openmp parallel for loop with two or more reductions

Question

Hi just wondering if this is the right way to go going about having a regular for loop but with two reductions , is this the right approach below? Would this work with more then two reductions as well. Is there a better way to do this? also is there any chance to integrate this with an MPI_ALLREDUCE command?

heres the psuedo code

      #pragma omp parallel for \
      default(shared) private(i) \
      //todo first  reduction(+:sum)
      //todo second reduction(+:result)

      for loop i < n; i ++; {
        y = fun(x,z,i)
        sum += fun2(y,x)
        result += fun3(y,z)
      }

Solution 1

You can do reduction by specifying more than one variable separated by a comma, i.e. a list:

#pragma omp parallel for default(shared) reduction(+:sum,result) ...

Private thread variables will be created for sum and result that will be combined using + and assigned to the original global variables at the end of the thread block.

Also, variable y should be marked private.

See https://computing.llnl.gov/tutorials/openMP/#REDUCTION

Solution 2

You can simply add another reduction clause:

#include <iostream>
#include <cmath>

int main(){
    double sum_i = 0, max_i = -1;
    #pragma omp parallel for reduction(+:sum_i) reduction(max:max_i)
    for (int i=0; i<5000; i++){
        sum_i += i;
        if (i > max_i)
            max_i = i;
    }
    std::cout << "Sum = " << sum_i << std::endl;
    std::cout << "Max = " << max_i << std::endl;
    return 0;
}

From OpenMP 4.5 Complete Specifications (Nov 2015)

Any number of reduction clauses can be specified on the directive, but a list item can appear only once in the reduction clauses for that directive.

The same works on Visual C++ that uses oMP v2.0: reduction VC++

Comments

[worenga]

What if there are different operations to be performed e.g. + and max ?

[Azmisov]

@mightyuhu See my answer

[Royi]

It doesn't work in GCC 7.1. Any idea how to apply 2 different reductions?

[Marv]

@Royi maybe it's because of the comma after the first reduction term? I don't recall commas being used to distinguish omp parameters.