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MySQL：query cache lock

近期发现管理的一些MySQL实例偶发会出现连接数飙高，响应时间增长，负载上升等问题，但是业务又没有任何的波动，经过一定的排查发现是query cache lock导致。

问题表现：

419 State: Waiting for query cache lock

4 State: statistics

2 State: storing result in query cache

2 State: Sending data

1 State: Waiting for master to send event

关闭query cache 后：

1067 State:

1 State: Waiting for master to send event

1 State: Slave has read all relay log; waiting for the slave I/O thread to update it

关闭的效果非常明显，于是研究了一下query cache lock的问题。

首先，如果开启query cache，mysql再执行一个query的时候会先锁住query cache，然后判断是否再query cache中存在，如果存在直接返回结果，如果不存在，则再进行引擎查询等操作

mysql> show profile for query 4;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000015 |
| Waiting for query cache lock   | 0.000003 |
| checking query cache for query | 0.000045 |
| checking permissions           | 0.000006 |
| Opening tables                 | 0.000027 |
| System lock                    | 0.000007 |
| Waiting for query cache lock   | 0.000032 |
| init                           | 0.000018 |
| optimizing                     | 0.000008 |
| statistics                     | 0.033109 |
| preparing                      | 0.000019 |
| executing                      | 0.000002 |
| Sending data                   | 4.575480 |
| Waiting for query cache lock   | 0.000005 |
| Sending data                   | 5.527728 |
| Waiting for query cache lock   | 0.000005 |
| Sending data                   | 5.743041 |
| Waiting for query cache lock   | 0.000004 |
| Sending data                   | 6.191706 |
| end                            | 0.000007 |
| query end                      | 0.000005 |
| closing tables                 | 0.000028 |
| freeing items                  | 0.000008 |
| Waiting for query cache lock   | 0.000002 |
| freeing items                  | 0.000182 |
| Waiting for query cache lock   | 0.000002 |
| freeing items                  | 0.000002 |
| storing result in query cache  | 0.000004 |
| logging slow query             | 0.000001 |
| logging slow query             | 0.000002 |
| cleaning up                    | 0.000003 |
+--------------------------------+----------+

其次，我们知道，所有insert、update和delete操作都会清空query cahce中被影响表的缓存，从而重新开始种cache。

The query cache always contains current and reliable data. Any insert, update, delete, or other modification to a table causes any relevant entries in the query cache to be flushed.

那么问题就来了，当我们的数据库不是那么频繁的更新的时候，query cache是个好东西，可以在本地形成一个cache，提高RT响应时间，但是如果反过来，如果写入非常频繁，并集中在某机张表上的时候。

那么query cache lock的锁机制会造成很频繁的锁冲突，对于这一张表的写和读会互相等待query cache lock解锁，导致select的查询效率下降。