Programming/OpenMP

Programming Details

OpenMP is designed for multi-processor/core, shared memory machines. The underlying architecture can be shared memory UMA or NUMA.

It is an Application Program Interface (API) that may be used to explicitly direct multi-threaded, shared memory parallelism. Comprised of three primary API components:

Compiler Directives
Runtime Library Routines
Environment Variables

OpenMP compiler directives are used for various purposes:

Spawning a parallel region
Dividing blocks of code among threads
Distributing loop iterations between threads
Serializing sections of code
Synchronization of work among threads

Programming Example

C Example


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

/* compile with gcc -o test2 -fopenmp test2.c */

int main(int argc, char** argv)
{
    int i = 0;
    int size = 20;
    int* a = (int*) calloc(size, sizeof(int));
    int* b = (int*) calloc(size, sizeof(int));
    int* c;

    for ( i = 0; i < size; i++ )
    {
        a[i] = i;
        b[i] = size-i;
        printf("[BEFORE] At %d: a=%d, b=%d\n", i, a[i], b[i]);
    }

    #pragma omp parallel shared(a,b) private(c,i)
    {
        c = (int*) calloc(3, sizeof(int));

        #pragma omp for
        for ( i = 0; i < size; i++ )
        {
            c[0] = 5*a[i];
            c[1] = 2*b[i];
            c[2] = -2*i;
            a[i] = c[0]+c[1]+c[2];

            c[0] = 4*a[i];
            c[1] = -1*b[i];
            c[2] = i;
            b[i] = c[0]+c[1]+c[2];
        }

        free(c);
    }

    for ( i = 0; i < size; i++ )
    {
        printf("[AFTER] At %d: a=%d, b=%d\n", i, a[i], b[i]);
    }
}

Fortran Example


program omp_par_do
  implicit none

  integer, parameter :: n = 100
  real, dimension(n) :: dat, result
  integer :: i

  !$OMP PARALLEL DO
  do i = 1, n
     result(i) = my_function(dat(i))
  end do
  !$OMP END PARALLEL DO

contains

  function my_function(d) result(y)
    real, intent(in) :: d
    real :: y

    ! do something complex with data to calculate y
  end function my_function

end program omp_par_do

Compilation

The program would be compiled in the following way, optional Intel compiler available too:


For C
[username@login01 ~]$  module add gcc/4.9.3
[username@login01 ~]$  gcc -o test2 -fopenmp test2.c

For Fortran
[username@login01 ~]$  module add gcc/4.9.3
[username@login01 ~]$  gfortran -o test2 -fopenmp test2.c

Modules Available

The following modules are available for OpenMP:

module add gcc/4.9.3 (GNU compiler)
module add intel/compiler/64/2016.2.181 (Intel compiler)

OpenMP is a library directive within the compiler and does not require any additional module to be loaded.

Usage Examples

Batch example


#!/bin/bash

#SBATCH -J openmpi-single-node
#SBATCH -N 1
#SBATCH --ntasks-per-node 28
#SBATCH -o %N.%j.%a.out
#SBATCH -e %N.%j.%a.err
#SBATCH -p compute
#SBATCH --exclusive

echo $SLURM_JOB_NODELIST

module purge
module add gcc/4.9.3

export I_MPI_DEBUG=5
export I_MPI_FABRICS=shm:tmi
export I_MPI_FALLBACK=no

/home/user/CODE_SAMPLES/OPENMP/demo

[username@login01 ~]$ sbatch demo.job
Submitted batch job 289552

Further Information

Specific

General

HPC