为什么 std::condition_variable 使调度不公平

我正在尝试创建一个简单的池对象，我想或多或少地公平地将对一组共享资源的访问权限分配给任何请求它的线程。 在Windows中，我通常会有一个互斥数组并执行WaitForMultipleObjects，bWaitAll=FALSE（见下文windows_pool_of_n_t）。 但我希望有一天能够将其移植到其他操作系统，所以我想坚持这个标准。 资源的数量，在size（）！=0上condition_variable似乎是显而易见的解决方案（见下面的pool_of_n_t）。

但由于我不明白的原因，该代码序列化线程访问。 我并不期望严格的公平性，但这几乎是最糟糕的情况 - 上次锁定的线程下次总是得到锁定。 这并不是说 std：：mutex 或多或少不符合 Windows 公平调度，因为只使用没有条件变量的互斥锁可以按预期工作，尽管当然只适用于一个池（请参阅下面的pool_of_one_t）。

谁能解释一下？ 有没有办法解决这个问题？

结果：

C:tempstdpool>binstdpool.exe
pool:pool_of_one_t
thread 0:19826 ms
thread 1:19846 ms
thread 2:19866 ms
thread 3:19886 ms
thread 4:19906 ms
thread 5:19926 ms
thread 6:19946 ms
thread 7:19965 ms
thread 8:19985 ms
thread 9:20004 ms
pool:windows_pool_of_n_t(1)
thread 0:19819 ms
thread 1:19838 ms
thread 2:19858 ms
thread 3:19878 ms
thread 4:19898 ms
thread 5:19918 ms
thread 6:19938 ms
thread 7:19958 ms
thread 8:19978 ms
thread 9:19997 ms
pool:pool_of_n_t(1)
thread 9:3637 ms
thread 0:4538 ms
thread 6:7558 ms
thread 4:9779 ms
thread 8:9997 ms
thread 2:13058 ms
thread 1:13997 ms
thread 3:17076 ms
thread 5:17995 ms
thread 7:19994 ms
pool:windows_pool_of_n_t(2)
thread 1:9919 ms
thread 0:9919 ms
thread 2:9939 ms
thread 3:9939 ms
thread 5:9958 ms
thread 4:9959 ms
thread 6:9978 ms
thread 7:9978 ms
thread 9:9997 ms
thread 8:9997 ms
pool:pool_of_n_t(2)
thread 2:6019 ms
thread 0:7882 ms
thread 4:8102 ms
thread 5:8182 ms
thread 1:8382 ms
thread 8:8742 ms
thread 7:9162 ms
thread 9:9641 ms
thread 3:9802 ms
thread 6:10201 ms
pool:windows_pool_of_n_t(5)
thread 4:3978 ms
thread 3:3978 ms
thread 2:3979 ms
thread 0:3980 ms
thread 1:3980 ms
thread 9:3997 ms
thread 7:3999 ms
thread 6:3999 ms
thread 5:4000 ms
thread 8:4001 ms
pool:pool_of_n_t(5)
thread 2:3080 ms
thread 0:3498 ms
thread 8:3697 ms
thread 3:3699 ms
thread 6:3797 ms
thread 7:3857 ms
thread 1:3978 ms
thread 4:4039 ms
thread 9:4057 ms
thread 5:4059 ms

代码：

#include <iostream>
#include <deque>
#include <vector>
#include <mutex>
#include <thread>
#include <sstream>
#include <chrono>
#include <iomanip>
#include <cassert>
#include <condition_variable>
#include <windows.h>
using namespace std;
class pool_t {
    public:
        virtual void check_in(int size) = 0;
        virtual int check_out() = 0;
        virtual string pool_name() = 0;
};
class pool_of_one_t : public pool_t {
    mutex lock;
public:
    virtual void check_in(int resource) {
        lock.unlock();
    }
    virtual int check_out() {
        lock.lock();
        return 0;
    }
    virtual string pool_name() {
        return "pool_of_one_t";
    }
};

class windows_pool_of_n_t : public pool_t {
    vector<HANDLE> resources;
public:
    windows_pool_of_n_t(int size) {
        for (int i=0; i < size; ++i)
            resources.push_back(CreateMutex(NULL, FALSE, NULL));
    }
    ~windows_pool_of_n_t() {
        for (auto resource : resources)
            CloseHandle(resource);
    }
    virtual void check_in(int resource) {
        ReleaseMutex(resources[resource]);
    }
    virtual int check_out() {
        DWORD result = WaitForMultipleObjects(resources.size(),
                resources.data(), FALSE, INFINITE);
        assert(result >= WAIT_OBJECT_0 
                && result < WAIT_OBJECT_0+resources.size());
        return result - WAIT_OBJECT_0;
    }
    virtual string pool_name() {
        ostringstream name;
        name << "windows_pool_of_n_t(" << resources.size() << ")";
        return name.str();
    }
};
class pool_of_n_t : public pool_t {
    deque<int> resources;
    mutex lock;
    condition_variable not_empty;
public:
    pool_of_n_t(int size) {
        for (int i=0; i < size; ++i)
            check_in(i);
    }
    virtual void check_in(int resource) {
        unique_lock<mutex> resources_guard(lock);
        resources.push_back(resource);
        resources_guard.unlock();
        not_empty.notify_one();
    }
    virtual int check_out() {
        unique_lock<mutex> resources_guard(lock);
        not_empty.wait(resources_guard,
                [this](){return resources.size() > 0;});
        auto resource = resources.front();
        resources.pop_front();
        bool notify_others = resources.size() > 0;
        resources_guard.unlock();
        if (notify_others)
            not_empty.notify_one();
        return resource;
    }
    virtual string pool_name() {
        ostringstream name;
        name << "pool_of_n_t(" << resources.size() << ")";
        return name.str();
    }
};

void worker_thread(int id, pool_t& resource_pool)
{
    auto start_time = chrono::system_clock::now();
    for (int i=0; i < 100; ++i) {
        auto resource = resource_pool.check_out();
        this_thread::sleep_for(chrono::milliseconds(20));
        resource_pool.check_in(resource);
        this_thread::yield();
    }
    static mutex cout_lock;
    {
        unique_lock<mutex> cout_guard(cout_lock);
        cout << "thread " << id << ":"
            << chrono::duration_cast<chrono::milliseconds>(
                    chrono::system_clock::now() - start_time).count()
            << " ms" << endl;
    }
}
void test_it(pool_t& resource_pool)
{
    cout << "pool:" << resource_pool.pool_name() << endl;
    vector<thread> threads;
    for (int i=0; i < 10; ++i)
        threads.push_back(thread(worker_thread, i, ref(resource_pool)));
    for (auto& thread : threads)
        thread.join();
}
int main(int argc, char* argv[])
{
    test_it(pool_of_one_t());
    test_it(windows_pool_of_n_t(1));
    test_it(pool_of_n_t(1));
    test_it(windows_pool_of_n_t(2));
    test_it(pool_of_n_t(2));
    test_it(windows_pool_of_n_t(5));
    test_it(pool_of_n_t(5));
    return 0;
}