Which is more efficient, basic mutex lock or atomic integer?

Question

For something simple like a counter if multiple threads will be increasing the number. I read that mutex locks can decrease efficiency since the threads have to wait. So, to

Answer 1

A minimal (standards compliant) mutex implementation requires 2 basic ingredients:

• A way to atomically convey a state change between threads (the 'locked' state)
• memory barriers to enforce memory operations protected by the mutex to stay inside the protected area.

There is no way you can make it any simpler than this because of the 'synchronizes-with' relationship the C++ standard requires.

A minimal (correct) implementation might look like this:

class mutex {
    std::atomic<bool> flag{false};

public:
    void lock()
    {
        while (flag.exchange(true, std::memory_order_relaxed));
        std::atomic_thread_fence(std::memory_order_acquire);
    }

    void unlock()
    {
        std::atomic_thread_fence(std::memory_order_release);
        flag.store(false, std::memory_order_relaxed);
    }
};

Due to its simplicity (it cannot suspend the thread of execution), it is likely that, under low contention, this implementation outperforms a std::mutex. But even then, it is easy to see that each integer increment, protected by this mutex, requires the following operations:

• an atomic store to release the mutex
• an atomic compare-and-swap (read-modify-write) to acquire the mutex (possibly multiple times)
• an integer increment

If you compare that with a standalone std::atomic<int> that is incremented with a single (unconditional) read-modify-write (eg. fetch_add), it is reasonable to expect that an atomic operation (using the same ordering model) will outperform the case whereby a mutex is used.