Thread. Многопоточность

mutex mtx;
 
class Make_timer
{
public:
    Make_timer() :t1(system_clock::now()) { }
    void reset() { t1 = system_clock::now(); }
 
    void operator()(const string& label)
    {
        auto t2 = system_clock::now();
        cout << "  " << label << " tool: "
            << duration_cast<milliseconds>(t2 - t1).count() << " ms.\n";
    }
private:
    system_clock::time_point t1;
};
 
 
string get_string()
{
    const unsigned int min = 1;
    const unsigned int max = 100;
    const unsigned int low = 33;
    const unsigned int high = 126;
 
    stringstream ss;
    thread_local static default_random_engine ran;
    auto len = uniform_int_distribution<>{ (int)min, (int)max }(ran);
 
    for (auto i = 0; i < len; ++i)
    {
        ss << uniform_int_distribution<>{(int)low, (int)high}(ran);
    }
    return ss.str();
}
 
template<typename C>
C random_fill(int n)
{
    C vs;
    for (auto i = 0; i < n; ++i)
        vs.insert(get_string());
    return vs;
}
 
 
void fill_task(set<string>& vs, int n, mutex& m)
{
    set<string> v = random_fill<set<string>>(n);
 
    unique_lock<mutex> lck(m);
    //copy(v.begin(), v.end(), back_inserter(vs));
    for (const auto& a : v)
        vs.insert(a);
}
 
set<string> threaded_fill(int n)
//fill a large vector with random strings.
{
    set<string> vs;
    const int num_threads = 8;
    
    //const int num_threads = thread::hardware_concurrency();
 
    //cout << "number of threds " << num_threads << "\n";
 
    mutex mtx;
    vector<thread> vt;
    for (auto i = 0; i < num_threads; ++i)
        vt.push_back(thread(fill_task, ref(vs), n / num_threads, ref(mtx)));
 
    for (auto& t : vt)
        t.join();
 
    cout << "vector size: " << vs.size() << '\n';
 
    return vs;
}

int main()
{
    //....
const int max1 = 500000;
        const int max2 = 5000000;
    Make_timer timer;
    cout << "Filling both vectors..\n";
    timer.reset();
    //vector<string> vs1 = random_fill(max1);
    //vector<string> vs2 = random_fill(max2);
    set<string> vs1 = threaded_fill(max1);
    set<string> vs2 = threaded_fill(max2);
    timer("set fills");
    //....
}