Question

我一直在努力应对多线程程序。我目前的理解是当我创建多个线程时，它将并行运行。但它没有发生。

这是我到目前为止的代码。

#!/usr/bin/python

import threading
import time

class myThread (threading.Thread):
    def __init__(self, threadID, name, counter):
        threading.Thread.__init__(self)
        self.threadID = threadID
        self.name = name
        self.counter = counter
    def run(self):
        print "Starting " + self.name
        # Get lock to synchronize threads
        threadLock.acquire()
        print_time(self.name, self.counter, 3)
        # Free lock to release next thread
        threadLock.release()
        print "Exiting " + self.name

def print_time(threadName, delay, counter):
    while counter:
        time.sleep(delay)
        print "%s: %s" % (threadName, time.ctime(time.time()))
        counter -= 1

threadLock = threading.Lock()
threads = []

# Create new threads
thread1 = myThread(1, "Thread-1", 1)
thread2 = myThread(2, "Thread-2", 2)

# Start new Threads
thread1.start()
thread2.start()

# Add threads to thread list
threads.append(thread1)
threads.append(thread2)

# Wait for all threads to complete
for t in threads:
    t.join()
print "Exiting Main Thread"

输出：

Starting Thread-1
Starting Thread-2
Thread-1: Mon Nov 16 23:49:33 2015
Thread-1: Mon Nov 16 23:49:34 2015
Thread-1: Mon Nov 16 23:49:35 2015
Exiting Thread-1
Thread-2: Mon Nov 16 23:49:37 2015
Thread-2: Mon Nov 16 23:49:39 2015
Thread-2: Mon Nov 16 23:49:41 2015
Exiting Thread-2
Exiting Main Thread

我除了两个线程都要并行运行。但它没有发生。请帮我理解多线程程序，因为我是python中多线程程序的新手。提前致谢

Answer 1

在每个帖子中打印之前，您获得了共享锁。所以他们当然没有交错print s，当第一个获取它的线程休眠时，锁定没有被释放，所以第二个线程一直等待锁定。“ / p>

如果目标是仅保护时间打印，而不是从开始到结束的打印循环，则您需要将run和print_time的代码更改为：

    def run(self):
        print "Starting " + self.name
        # No locking now, letting print_time do it
        print_time(self.name, self.counter, 3)
        print "Exiting " + self.name

def print_time(threadName, delay, counter):
    while counter:
        time.sleep(delay)
        # Use a with statement to avoid problems with lock being held forever
        # if release() call skipped due to an exception (or coding error)
        with threadLock: # Hold lock during time and print, and at no other time
            print "%s: %s" % (threadName, time.ctime(time.time()))
        counter -= 1

Answer 2

顺便说一下：

CPython实现细节：在CPython中，由于Global Interpreter Lock，只有一个线程可以同时执行Python代码（即使某些面向性能的库可能会克服此限制）。如果您希望应用程序更好地利用多核计算机的计算资源，建议您使用多处理或concurrent.futures.ProcessPoolExecutor。但是，如果要同时运行多个I / O绑定任务，则线程仍然是一个合适的模型。

Python threading

无论有没有锁，都无法获得真正的并行性。您必须使用multiprocessing。

多线程程序执行不是并行发生的，而是在python中顺序发生

2 个答案: