我想在我的多线程Python应用程序中定期执行操作。我已经看到了两种不同的方法
exit = False
def thread_func():
while not exit:
action()
time.sleep(DELAY)
要么
exit_flag = threading.Event()
def thread_func():
while not exit_flag.wait(timeout=DELAY):
action()
一种方法比另一种方法有优势吗？是使用更少的资源还是与其他线程和GIL更好地协作？哪一个使我的应用程序中的其余线程更具响应性？
（假设有一些外部事件集exit或exit_flag，我愿意在关闭时等待完整的延迟）
Event.wait在这种情况下使用，即使python 2.x在后台轮询。以1秒为间隔的睡眠时间可以合理地做出反应，并且不那么刺耳。
Answers:
使用exit_flag.wait(timeout=DELAY)将具有更高的响应速度，因为在exit_flag设置时，您将立即退出while循环。使用time.sleep，即使在设置了事件之后，您也将在time.sleep通话中等待直到睡了DELAY几秒钟。
在实现方面，Python 2.x和Python 3.x具有非常不同的行为。在Python 2.xEvent.wait中，使用大量的小time.sleep调用在纯Python中实现：
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)
实际上，这意味着使用wait它可能比仅DELAY无条件地休眠整个服务器要消耗更多的CPU ，但这样做的好处是（DELAY响应时间长很多，取决于时间长短）。这也意味着需要经常重新获取GIL，以便可以安排下次睡眠，同时time.sleep可以释放GIL的全部时间。DELAY。现在，更频繁地获取GIL是否会对您应用程序中的其他线程产生明显影响？也许不是。这取决于正在运行的其他线程数以及它们承担的工作量。我的猜测是，除非您有大量的线程，或者另一个线程正在执行大量CPU限制的工作，否则它不会特别引人注目，但是它很容易同时尝试和观察。
在Python 3.x中，大部分实现都移至纯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()
和获得锁的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;
}
该实现是响应式的，不需要频繁唤醒即可重新获取GIL，因此您可以两全其美。
sleep(DELAY)GIL较轻？尽管不那么准确？
wait都需要重新获取GIL，在那里sleep可以将其全部释放DELAY。
Python 2. *
就像@dano所说的那样，event.wait的响应速度更快，
但是当系统时间向后更改时，它可能会很危险！错误＃1607041：由于时钟更改，Condition.wait超时失败
看到这个例子：
def someHandler():
while not exit_flag.wait(timeout=0.100):
action()
通常action()会在100ms间隔内调用。
但是，当您更改时间时。一个小时，则两个动作之间要暂停一小时。
结论：如果可以更改时间，则应避免 event.wait
有趣的是，可以单独调用event.wait（）方法：
from threading import Event # Needed for the wait() method
from time import sleep
print("\n Live long and prosper!")
sleep(1) # Conventional sleep() Method.
print("\n Just let that soak in..")
Event().wait(3.0) # wait() Method, useable sans thread.
print("\n Make it So! = )\n")
那么，为什么在多线程之外不使用wait（）替代sleep（）？一句话，禅。（当然。）代码的清晰是很重要的。
exit标志的代码在哪里？是在action()调用中还是在另一个线程中，或者可能是由信号处理程序调用的？