c++ - openmp : check if nested parallesim

Question

Assume I have a method that multiplies two std::vector :

double multiply(std::vector<double> const& a, std::vector<double> const& b){
    double tmp(0);
    /*here I could easily do a parallelization with*/
    /*#pragma omp parallel loop for*/
    for(unsigned int i=0;i<a.size();i++){
        tmp += a[i]*b[i];
    }
    return tmp;
}

If I set in this function the pragma macro, a call to multiply(...) will run on all threads.

Now assume that somewehere else I want to do many vector multiplication :

void many_multiplication(std::vector<double>* a, std::vector<double>* b, unsigned int N){
    /*here I could easily do a parallelization with*/
    /*#pragma omp parallel loop for*/
    for(unsigned int i=0;i<N;i++){
        for(unsigned int j=0;j<N;j++){
            multiply(a[i],b[j]);
        }
    }
}

I could also do the parallelization the same way. But this will lead to unwanted nested parallelism.

How can I check that if multiply(..) is called within a parallel region, then the pragma macro of multiply(...) is "turn off". And if it's called from a non-parallel region, then it's "turn on".

Answer 1

Nested parallelism is disabled by default, unless enabled specificially by setting OMP_NESTED to true or by calling omp_set_nested(1); (§2.3.2 of the OpenMP specification) Explicitly modifying the settings for nesting as suggested by Avi Ginsburg is a bad idea. Instead, you should use conditional parallel execution based on the level of nesting:

double multiply(std::vector<double> const& a, std::vector<double> const& b){
    double tmp(0);
    int active_levels = omp_get_active_level();
    #pragma omp parallel for reduction(+:tmp) if(active_level < 1)
    for(unsigned int i=0;i<a.size();i++){
        tmp += a[i]+b[i];
    }
    return tmp;
}

omp_get_active_level() returns the number of active parallel regions that enclose the thread at the moment the call is made. It returns 0 if called from outside a parallel region or with inactive outer region(s). Thanks to the if(active_level < 1) clause, the parallel region will only be activated, i.e. run in parallel, if it is not enclosed in an active region, regardless of the setting for nesting.

If your compiler does not support OpenMP 3.0 or higher (e.g. with any version of MS Visual C/C++ Compiler), then omp_in_parallel() call can be used instead:

double multiply(std::vector<double> const& a, std::vector<double> const& b){
    double tmp(0);
    int in_parallel = omp_in_parallel();
    #pragma omp parallel for reduction(+:tmp) if(in_parallel == 0)
    for(unsigned int i=0;i<a.size();i++){
        tmp += a[i]+b[i];
    }
    return tmp;
}

omp_in_parallel() returns non-zero if at least one enclosing parallel region is active, but does not provide information about the depth of nesting, i.e. is a bit less flexible.

In any case, writing such code is a bad practice. You should simply leave the parallel regions as they are and allow the end user choose whether nested parallelism should be enabled or not.