最近突然对MySQL的连接非常感兴趣，从status根据thread关键字可以查出如下是个状态
show global status like 'thread%'; +-------------------+-------+ | Variable_name | Value | +-------------------+-------+ | Threads_cached | 57 | | Threads_connected | 1268 | | Threads_created | 31715 | | Threads_running | 1 | +-------------------+-------+
Thread_cached：The number of threads in the thread cache
Thread_connected：The number of currently open connections.
Thread_created：The number of threads created to handle connections.
Thread_running：The number of threads that are not sleeping.
以上是这4个状态的含义，thread_connected等于show processlist，thread_running代表真正在运行的（等于1一般就是这个show status命令本身），thread_cached代表mysql管理的线程池中还有多少可以被复用的资源，thread_created代表新创建的thread（根据官方文档，如果thread_created增大迅速，需要适当调高thread_cache_size）。
我们先来实际看下这4个状态之间的直观关系。
从上面这个图，我们可以总结出来一个公式：running和其他三个状态关系不大，但肯定不会超过thread_connected
(new_con-old_con)=create+(old_cache-new_cache)
从上面公式可以看出，如果create等于0，那么thread_connected减少的和thread_cached增加的相等，thread_connected增加的和thread_cached减少的相等。（其实这也就是thread_cached存在的意义，资源可以复用）
我们来看眼影响thread_cached的参数thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. Normally, this does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.6, “Server Status Variables”.
众所周知，mysql建立连接非常消耗资源，所以就有了thread_cache，当已有连接不再使用之后，mysql server不是直接断开连接，而是将已有连接转入到thread_cache中，以便下次在有create thread的需求时，可以在cache中复用，提高性能，降低资源消耗。
当然，如果已经有了中间件或者其他的连接池管理，那么这个参数就没有那么重要了，但是如果没有其他的连接池管理，那么优化这个参数还是可以得到不错的回报的。