python - 了解使用锁和 Python GIL 的抢占式多任务处理？

Question

我正在阅读Grok The GIL在关于锁定的讨论中，有如下的表述。

So long as no thread holds a lock while it sleeps, does I/O, or some other GIL-dropping operation, you should use the coarsest, simplest locks possible. Other threads couldn't have run in parallel anyway.

它是在关于抢占式多任务处理的讨论之后出现的。当你有锁时，什么可以防止 GIL 被抢先丢弃？或者这不是该声明所指的内容？

Answer 1

我问了这篇文章的作者，它归结为因为等待外部操作而删除 GIL 与内部抢占之间的区别:https://opensource.com/article/17/4/grok-gil#comment-136186

Hi! Nothing prevents a thread from preemptively dropping the GIL whileit holds a lock. Let's call that Thread A, and let's say there's alsoa Thread B. If Thread A holds a lock and gets preempted, then maybeThread B could run instead of Thread A.

If Thread B is waiting for the lock that Thread A is holding, then Thread B is not waiting for the GIL. In that case Thread A reacquires the GIL immediately after dropping it, and Thread A continues.

If Thread B is not waiting forthe lock that Thread A is holding, then Thread B might acquire the GILand run.

My point about coarse locks, however, is this: no two threadscan ever execute Python in parallel, because of the GIL. So usingfine-grained locks doesn't improve throughput. This is in contrast toa language like Java or C, where fine-grained locks allow greaterparallelism, and therefore greater throughput.

我仍然需要一些澄清，他确实证实了这一点:

If I'm understanding you correctly, the intent of the statement I referenced was to avoid using locks around external operations, where you could then block multiple threads, if they all depended on that lock.

For the preemptive example, Thread A isn't blocked by anything externally, so the processing just goes back and forth similar to cooperative multitasking.