最近突然對MySQL的連接非常感興趣，從status根據thread關鍵字可以查出如下是個狀態
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.
以上是這4個狀態的含義，thread_connected等於show processlist，thread_running代表真正在運行的（等於1一般就是這個show status命令本身），thread_cached代表mysql管理的線程池中還有多少可以被復用的資源，thread_created代表新創建的thread（根據官方文檔，如果thread_created增大迅速，需要適當調高thread_cache_size）。
我們先來實際看下這4個狀態之間的直觀關系。
從上面這個圖，我們可以總結出來一個公式：running和其他三個狀態關系不大，但肯定不會超過thread_connected
(new_con-old_con)=create+(old_cache-new_cache)
從上面公式可以看出，如果create等於0，那么thread_connected減少的和thread_cached增加的相等，thread_connected增加的和thread_cached減少的相等。（其實這也就是thread_cached存在的意義，資源可以復用）
我們來看眼影響thread_cached的參數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”.
眾所周知，mysql建立連接非常消耗資源，所以就有了thread_cache，當已有連接不再使用之后，mysql server不是直接斷開連接，而是將已有連接轉入到thread_cache中，以便下次在有create thread的需求時，可以在cache中復用，提高性能，降低資源消耗。
當然，如果已經有了中間件或者其他的連接池管理，那么這個參數就沒有那么重要了，但是如果沒有其他的連接池管理，那么優化這個參數還是可以得到不錯的回報的。