Question

I was searching Google for a page offering some simple OpenMp algorithms.
Probably there is an example to calculate min, max, median, average from a huge data array but I am not capable to find it.

At least I would normally try to divide the array into one chunk for each core and do some boundary calculation afterwards to get the result for the complete array.

I just didn’t want to reinvent the wheel.

---

Additional Remark:
I know that there are thousands of examples that work with simple reduction.
e.g. Calculating PI.

const int num_steps = 100000; 
double x, sum = 0.0; 
const double step = 1.0/double(num_steps); 
#pragma omp parallel for reduction(+:sum) private(x) 
for (int i=1;i<= num_steps; i++){ 
  x = double(i-0.5)*step; 
  sum += 4.0/(1.0+x*x); 
} 
const double pi = step * sum;

but when these kind of algorithms aren’t usable there are almost no examples left for reducing algorithms.

Answer 1

OpenMP (at least 2.0) supports reduction for some simple operations, but not for max and min.

In the following example the reduction clause is used to make a sum and a critical section is used to update a shared variable using a thread-local one without conflicts.

#include <iostream>
#include <cmath>

int main()
{
  double sum = 0;
  uint64_t ii;
  uint64_t maxii = 0;
  uint64_t maxii_shared = 0;
#pragma omp parallel shared(maxii_shared) private(ii) firstprivate(maxii)
  {
#pragma omp for reduction(+:sum) nowait
    for(ii=0; ii<10000000000; ++ii)
      {
        sum += std::pow((double)ii, 2.0);
        if(ii > maxii) maxii = ii;
      }
#pragma omp critical 
    {
      if(maxii > maxii_shared) maxii_shared = maxii;
    }
  }
  std::cerr << "Sum: " << sum << " (" << maxii_shared << ")" << std::endl;
}

EDIT: a cleaner implementation:

#include <cmath>
#include <limits>
#include <vector>
#include <iostream>
#include <algorithm>
#include <tr1/random>

// sum the elements of v
double sum(const std::vector<double>& v)
{
  double sum = 0.0;
#pragma omp parallel for reduction(+:sum)
  for(size_t ii=0; ii< v.size(); ++ii)
    {
      sum += v[ii];
    }
  return sum;
}

// extract the minimum of v
double min(const std::vector<double>& v)
{
  double shared_min;
#pragma omp parallel 
  {
    double min = std::numeric_limits<double>::max();
#pragma omp for nowait
    for(size_t ii=0; ii<v.size(); ++ii)
      {
        min = std::min(v[ii], min);
      }
#pragma omp critical 
    {
      shared_min = std::min(shared_min, min);
    }
  }
  return shared_min;
}

// generate a random vector and use sum and min functions.
int main()
{
  using namespace std;
  using namespace std::tr1;

  std::tr1::mt19937 engine(time(0));
  std::tr1::uniform_real<> unigen(-1000.0,1000.0);
  std::tr1::variate_generator<std::tr1::mt19937, 
    std::tr1::uniform_real<> >gen(engine, unigen);

  std::vector<double> random(1000000);
  std::generate(random.begin(), random.end(), gen);

  cout << "Sum: " << sum(random) << " Mean:" << sum(random)/random.size()
       << " Min:" << min(random) << endl;
}