waiting in InnoDB queue

有的应用的查询非常慢，手工访问某库的表也非常慢。看了一个最慢的SQL，访问的都是innodb的表。

从innodb status中看到有一些线程waiting in InnoDB queue，有的明明是select语句也在等待。

搜了一下，估计是下面这个参数的问题

innodb_thread_concurrency

InnoDB tries to keep the number of operating system threads concurrently inside InnoDB less than or equal to the limit given by this variable. Once the number of threads reaches this limit, additional threads are placed into a wait state within a FIFO queue for execution. Threads waiting for locks are not counted in the number of concurrently executing threads.

The correct value for this variable is dependent on environment and workload. You will need to try a range of different values to determine what value works for your application.

The range of this variable is 0 to 1000. A value of 20 or higher is interpreted as infinite concurrency before MySQL 5.1.12. From 5.1.12 on, you can disable thread concurrency checking by setting the value to 0, which allows InnoDB to create as many threads as it needs.

The default value is 20 before MySQL 5.1.11, and 8 from 5.1.11 on.