如何使boost :: thread_group执行固定数量的并行线程

This is the code to create a thread_group and execute all threads in parallel:

这是创建thread_group并并行执行所有线程的代码：

boost::thread_group group;
for (int i = 0; i < 15; ++i)
    group.create_thread(aFunctionToExecute);
group.join_all();

This code will execute all threads at once. What I want to do is to execute them all but 4 maximum in parallel. When on is terminated, another one is executed until there are no more to execute.

此代码将立即执行所有线程。我想要做的是并行执行除4最大值以外的所有操作。当on终止时，执行另一个，直到不再执行为止。

4 个解决方案

#1

Another, more efficient solution would be to have each thread callback to the primary thread when they are finished, and the handler on the primary thread could launch a new thread each time. This prevents the repetitive calls to timed_join, as the primary thread won't do anything until the callback is triggered.

另一个更有效的解决方案是让每个线程在完成后回调到主线程，并且主线程上的处理程序每次都可以启动一个新线程。这可以防止重复调用timed_join，因为主要线程在触发回调之前不会执行任何操作。

#2

I have something like this:

我有这样的事情：

    boost::mutex mutex_;
    boost::condition_variable condition_;
    const size_t throttle_;
    size_t size_;
    bool wait_;
    template <typename Env, class F>
    void eval_(const Env &env, const F &f) {
        {   
            boost::unique_lock<boost::mutex> lock(mutex_);
            size_ = std::min(size_+1, throttle_);
            while (throttle_ <= size_) condition_.wait(lock);
        }
        f.eval(env);
        {
            boost::lock_guard<boost::mutex> lock(mutex_);
            --size_; 
        }
        condition_.notify_one();
    }

#3

I think you are looking for a thread_pool implementation, which is available here.

我想你正在寻找一个thread_pool实现，可以在这里找到。

Additionally I have noticed that if you create a vector of std::future and store futures of many std::async_tasks in it and you do not have any blocking code in the function passed to the thread, VS2013 (atleast from what I can confirm) will launch exactly the appropriate no of threads your machine can handle. It reuses the threads once created.

另外我注意到如果你创建一个std :: future的向量并在其中存储许多std :: async_tasks的未来，并且在传递给该线程的函数中没有任何阻塞代码，VS2013（至少我可以确认））将准确启动您的机器可以处理的相应的线程数。它重用一次创建的线程。

#4

I created my own simplified interface of boost::thread_group to do this job:

我创建了自己的boost :: thread_group简化接口来完成这项工作：

class ThreadGroup : public boost::noncopyable
{
    private:
        boost::thread_group        group;
        std::size_t                maxSize;
        float                      sleepStart;
        float                      sleepCoef;
        float                      sleepMax;
        std::set<boost::thread*>   running;

    public:
        ThreadGroup(std::size_t max_size = 0,
                    float max_sleeping_time = 1.0f,
                    float sleeping_time_coef = 1.5f,
                    float sleeping_time_start = 0.001f) :
            boost::noncopyable(),
            group(),
            maxSize(max_size),
            sleepStart(sleeping_time_start),
            sleepCoef(sleeping_time_coef),
            sleepMax(max_sleeping_time),
            running()
        {
            if(max_size == 0)
                this->maxSize = (std::size_t)std::max(boost::thread::hardware_concurrency(), 1u);
            assert(max_sleeping_time >= sleeping_time_start);
            assert(sleeping_time_start > 0.0f);
            assert(sleeping_time_coef > 1.0f);
        }

        ~ThreadGroup()
        {
            this->joinAll();
        }

        template<typename F> boost::thread* createThread(F f)
        {
            float sleeping_time = this->sleepStart;
            while(this->running.size() >= this->maxSize)
            {
                for(std::set<boost::thread*>::iterator it = running.begin(); it != running.end();)
                {
                    const std::set<boost::thread*>::iterator jt = it++;
                    if((*jt)->timed_join(boost::posix_time::milliseconds((long int)(1000.0f * sleeping_time))))
                        running.erase(jt);
                }
                if(sleeping_time < this->sleepMax)
                {
                    sleeping_time *= this->sleepCoef;
                    if(sleeping_time > this->sleepMax)
                        sleeping_time = this->sleepMax;
                }
            }
            return *this->running.insert(this->group.create_thread(f)).first;
        }

        void joinAll()
        {
            this->group.join_all();
        }

        void interruptAll()
        {
#ifdef BOOST_THREAD_PROVIDES_INTERRUPTIONS
            this->group.interrupt_all();
#endif
        }

        std::size_t size() const
        {
            return this->group.size();
        }
    };

Here is an example of use, very similar to boost::thread_group with the main difference that the creation of the thread is a waiting point:

这是一个使用示例，与boost :: thread_group非常相似，主要区别在于线程的创建是一个等待点：

{
  ThreadGroup group(4);
  for(int i = 0; i < 15; ++i)
    group.createThread(aFunctionToExecute);
} // join all at destruction

#1

#2