C++ – OpenMP – Sum reduction example (simple example)

Here is an example of a reduction on a variable to sum the result from each thread.

The code

You can change the number of thread by changing “NB_THREAD_TEST”.

#include "stdafx.h"

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

// number of thread for this simu
// we use it only to use many thread (more than cpu nb)
const int NB_THREAD_TEST = 6;

int _tmain(int argc, _TCHAR* argv[])
{
	// sum of each thread
	int sum = 0;

	// parallel section with NB_THREAD_TEST and recution for the sum variable
	// the reduction cumulate each results of each threads
	#pragma omp parallel num_threads(NB_THREAD_TEST) reduction(+:sum)
    {
		// master thread prints the number of working thread
		#pragma omp master
        {
			std::cout << "Number of threads = " << omp_get_num_threads() << std::endl;
		}
		// put the thread id into sum
		sum = omp_get_thread_num();
    }

	// withou redution the sum will be the last thread id that wrote into the variable
	// but here this is : sum = 4 + 3 + 2 + 1 + 0
	std::cout << "Sum = " << sum << std::endl;
	std::cout << "Sum should be = " << ( ((NB_THREAD_TEST)>>1) * ((NB_THREAD_TEST&0x01) + NB_THREAD_TEST - 1) ) << std::endl;

	return 0;
}

Output

Number of threads = 6
Sum = 15
Sum should be = 15
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