Producer consumer with threads and using boost ring buffer

Question

I have two threads, one is the producer and other is consumer. My consumer is always late (due to some costly function call, simulated in below code using sleeps) so I have used ring buffer as I can afford to loose some events.

I am looking to see if my locking looks alright and general c++ review comments.

#include <iostream>
#include <thread>
#include <chrono>
#include <vector>
#include <atomic>
#include <boost/circular_buffer.hpp>
#include <condition_variable>
#include <functional>

std::atomic<bool> mRunning;
std::mutex m_mutex;
std::condition_variable m_condVar;

class VecBuf {
    private:
    std::vector<int8_t> vec;

    public:
    VecBuf() = default;
    VecBuf(std::vector<int8_t> v)
    {
        vec = v;
    }
};

std::vector<int8_t> data{ 10, 20, 30 };

class Detacher {
    public:
    template<typename Function, typename ... Args>
    void createTask(Function &&func, Args&& ... args) {
        m_threads.emplace_back(std::forward<Function>(func), std::forward<Args>(args)...);
    }

    Detacher() = default;
    Detacher(const Detacher&) = delete;
    Detacher & operator=(const Detacher&) = delete;
    Detacher(Detacher&&) = default;
    Detacher& operator=(Detacher&&) = default;

    ~Detacher() {
        for (auto& thread : m_threads) {
            thread.join();
        }
    }

    private:
    std::vector<std::thread> m_threads;
};

void foo_1(boost::circular_buffer<VecBuf> *cb)
{
    while (mRunning) {
        std::unique_lock<std::mutex> mlock(m_mutex);

        m_condVar.wait(mlock, [=]() { return !cb->empty(); });

        VecBuf local_data(cb->front());
        cb->pop_front();
        mlock.unlock();
        if (!mRunning) {
            break;
        }
        //simulate time consuming function call and consume local_data here
        std::this_thread::sleep_for(std::chrono::milliseconds(16));
    }

    while (cb->size()) {
        VecBuf local_data(cb->front());
        cb->pop_front();
        if (!mRunning) {
            break;
        }
    }
}

void foo_2(boost::circular_buffer<VecBuf> *cb)
{
    while (mRunning) {
        std::unique_lock<std::mutex> mlock(m_mutex);

        while (cb->full()) {
            mlock.unlock();
            /* can we do better than this? */
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
            mlock.lock();
        }
        cb->push_back(VecBuf(data));
        m_condVar.notify_one();
    }
}

int main()
{
    mRunning = true;
    boost::circular_buffer<VecBuf> cb(100);
    Detacher thread_1;
    thread_1.createTask(foo_1, &cb);
    Detacher thread_2;
    thread_2.createTask(foo_2, &cb);
    std::this_thread::sleep_for(std::chrono::milliseconds(20000));
    mRunning = false;
}

Answer 1

       /* can we do better than this? */

In a circular buffer context, the standard way to avoid busy waiting is to have two semaphores. First to block a producer when a buffer is full, and second to block a consumer when the buffer is empty. Once a process passes its semaphore, and does its job, it should signal the peer.

---

The circular buffer is good when the consumer is only sometimes late and you cannot afford loosing data. In your situation it looks like a wrong solution: the producer becomes throttled by the rate of consumption, and the consumer is a presented with the stale data.

A typical answer is to let the producer run at full speed, and triple-buffer the production (at least, it guarantees that the consumer would get the most recently produced data). Please forgive the shameless self promotion.