高级队列锁Lock与低级内存锁的区别?

高级队列锁Lock与低级内存锁的区别：

1.Lock的种类多，如TM，TX，TT等等；

2.Lock作用于数据库Object，Lacth只作用于SGA内存中；

3.Lock的释放需要等到事务结束，Latch是瞬间的占用和释放；

4.Lock存在死锁，Latch不存在死锁（因free lock机制）

 

低级内存锁与中级内存锁的区别，主要在于低级内存锁有lock free机制（无锁式编程）：即请求latch时，如果该latch不可用，进程会在CPU中等待小段时间（spin）然后重新请求latch。如果依然不可用，该进程会重复以上步骤去请求latch。重复的次数默认值为2000（mutex为500）。如果在重复次数内还是请求不到latch，进程会在v$session_wait中记录latch free等待事件，然后释放cpu，转入睡眠状态。当睡眠一定时间后，进程被唤醒并重复上面的过程，一直到成功获取latch。因此低级内存锁避免了死锁的存在。此外，中级内存锁往往执行多条记录才会释放，而低级内存锁执行几条就可能会释放。

 

 

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