c++中的多线程快速排序

我用multi-thread方法在C++中用Portfolio任务实现了一个quicksort程序。

公文包任务的方法是维护一个任务队列。每个自由线程从公文包中选择一个任务，并在必要时执行它生成新的子任务并将其放入公文包

但我不确定什么是正确的！在我看来，在一个thread中，该算法的工作速度比两个或四个thread快。我能以某种方式打乱同步吗？

谢谢任何人帮助我。

代码：

#include <thread>
#include <chrono>
#include <mutex>
#include <condition_variable>
#include <iostream>
#include <queue>
#include <vector>
#include <set>
#include <ctime>
#include <algorithm>
using namespace std;
//print array
template <typename T>
void print(const vector<T> &arr)
{
for (size_t i = 0; i < arr.size(); i++)
cout << arr[i] << " ";
cout << endl;
}
//queue tasks
queue< pair<int, int> > tasks;
//mutexs for set and queue task
mutex q_mutex, s_mutex;
//condition variable
condition_variable cv;
//set
set<int> ss;
//partition algorithm
template <typename T>
int partition(vector<T> &arr, int l, int r)
{
T tmp = arr[r]; //as pivot element
int i = l - 1;
for (int j = l; j <= r - 1; j++)
if (arr[j] < tmp)
{
i++;
swap(arr[i], arr[j]);       
}
swap(arr[i + 1], arr[r]);
i++;
return i;
}
//quick sort
template <typename T>
void quick_sort(vector<T> &arr)
{
while (true)
{
unique_lock<mutex> u_lock(q_mutex); //lock mutex
//sort is fineshed
if ( ss.size() == arr.size() ) //u_lock.unlock()
return;
//if queue task is not empty 
if ( tasks.size() > 0 )
{
//get task from queue
pair<int, int> cur_task = tasks.front();            
tasks.pop();
int l = cur_task.first, r = cur_task.second;        
if (l < r)
{
int q = partition(arr, l, r); //split array
//Add indexes in set
s_mutex.lock();
ss.insert(q);
ss.insert(l);
ss.insert(r);
s_mutex.unlock();
//push new tasks for left and right part
tasks.push( make_pair(l, q - 1) );
tasks.push( make_pair(q + 1, r) );
//wakeup some thread which waiting 
cv.notify_one();
}
}
else
//if queue is empty
cv.wait(u_lock);
}
}
//Size array
const int ARR_SIZE = 100000;
//Count threads
const int THREAD_COUNT = 8;
thread thrs[THREAD_COUNT];
//generatin array
void generate_arr(vector<int> &arr)
{
srand(time( NULL ));
std::generate(arr.begin(), arr.end(), [](){return rand() % 10000; });
}
//check for sorting
bool is_sorted(const vector<int> &arr)
{
for (size_t i = 0; i < arr.size() - 1; i++)
if ( ! (arr[i] <= arr[i + 1]) ) 
return false;
return true;
}
int main()
{
//time
clock_t start, finish;
vector<int> arr(ARR_SIZE);
//generate array
generate_arr(arr);
cout << endl << "Generating finished!" << endl << endl;
cout << "Array before sorting" << endl << endl;
//Before sorting
print(arr);
cout << endl << endl;
cout << "Checking is_sorted finished! The result is " << (is_sorted(arr) == 0? "false": "true") << "." << endl << endl;
//add task
tasks.push( make_pair(0, arr.size() - 1) );
//==================================================
start = clock();
for (int i = 0; i < THREAD_COUNT; i++)
thrs[i] = thread( quick_sort<int>, ref(arr) );
finish = clock();
//==================================================
for (auto& th : thrs)
th.join();
cout << "Sorting finished!" << endl << endl;
cout << "Array after sorting" << endl << endl;
//After sorting
print(arr);
cout << endl << endl;
cout << "Checking is_sorted finished! The result is " << (is_sorted(arr) == 0? "false": "true") << "." << endl << endl;
cout << "Runtime: " << (double)(finish - start) / CLOCKS_PER_SEC << endl;
return 0;
}