GNU OpenMP Example

Disclaimer: This is NOT a course on learning OpenMP. This is a very basic example on compiling an OpenMP example using the GNU compilers.

1 What is OpenMP? | 1.1 Example | 1.2 Compile and Link Libraries | 1.3 Using OMP_NUM_THREADS Environment Variable

What is OpenMP?

OpenMP is a “specification for a set of compiler directives, library routines, and environment variables that can be used to specify high-level parallelism in Fortran and C/C++ programs.”

Example

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

int main(int argc, char* argv[])
{
    // Start of parallel region
    #pragma omp parallel
    {
        printf("Ths is OMP thread = %d\n", omp_get_thread_num());
    }
    // End of parallel region
}

Compile and Link Libraries

To indicate to the compile you’re using OpenMP use the -fopenmp compile option and link the gomp library.

# Create an interactive session with 8 cores:
[]$ salloc -A <your project> -t 2:00:00 -c 8

# Load the specific GNU compiler version.
[]$ module load gcc/12.2.0

# Compile and link in required libraries:
[]$ gcc -O3 -w -g -fopenmp -lgomp openmp_ex01.c -o openmp_ex01
[]$ ./openmp_ex01
Ths is OMP thread = 0
Ths is OMP thread = 6
Ths is OMP thread = 5
Ths is OMP thread = 2
Ths is OMP thread = 3
Ths is OMP thread = 7
Ths is OMP thread = 4
Ths is OMP thread = 1

Do not link using -lomp, this is related to the LLVM/Clang project and not GNU.

This library is not part of the standard compute node image.

Using OMP_NUM_THREADS Environment Variable

We can limit the number of cores for OpenMP to use by setting the OMP_NUM_THREADS environment variable:

# Create an interactive session with 8 cores:
[]$ salloc -A <your project> -t 2:00:00 -c 8

[]$ module load gcc/12.2.0

# Although the session has 8 cores, we can limit our application to only use 4:
[salexan5@mtest2 src]$ export OMP_NUM_THREADS=4
[salexan5@mtest2 src]$ ./openmp_ex01
Ths is OMP thread = 0
Ths is OMP thread = 1
Ths is OMP thread = 2
Ths is OMP thread = 3

[salexan5@mtest2 src]$ unset OMP_NUM_THREADS
[salexan5@mtest2 src]$ ./openmp_ex01
Ths is OMP thread = 0
Ths is OMP thread = 5
Ths is OMP thread = 7
Ths is OMP thread = 6
Ths is OMP thread = 4
Ths is OMP thread = 3
Ths is OMP thread = 2
Ths is OMP thread = 1