C++mutex


Osservazioni

È meglio usare std :: shared_mutex di std :: shared_timed_mutex .

La differenza di prestazioni è più del doppio.

Se vuoi usare RWLock, troverai che ci sono due opzioni.
È std :: shared_mutex e shared_timed_mutex.
potresti pensare che std :: shared_timed_mutex sia solo la versione 'std :: shared_mutex + time method'.

Ma l'implementazione è completamente diversa.

Il codice seguente è l'implementazione di MSVC14.1 di std :: shared_mutex.

class shared_mutex
{
public: 
typedef _Smtx_t * native_handle_type;

shared_mutex() _NOEXCEPT
    : _Myhandle(0)
    {    // default construct
    }

~shared_mutex() _NOEXCEPT
    {    // destroy the object
    }

void lock() _NOEXCEPT
    {    // lock exclusive
    _Smtx_lock_exclusive(&_Myhandle);
    }

bool try_lock() _NOEXCEPT
    {    // try to lock exclusive
    return (_Smtx_try_lock_exclusive(&_Myhandle) != 0);
    }

void unlock() _NOEXCEPT
    {    // unlock exclusive
    _Smtx_unlock_exclusive(&_Myhandle);
    }

void lock_shared() _NOEXCEPT
    {    // lock non-exclusive
    _Smtx_lock_shared(&_Myhandle);
    }

bool try_lock_shared() _NOEXCEPT
    {    // try to lock non-exclusive
    return (_Smtx_try_lock_shared(&_Myhandle) != 0);
    }

void unlock_shared() _NOEXCEPT
    {    // unlock non-exclusive
    _Smtx_unlock_shared(&_Myhandle);
    }

native_handle_type native_handle() _NOEXCEPT
    {    // get native handle
    return (&_Myhandle);
    }

shared_mutex(const shared_mutex&) = delete;
shared_mutex& operator=(const shared_mutex&) = delete;
private: 
    _Smtx_t _Myhandle;
};

void __cdecl _Smtx_lock_exclusive(_Smtx_t * smtx)
{    /* lock shared mutex exclusively */
AcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(smtx));
}

void __cdecl _Smtx_lock_shared(_Smtx_t * smtx)
{    /* lock shared mutex non-exclusively */
AcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(smtx));
}

int __cdecl _Smtx_try_lock_exclusive(_Smtx_t * smtx)
{    /* try to lock shared mutex exclusively */
return (TryAcquireSRWLockExclusive(reinterpret_cast<PSRWLOCK>(smtx)));
}

int __cdecl _Smtx_try_lock_shared(_Smtx_t * smtx)
{    /* try to lock shared mutex non-exclusively */
return (TryAcquireSRWLockShared(reinterpret_cast<PSRWLOCK>(smtx)));
}

void __cdecl _Smtx_unlock_exclusive(_Smtx_t * smtx)
{    /* unlock exclusive shared mutex */
ReleaseSRWLockExclusive(reinterpret_cast<PSRWLOCK>(smtx));
}

void __cdecl _Smtx_unlock_shared(_Smtx_t * smtx)
{    /* unlock non-exclusive shared mutex */
ReleaseSRWLockShared(reinterpret_cast<PSRWLOCK>(smtx));
}

Puoi vedere che std :: shared_mutex è implementato in Windows Slim Reader / Write Locks ( https://msdn.microsoft.com/ko-kr/library/windows/desktop/aa904937(v=vs.85).aspx)

Ora esaminiamo l'implementazione di std :: shared_timed_mutex.

Il codice seguente è l'implementazione di MSVC14.1 di std :: shared_timed_mutex.

class shared_timed_mutex
{
typedef unsigned int _Read_cnt_t;
static constexpr _Read_cnt_t _Max_readers = _Read_cnt_t(-1);
public:
shared_timed_mutex() _NOEXCEPT
    : _Mymtx(), _Read_queue(), _Write_queue(),
        _Readers(0), _Writing(false)
    {    // default construct
    }

~shared_timed_mutex() _NOEXCEPT
    {    // destroy the object
    }

void lock()
    {    // lock exclusive
    unique_lock<mutex> _Lock(_Mymtx);
    while (_Writing)
        _Write_queue.wait(_Lock);
    _Writing = true;
    while (0 < _Readers)
        _Read_queue.wait(_Lock);    // wait for writing, no readers
    }

bool try_lock()
    {    // try to lock exclusive
    lock_guard<mutex> _Lock(_Mymtx);
    if (_Writing || 0 < _Readers)
        return (false);
    else
        {    // set writing, no readers
        _Writing = true;
        return (true);
        }
    }

template<class _Rep,
    class _Period>
    bool try_lock_for(
        const chrono::duration<_Rep, _Period>& _Rel_time)
    {    // try to lock for duration
    return (try_lock_until(chrono::steady_clock::now() + _Rel_time));
    }

template<class _Clock,
    class _Duration>
    bool try_lock_until(
        const chrono::time_point<_Clock, _Duration>& _Abs_time)
    {    // try to lock until time point
    auto _Not_writing = [this] { return (!_Writing); };
    auto _Zero_readers = [this] { return (_Readers == 0); };
    unique_lock<mutex> _Lock(_Mymtx);

    if (!_Write_queue.wait_until(_Lock, _Abs_time, _Not_writing))
        return (false);

    _Writing = true;

    if (!_Read_queue.wait_until(_Lock, _Abs_time, _Zero_readers))
        {    // timeout, leave writing state
        _Writing = false;
        _Lock.unlock();    // unlock before notifying, for efficiency
        _Write_queue.notify_all();
        return (false);
        }

    return (true);
    }

void unlock()
    {    // unlock exclusive
        {    // unlock before notifying, for efficiency
        lock_guard<mutex> _Lock(_Mymtx);

        _Writing = false;
        }

    _Write_queue.notify_all();
    }

void lock_shared()
    {    // lock non-exclusive
    unique_lock<mutex> _Lock(_Mymtx);
    while (_Writing || _Readers == _Max_readers)
        _Write_queue.wait(_Lock);
    ++_Readers;
    }

bool try_lock_shared()
    {    // try to lock non-exclusive
    lock_guard<mutex> _Lock(_Mymtx);
    if (_Writing || _Readers == _Max_readers)
        return (false);
    else
        {    // count another reader
        ++_Readers;
        return (true);
        }
    }

template<class _Rep,
    class _Period>
    bool try_lock_shared_for(
        const chrono::duration<_Rep, _Period>& _Rel_time)
    {    // try to lock non-exclusive for relative time
    return (try_lock_shared_until(_Rel_time
        + chrono::steady_clock::now()));
    }

template<class _Time>
    bool _Try_lock_shared_until(_Time _Abs_time)
    {    // try to lock non-exclusive until absolute time
    auto _Can_acquire = [this] {
        return (!_Writing && _Readers < _Max_readers); };

    unique_lock<mutex> _Lock(_Mymtx);

    if (!_Write_queue.wait_until(_Lock, _Abs_time, _Can_acquire))
        return (false);

    ++_Readers;
    return (true);
    }

template<class _Clock,
    class _Duration>
    bool try_lock_shared_until(
        const chrono::time_point<_Clock, _Duration>& _Abs_time)
    {    // try to lock non-exclusive until absolute time
    return (_Try_lock_shared_until(_Abs_time));
    }

bool try_lock_shared_until(const xtime *_Abs_time)
    {    // try to lock non-exclusive until absolute time
    return (_Try_lock_shared_until(_Abs_time));
    }

void unlock_shared()
    {    // unlock non-exclusive
    _Read_cnt_t _Local_readers;
    bool _Local_writing;

        {    // unlock before notifying, for efficiency
        lock_guard<mutex> _Lock(_Mymtx);
        --_Readers;
        _Local_readers = _Readers;
        _Local_writing = _Writing;
        }

    if (_Local_writing && _Local_readers == 0)
        _Read_queue.notify_one();
    else if (!_Local_writing && _Local_readers == _Max_readers - 1)
        _Write_queue.notify_all();
    }

shared_timed_mutex(const shared_timed_mutex&) = delete;
shared_timed_mutex& operator=(const shared_timed_mutex&) = delete;
private:
mutex _Mymtx;
condition_variable _Read_queue, _Write_queue;
_Read_cnt_t _Readers;
bool _Writing;
};

class stl_condition_variable_win7 final : public stl_condition_variable_interface
{
public:
    stl_condition_variable_win7()
    {
        __crtInitializeConditionVariable(&m_condition_variable);
    }

    ~stl_condition_variable_win7() = delete;
    stl_condition_variable_win7(const stl_condition_variable_win7&) = delete;
    stl_condition_variable_win7& operator=(const stl_condition_variable_win7&) = delete;

    virtual void destroy() override {}

    virtual void wait(stl_critical_section_interface *lock) override
    {
        if (!stl_condition_variable_win7::wait_for(lock, INFINITE))
            std::terminate();
    }

    virtual bool wait_for(stl_critical_section_interface *lock, unsigned int timeout) override
    {
        return __crtSleepConditionVariableSRW(&m_condition_variable, static_cast<stl_critical_section_win7 *>(lock)->native_handle(), timeout, 0) != 0;
    }

    virtual void notify_one() override
    {
        __crtWakeConditionVariable(&m_condition_variable);
    }

    virtual void notify_all() override
    {
        __crtWakeAllConditionVariable(&m_condition_variable);
    }

private:
    CONDITION_VARIABLE m_condition_variable;
};

Puoi vedere che std :: shared_timed_mutex è implementato in std :: condition_value.

Questa è un'enorme differenza.

Quindi controlliamo le prestazioni di due di loro.

RISULTATI DEL TEST

Questo è il risultato del test di lettura / scrittura per 1000 millisecondi.

std :: shared_mutex ha elaborato read / write più di 2 volte rispetto a std :: shared_timed_mutex.

In questo esempio, il rapporto di lettura / scrittura è lo stesso, ma la frequenza di lettura è più frequente della velocità di scrittura in tempo reale.
Pertanto, la differenza di prestazioni può essere maggiore.

il codice qui sotto è il codice in questo esempio.

void useSTLSharedMutex()
{
    std::shared_mutex shared_mtx_lock;

    std::vector<std::thread> readThreads;
    std::vector<std::thread> writeThreads;

    std::list<int> data = { 0 };
    volatile bool exit = false;

    std::atomic<int> readProcessedCnt(0);
    std::atomic<int> writeProcessedCnt(0);

    for (unsigned int i = 0; i < std::thread::hardware_concurrency(); i++)
    {

        readThreads.push_back(std::thread([&data, &exit, &shared_mtx_lock, &readProcessedCnt]() {
            std::list<int> mydata;
            int localProcessCnt = 0;

            while (true)
            {
                shared_mtx_lock.lock_shared();

                mydata.push_back(data.back());
                ++localProcessCnt;

                shared_mtx_lock.unlock_shared();

                if (exit)
                    break;
            }

            std::atomic_fetch_add(&readProcessedCnt, localProcessCnt);

        }));

        writeThreads.push_back(std::thread([&data, &exit, &shared_mtx_lock, &writeProcessedCnt]() {

            int localProcessCnt = 0;

            while (true)
            {
                shared_mtx_lock.lock();

                data.push_back(rand() % 100);
                ++localProcessCnt;

                shared_mtx_lock.unlock();

                if (exit)
                    break;
            }

            std::atomic_fetch_add(&writeProcessedCnt, localProcessCnt);

        }));
    }

    std::this_thread::sleep_for(std::chrono::milliseconds(MAIN_WAIT_MILLISECONDS));
    exit = true;

    for (auto &r : readThreads)
        r.join();

    for (auto &w : writeThreads)
        w.join();

    std::cout << "STLSharedMutex READ :           " << readProcessedCnt << std::endl;
    std::cout << "STLSharedMutex WRITE :          " << writeProcessedCnt << std::endl;
    std::cout << "TOTAL READ&WRITE :              " << readProcessedCnt + writeProcessedCnt << std::endl << std::endl;
}

void useSTLSharedTimedMutex()
{
    std::shared_timed_mutex shared_mtx_lock;

    std::vector<std::thread> readThreads;
    std::vector<std::thread> writeThreads;

    std::list<int> data = { 0 };
    volatile bool exit = false;

    std::atomic<int> readProcessedCnt(0);
    std::atomic<int> writeProcessedCnt(0);

    for (unsigned int i = 0; i < std::thread::hardware_concurrency(); i++)
    {

        readThreads.push_back(std::thread([&data, &exit, &shared_mtx_lock, &readProcessedCnt]() {
            std::list<int> mydata;
            int localProcessCnt = 0;

            while (true)
            {
                shared_mtx_lock.lock_shared();

                mydata.push_back(data.back());
                ++localProcessCnt;

                shared_mtx_lock.unlock_shared();

                if (exit)
                    break;
            }

            std::atomic_fetch_add(&readProcessedCnt, localProcessCnt);

        }));

        writeThreads.push_back(std::thread([&data, &exit, &shared_mtx_lock, &writeProcessedCnt]() {

            int localProcessCnt = 0;

            while (true)
            {
                shared_mtx_lock.lock();

                data.push_back(rand() % 100);
                ++localProcessCnt;

                shared_mtx_lock.unlock();

                if (exit)
                    break;
            }

            std::atomic_fetch_add(&writeProcessedCnt, localProcessCnt);

        }));
    }

    std::this_thread::sleep_for(std::chrono::milliseconds(MAIN_WAIT_MILLISECONDS));
    exit = true;

    for (auto &r : readThreads)
        r.join();

    for (auto &w : writeThreads)
        w.join();

    std::cout << "STLSharedTimedMutex READ :      " << readProcessedCnt << std::endl;
    std::cout << "STLSharedTimedMutex WRITE :     " << writeProcessedCnt << std::endl;
    std::cout << "TOTAL READ&WRITE :              " << readProcessedCnt + writeProcessedCnt << std::endl << std::endl;
}

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