OpenMP (Processes vs. Threads)

COMP328 Lectures 2023-03-14

Process

The basic unit of work for the operating system.

Processes have a lot of overhead due to their isolation.

Thread

A portion of a process that can be run concurrently with another.

Threads have low overhead as there is implicit trust. They share a heap but have their own stack.

OpenMP

OpenMP, available at https://www.openmp.org, is a parallel library that operates on a thread level.

This is unlike OpenMPI that operates on a process level.

Consider the following example program:

#include <omp.h>
#include <stdio.h>

int main(void) {
    #pragma omp parallel
    {
        printf("Thread id is %d\n", omp_get_thread_num());
    }
    return 0;
} 

We can compile this with the following flags:

$ gcc -fopenmp thread.c
$ ./a.out

We can define the number of threads to spawn with the following variable:

OMP_NUM_THREADS=200 ./a.out

Otherwise the total number of system threads are used.