什么是计数信号量？ [英] what is Counting Semaphore?
问题描述
我确实知道计数信号量是如何工作的？请帮助我理解。
Hi I did get how the Counting Semaphore works? Please help me in understanding.
根据我的理解，如果我们将count设置为3，则进程可以使用3个线程来访问资源。因此，这里只有3个线程可以访问资源。当1个线程离开时，另一个等待线程进入。如果我的理解是正确的，那么这3个线程也会破坏共享数据。那么，它的用途是什么？
As per my understanding if we set count as 3, then process can use 3 threads to access the resource. so, here just 3 threads have access on the resource. When 1 thread leaves the other waiting thread comes in. If my understanding is correct, these 3 thread can corrupt shared data too. Then what is use of it?
推荐答案
您的观察是正确的；通常，资源要么需要限制为一个线程（例如，正在被写入），要么可以安全地与无限数量的线程一起使用（例如，它是只读的）。限制要由5个线程使用的资源很少有用。
Your observations are correct; typically a resource either needs to be restricted to one thread (e.g. it is being written to), or is safe to use with an unlimited number of threads (e.g. it is read-only). Restricting a resource to be used by say 5 threads is rarely useful.
因此，计数为 N 的计数信号量通常用于限制访问到 N 个资源池中...当计数达到零时，下一个线程必须等待从该池中获取资源。
Thus a counting semaphore with count N is most often used to restrict access to a pool of N resources...when the count reaches zero the next thread has to wait to obtain a resource from the pool.
但是，在实践中，我通常并不觉得这有用，因为仅控制访问资源池的线程的 number 是不够的，您还需要自己管理资源。因此，我通常以阻塞队列结尾，该队列包含线程可以从中获取的托管资源。当线程用完资源后，它将该资源（例如对象）返回到队列，以便等待的线程可以使用它。
However, I don't commonly find this useful in practice because simply controlling the number of threads accessing a pool of resources isn't sufficient, you need to manage the resources themselves as well. So I typically end up with a blocking queue containing the managed resources that threads can take from. When a thread is done with a resource, it returns that resource (e.g. an object) to the queue so that a waiting thread can take it.
队列可能在内部使用信号量来控制对内部缓冲区的访问，但这通常是由队列用户封装的。
The queue might internally use a semaphore to control access to the internal buffer, but that is usually encapsulated from the user of the queue.
- Wikipedia: Semaphore - Important Observations
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