Python time.sleep() 与 event.wait()

Question

我想在我的多线程 Python 应用程序中定期执行一项操作。我见过两种不同的做法

exit = False
def thread_func(): 
    while not exit:
       action()
       time.sleep(DELAY)

或

exit_flag = threading.Event()
def thread_func(): 
    while not exit_flag.wait(timeout=DELAY):
       action()

一种方式比另一种方式有优势吗？是使用更少的资源，还是与其他线程和 GIL 一起玩得更好？哪一个使我的应用程序中的剩余线程更具响应性？

(假设一些外部事件集exit或exit_flag，我愿意在关机时等待完全延迟)

Answer 1

使用 exit_flag.wait(timeout=DELAY) 会更灵敏，因为当 exit_flag 设置时，您会立即跳出 while 循环。使用 time.sleep，即使在设置了事件之后，你也会在 time.sleep 调用中等待，直到你睡了 DELAY 秒。

在实现方面，Python 2.x 和 Python 3.x 有非常不同的行为。在 Python 2.x 中，Event.wait 是在纯 Python 中使用一堆小的 time.sleep 调用实现的:

from time import time as _time, sleep as _sleep

....
# This is inside the Condition class (Event.wait calls Condition.wait).
def wait(self, timeout=None):
    if not self._is_owned():
        raise RuntimeError("cannot wait on un-acquired lock")
    waiter = _allocate_lock()
    waiter.acquire()
    self.__waiters.append(waiter)
    saved_state = self._release_save()
    try:    # restore state no matter what (e.g., KeyboardInterrupt)
        if timeout is None:
            waiter.acquire()
            if __debug__:
                self._note("%s.wait(): got it", self)
        else:
            # Balancing act:  We can't afford a pure busy loop, so we
            # have to sleep; but if we sleep the whole timeout time,
            # we'll be unresponsive.  The scheme here sleeps very
            # little at first, longer as time goes on, but never longer
            # than 20 times per second (or the timeout time remaining).
            endtime = _time() + timeout
            delay = 0.0005 # 500 us -> initial delay of 1 ms
            while True:
                gotit = waiter.acquire(0)
                if gotit:
                    break
                remaining = endtime - _time()
                if remaining <= 0:
                    break
                delay = min(delay * 2, remaining, .05)
                _sleep(delay)
            if not gotit:
                if __debug__:
                    self._note("%s.wait(%s): timed out", self, timeout)
                try:
                    self.__waiters.remove(waiter)
                except ValueError:
                    pass
            else:
                if __debug__:
                    self._note("%s.wait(%s): got it", self, timeout)
    finally:
        self._acquire_restore(saved_state)

这实际上意味着使用 wait 可能比仅仅无条件地 sleep 完整的 DELAY 更占用 CPU 资源，但好处是(可能很多，取决于DELAY 多长时间)响应更快。这也意味着需要频繁地重新获取 GIL，以便安排下一次 sleep ，而 time.sleep 可以释放 GIL 以获得完整的 DELAY。现在，更频繁地获取 GIL 会对应用程序中的其他线程产生显着影响吗？也许也许不是。这取决于有多少其他线程正在运行以及它们具有什么样的工作负载。我的猜测是它不会特别引人注目，除非你有大量线程，或者可能有另一个线程在做大量 CPU 密集型工作，但它很容易以两种方式尝试并查看。

在 Python 3.x 中，大部分实现都转移到了纯 C 代码:

import _thread # C-module
_allocate_lock = _thread.allocate_lock

class Condition:
    ...
    def wait(self, timeout=None):
        if not self._is_owned():
            raise RuntimeError("cannot wait on un-acquired lock")
        waiter = _allocate_lock()
        waiter.acquire()
        self._waiters.append(waiter)
        saved_state = self._release_save()
        gotit = False
        try:    # restore state no matter what (e.g., KeyboardInterrupt)
            if timeout is None:
                waiter.acquire()
                gotit = True
            else:
                if timeout > 0:
                    gotit = waiter.acquire(True, timeout)  # This calls C code
                else:
                    gotit = waiter.acquire(False)
            return gotit
        finally:
            self._acquire_restore(saved_state)
            if not gotit:
                try:
                    self._waiters.remove(waiter)
                except ValueError:
                    pass

class Event:
    def __init__(self):
        self._cond = Condition(Lock())
        self._flag = False

    def wait(self, timeout=None):
        self._cond.acquire()
        try:
            signaled = self._flag
            if not signaled:
                signaled = self._cond.wait(timeout)
            return signaled
        finally:
            self._cond.release()

以及获取锁的C代码:

/* Helper to acquire an interruptible lock with a timeout.  If the lock acquire
 * is interrupted, signal handlers are run, and if they raise an exception,
 * PY_LOCK_INTR is returned.  Otherwise, PY_LOCK_ACQUIRED or PY_LOCK_FAILURE
 * are returned, depending on whether the lock can be acquired withing the
 * timeout.
 */
static PyLockStatus
acquire_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds)
{
    PyLockStatus r;
    _PyTime_timeval curtime;
    _PyTime_timeval endtime;


    if (microseconds > 0) {
        _PyTime_gettimeofday(&endtime);
        endtime.tv_sec += microseconds / (1000 * 1000);
        endtime.tv_usec += microseconds % (1000 * 1000);
    }


    do {
        /* first a simple non-blocking try without releasing the GIL */
        r = PyThread_acquire_lock_timed(lock, 0, 0);
        if (r == PY_LOCK_FAILURE && microseconds != 0) {
            Py_BEGIN_ALLOW_THREADS  // GIL is released here
            r = PyThread_acquire_lock_timed(lock, microseconds, 1);
            Py_END_ALLOW_THREADS
        }

        if (r == PY_LOCK_INTR) {
            /* Run signal handlers if we were interrupted.  Propagate
             * exceptions from signal handlers, such as KeyboardInterrupt, by
             * passing up PY_LOCK_INTR.  */
            if (Py_MakePendingCalls() < 0) {
                return PY_LOCK_INTR;
            }

            /* If we're using a timeout, recompute the timeout after processing
             * signals, since those can take time.  */
            if (microseconds > 0) {
                _PyTime_gettimeofday(&curtime);
                microseconds = ((endtime.tv_sec - curtime.tv_sec) * 1000000 +
                                (endtime.tv_usec - curtime.tv_usec));

                /* Check for negative values, since those mean block forever.
                 */
                if (microseconds <= 0) {
                    r = PY_LOCK_FAILURE;
                }
            }
        }
    } while (r == PY_LOCK_INTR);  /* Retry if we were interrupted. */

    return r;
}

此实现具有响应性，不需要频繁唤醒重新获取 GIL，因此您可以两全其美。