MySQL 5.0.19中innodb_thread_concurrency变量的意义发生了变化。设置为0表示不受限制的并发线程数。从5.0.8开始，默认值为20，设置大于等于20将禁用并发检查。但在5.0.19中，又回到了默认值8，而0禁用了并发检查。若之前设置超过500或使用默认值，在新版本中需改为0以避免性能下降。
Posted by
Vadim
In MySQL 5.0.19 the meaning of innodb_thread_concurrency variable was changed (yeah, again).
Now innodb_thread_concurrency=0 means unlimitied count of concurrent threads inside InnoDB. It’s logical, but there was long way. In MySQL versions below 5.0.8 for unlimited threads you had to set innodb_thread_concurrency over 500 (and default value for innodb_thread_concurrency was 8 ).
Starting with MySQL 5.0.8, the default value is 20, and concurrency checking will be disabled if the setting is greater than or equal to 20. But due to different bugs, in 5.0.19 the default value is 8 again, and 0 disables concurrency checking.
So if you used innodb_thread_concurrency over 500 (or default value in 5.0.8-5.0.18), don’t forget to change it to 0 in new versions, otherwise you can experience leaps of performance.
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[...] So what is the best value for innodb_thread_concurrency? For 1-CPU and 2-CPU I think it can be just disabled (innodb_thread_concurrency=0 since 5.0.19, more about default values). [...]
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