crude read/write mutex benchmark...

/* Simple, crude read/write mutex test
   by: Chris M. Thomasson
__________________________________________*/



#include <thread>
#include <atomic>
#include <shared_mutex>
#include <condition_variable>
#include <iostream>
#include <functional>
#include <cassert>
#include <cstdlib>
#include <ctime>
#include <climits>


#define THREADS 16UL
#define ITERS 10000000UL
#define COUNT (THREADS * ITERS)


// undefine to test std::shared_mutex
#define CT_TEST_FAST_MUTEX 1


// bare bones mutex/condvar based semaphore
struct ct_slow_semaphore
{
    unsigned long m_state;
    std::mutex m_mutex;
    std::condition_variable m_cond;

    ct_slow_semaphore(unsigned long state) : m_state(state) {}

    void inc()
    {
        {
            std::unique_lock<std::mutex> lock(m_mutex);
            ++m_state;
        }

        m_cond.notify_one();
    }

    void add(unsigned long addend)
    {
        {
            std::unique_lock<std::mutex> lock(m_mutex);
            m_state += addend;
        }

        m_cond.notify_all();
    }

    void dec()
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        while (m_state == 0) m_cond.wait(lock);
        --m_state;
    }
};




// bin-sema
struct ct_auto_reset_event
{
    bool m_state;
    std::mutex m_mutex;
    std::condition_variable m_cond;

    ct_auto_reset_event() : m_state(false) {}

    void signal()
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        m_state = true;
        m_cond.notify_one();
    }

    void wait()
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        while (m_state == false) m_cond.wait(lock);
        m_state = false; // auto-reset
    }
};


// just a layer over an auto-reset event
struct ct_fast_mutex
{
    std::atomic<unsigned int> m_state;
    ct_auto_reset_event m_waitset;

    ct_fast_mutex() : m_state(0) {}

    void lock()
    {
        if (m_state.exchange(1, std::memory_order_acquire))
        {
            while (m_state.exchange(2, std::memory_order_acquire))
            {
                m_waitset.wait();
            }
        }
    }

    void unlock()
    {
        if (m_state.exchange(0, std::memory_order_release) == 2)
        {
            m_waitset.signal();
        }
    }
};



// Chris M. Thomassons Experimental Read/Write Mutex
// Yeah, it is pretty damn fat wrt the state, however
// it has some interesting properties...
// The state can be compressed a bit...
// btw, it has no loops...
// Take a look at the lock_shared and unlock_shared functions

#define RWMUTEX_COUNT_MAX LONG_MAX

struct ct_rwmutex
{
    // shared state
    std::atomic<long> m_wrstate;
    std::atomic<long> m_count;
    std::atomic<long> m_rdwake;

    ct_slow_semaphore m_rdwset;
    ct_slow_semaphore m_wrwset;
    ct_fast_mutex m_wrlock;


    ct_rwmutex() :
        m_wrstate(1),
        m_count(RWMUTEX_COUNT_MAX),
        m_rdwake(0),
        m_rdwset(0),
        m_wrwset(0) {
    }


    // READ, pretty slim...
    void lock_shared()
    {
        if (m_count.fetch_add(-1, std::memory_order_acquire) < 1)
        {
            m_rdwset.dec();
        }
    }

    void unlock_shared()
    {
        if (m_count.fetch_add(1, std::memory_order_release) < 0)
        {
            if (m_rdwake.fetch_add(-1, std::memory_order_acq_rel) == 1)
            {
                m_wrwset.inc();
            }
        }
    }


    // WRITE, more hefty
    void lock()
    {
        m_wrlock.lock();

        long count = m_count.fetch_add(-RWMUTEX_COUNT_MAX, std::memory_order_acquire);

        if (count < RWMUTEX_COUNT_MAX)
        {
            long rdwake = m_rdwake.fetch_add(RWMUTEX_COUNT_MAX - count, std::memory_order_acquire);

            if (rdwake + RWMUTEX_COUNT_MAX - count)
            {
                m_wrwset.dec();
            }
        }
    }

    // write unlock
    void unlock()
    {
        long count = m_count.fetch_add(RWMUTEX_COUNT_MAX, std::memory_order_release);

        if (count < 0)
        {
            m_rdwset.add(-count);
        }

        m_wrlock.unlock();
    }
};


struct ct_shared
{
    std::atomic<unsigned long> m_state;

#if defined (CT_TEST_FAST_MUTEX)
    ct_rwmutex m_std_rwmutex;
#else
    std::shared_mutex m_std_rwmutex;
#endif

    ct_shared() : m_state(0) {}
};


void ct_thread(ct_shared& shared, std::size_t index)
{
    for (unsigned int i = 0; i < ITERS; ++i)
    {

        shared.m_std_rwmutex.lock();
        if (i % 256 == 0) std::this_thread::yield();
        shared.m_state += 1;
        shared.m_std_rwmutex.unlock();


        shared.m_std_rwmutex.lock_shared();
        if (i % 512 == 0) std::this_thread::yield();
        //shared.m_state += 1;
        shared.m_std_rwmutex.unlock_shared();

    }
}


int main()
{
    ct_shared shared;

    std::cout << "Testing: ";

#if defined (CT_TEST_FAST_MUTEX)
    std::cout << "Chris M. Thomasson's Experimental Read/Write Mutex\n\n";
#else
    std::cout << "std::shared_mutex\n\n";
#endif

    {
        std::thread threads[THREADS];

        std::clock_t start = std::clock();

        for (std::size_t i = 0; i < THREADS; ++i)
        {
            threads[i] = std::thread(ct_thread, std::ref(shared), i);
        }

        for (std::size_t i = 0; i < THREADS; ++i)
        {
            threads[i].join();
        }

        std::clock_t diff = clock() - start;

        unsigned long msec = diff * 1000 / CLOCKS_PER_SEC;

        std::cout << "msec = " << msec << "\n";
    }

    std::cout << "shared.m_state = " << shared.m_state << "\n";
    std::cout << "\n\nFin!\n";

    assert(shared.m_state == COUNT);

    return 0;
}