互斥-同步如何工作? [英] Mutual exclusion - How synchronized works?

问题描述

如 answer 中所述，synchronized是使用 compareAndSwap 实现的，阻止算法.

As mentioned in answer, synchronized is implemented using compareAndSwap, which is non-blocking algorithm.

1. 在不使用wait()的情况下在synchronized上，线程状态是否设置为BLOCKED?

1. On synchronized without using wait(), Does a thread state set to BLOCKED?

在BLOCKED&中执行线程吗? WAITING状态会消耗CPU周期吗?

Does a thread in BLOCKED & WAITING state consume CPU cycles?

推荐答案

如答案中所述，同步是使用compareAndSwap实现的，compareAndSwap是一种非阻塞算法.

As mentioned in answer, synchronized is implemented using compareAndSwap, which is non-blocking algorithm.

我认为您在误解该答案. synchronized当然不是通过Java级compareAndSwap调用实现的.它将由JVM中的解释器和JIT以本机代码实现.在幕后，它可能使用比较和交换指令，或者它可以使用其他指令(原子测试和设定以及原子交换也很常见-某些平台甚至没有CAS原语).

I think you are misreading that answer. synchronized is certainly not implemented with the Java-level compareAndSwap call. It will be implemented in native code by the interpreter and JIT in your JVM. Under the covers it might use the compare-and-swap instruction, or it may use something else (atomic test-and-set and atomic exchange are also common - and some platforms don't even have a CAS primitive).

这绝对不是非阻塞算法"-根据定义，synchronized实现了一种临界区，如果第二个线程试图进入另一个临界区，则它暗示了阻塞.在里面.

This is definitely not a "non-blocking algorithm" - by definition synchronized implements a type of critical section which implies blocking if a second thread tries to enter the critical section while another thread is inside it.

1)在不使用wait()的情况下进行同步时，线程状态是否设置为已阻塞"?

1) On synchronized without using wait(), Does a thread state set to BLOCKED?

是的，如果线程正在等待进入synchronized部分，则其状态将设置为BLOCKED.

Yes, if a thread is waiting to enter a synchronized section, its state is set to BLOCKED.

2)在BLOCKED&中是否有线程?等待状态会消耗CPU周期吗?

2) Does a thread in BLOCKED & WAITING state consume CPU cycles?

通常不，至少不是持续进行.进入和退出状态 ¹ 会带来CPU成本，但是一旦线程被阻塞，它通常会保持不可运行状态，直到监视器空闲时它被唤醒为止，并且不会在这段时间内不要使用CPU资源.

Generally no, at least not in an ongoing manner. There is a CPU cost associated with entering and exiting the state¹, but once the thread is blocked it is generally held in a non-runnable state until it is awoken when the monitor becomes free, and doesn't use CPU resources during that time.

¹ 这就是为什么良好的实现通常会在睡眠之前旋转"一点的原因，以防关键部分较短且保持线程可能很快退出-在这种情况下旋转避免了过渡到睡眠状态需要1000多个周期的开销(这涉及操作系统等).

¹ Which is exactly why good implementations will generally "spin" a bit before going to sleep, in case the critical section is short and the holding thread may soon exit - spinning in this case avoids the 1000+ cycle overhead of making the transition to sleep (which involves the OS, etc).

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