由innodb_purge_threads控制purge线程数。
(>= 5.6.5)的版本号中该值默觉得1。最大值为32。默认值1表示innodb的purge操作被分离到purge线程中，master
thread不再做purge操作。
The
number of background threads devoted to the InnoDB purge operation.
The new default and minimum value of 1 in MySQL 5.6.5 signifies that the purge operation is always performed by background threads, never as part of the master
thread. Non-zero values runs the purge operation in one or more background threads, which can reduce internal contention within InnoDB,
improving scalability. Increasing the value to greater than 1 creates that many separate purge threads, which can improve efficiency on systems where DML operations
are performed on multiple tables. The maximum is 32.