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mysql之thread cache讲解

时间:2015-08-14 13:56:21   编辑:一切随缘   文章来源:php教程网 已阅读:1934 次

  mysql之thread cache讲解

  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。

  

mysql之thread cache讲解

  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

  1How 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。

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