由innodb_purge_threads控制purge線程數。
(>= 5.6.5)的版本中該值默認為1，最大值為32。默認值1表示innodb的purge操作被分離到purge線程中，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.
