations, not by passing messages to specified threads.
In passing, you're not really writing an MPI program, just using one of the facilities its environment provides for making your shell-script writing a little easier. You could almost as easily write a shell-script to send a different id to each invocation of the program without the MPI environment - and you could do the same for invocations of an OpenMP program. Though why you would do either if your programs really are independent I don't know.

Answer 2

Too long to post it as a comment. Actually, this kind of parallel processing is useful for ensemble modeling. When your program runs for different initial condition, and each run creates different folder or output file.
I came up to this :
export OMP_NUM_THREADS=4
g++ prog.cpp -fopenmp
./a.out
in prog.cpp we have:
#include<stdio.h>
#include<stdlib.h>
#include<omp.h>
#include<string>
using std::string;
int main(){
int nthreads, tid;
string cmd;
#pragma omp parallel private(tid)
{
tid = omp_get_thread_num();
cmd = "echo 'this is a test. I am thread :' " + std::to_string(tid) ;
system(cmd.c_str()); //each thread can creat a folder and run a script
// system("./my_script.sh");
}
return 0;
}
with the output as follows:
this is a test. I am thread : 0
this is a test. I am thread : 3
this is a test. I am thread : 2
this is a test. I am thread : 1