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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高级队列锁(Lock)和低级内存锁的主要区别在于它们的使用场景、机制以及对性能的影响。
1. **使用场景**:高级队列锁通常用于更复杂的同步场景,如在多线程环境中保护共享资源,确保数据的一致性和完整性。而低级内存锁,如自旋锁,更适用于短时间的锁定,通常用于保护小范围的内存区域,避免线程间的竞争。
2. **机制**:高级队列锁可能涉及更复杂的锁机制,如公平锁、非公平锁、读写锁等,这些锁机制可以提供更细粒度的控制,减少线程间的等待时间。低级内存锁,如自旋锁,当一个线程试图获取已被占用的锁时,它不会立即阻塞,而是继续执行一个循环(自旋),直到锁被释放。这种机制在锁的持有时间很短时可以减少上下文切换的开销。
3. **性能影响**:高级队列锁在锁的获取和释放过程中可能涉及更多的系统调用和上下文切换,这在锁的持有时间较长时可以减少资源浪费。而低级内存锁在锁的持有时间很短时可以减少上下文切换的开销,但在锁的持有时间较长时,自旋锁会消耗大量的CPU资源,导致其他线程无法运行。
4. **数据落盘机制与缓存使用**:高级队列锁和低级内存锁在数据的落盘机制和缓存使用方面没有直接关系,但它们可能间接影响到这些机制的效率。例如,如果锁保护的资源是需要频繁访问的缓存数据,那么锁的效率将直接影响到缓存的访问性能。
5. **PFC死锁与恢复**:PFC(Priority-Flow Control)死锁和恢复次数与队列锁的使用有关,但更直接地与网络设备的流量控制机制相关。PFC死锁通常发生在网络设备的队列中,当队列满时,设备会发送PFC帧停止上游设备的发送,如果多个设备同时发生这种情况,可能会导致死锁。死锁恢复次数则反映了网络设备从死锁状态中恢复的能力。
综上所述,高级队列锁和低级内存锁在使用场景、机制、性能影响等方面存在显著差异,选择哪种锁取决于具体的应用需求和性能要求。
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