Latest revision as of 15:29, 23 August 2023

Programming Details

C++ is a general-purpose, imperative computer programming language. It is derived from the C language and shares many constructs with it, it also expands into object-orientated programming paradigms.

There is direct support for openMP and openMPI.

Programming example

#include <omp.h>
#include <iostream> 

int main () 
{
    int thread_number;
    #pragma omp parallel private(thread_number)
    {
        #pragma omp for schedule(static) nowait
        for (int i = 0; i < 100; i++) 
        {
            thread_number = omp_get_thread_num();
            cout << "Thread " << thread_number << " says " << i << endl;
        }
    }
    return 0;
}

#include <stdlib.h>
#include <stdio.h>
#include <omp.h>
#include <iostream>
#include <chrono>
#include <vector>

using namespace std;

#pragma optimize( "[t]", on)
int main(int argc, char *argv[]){
	const int size = atoi(argv[1]);
	const int maxVal = 255;
	int row, col, ele;	
	
	double** m1 =  new double*[size];
	double** m2 =  new double*[size];
	double** res =  new double*[size];

	for (int i = 0; i < size; i++){
		m1[i] = new double[size];
		m2[i] = new double[size];
		res[i] = new double[size];
	}

	int procs = omp_get_num_procs();
	omp_set_num_threads(procs);

	std::chrono::duration<double, std::micro> time;	
	
	for (int row = 0; row < size; row++){
		for (int col = 0; col < size; col++){
			m1[row][col] = 0 + (rand() % maxVal);
			m2[row][col] = 0 + (rand() % maxVal);
		}
	}
	
	auto start = std::chrono::high_resolution_clock::now();

	#pragma omp parallel shared(m1, m2, res)
	{

		#pragma omp for nowait private(row, col, ele) schedule(dynamic)
		for (int row = 0; row < size; row++) {
			for (int ele = 0; ele < size; ele++){
				double temp = 0;
				for (int col = 0; col < size; col++){
					temp += m1[row][ele] * m2[ele][col];
				}
				res[row][col] += temp;
			}
		}
	}

	auto end = std::chrono::high_resolution_clock::now();
		

	time = end - start;
	
	cout << (time.count() / 1000 / 1000) << endl;

	

	return 0;
}

Compilation

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

[username@login01 ~]$  module add gcc/8.2.0
[username@login01 ~]$  g++ -o test2 -fopenmp test2.c (openMP support)
[username@login01 ~]$ ./test2

Thread 22 says 32
Thread 11 says 39
.....

Modules Available

The following modules are available for C

• module add gcc/8.2.0 (GNU compiler)
• module add intel/2018 (Intel compiler)

Note: 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
#SBATCH --mail-user= your email address here

echo $SLURM_JOB_NODELIST

module purge
module add gcc/8.2.0

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 234552

Next Steps

• https://en.wikipedia.org/wiki/C%2B%2B
• https://en.wikipedia.org/wiki/OpenMP
• http://www.openmp.org/
• https://computing.llnl.gov/tutorials/openMP/

Languages | Main Page | Further Topics