学步园

 Lock

当多个进程需要访问共享资源的时候，Lock可以用来避免访问的冲突。

import multiprocessing
import sys

def worker_with(lock, f):
    with lock:
        fs = open(f,"a+")
        fs.write('Lock acquired via with\n')
        fs.close()
        
def worker_no_with(lock, f):
    lock.acquire()
    try:
        fs = open(f,"a+")
        fs.write('Lock acquired directly\n')
        fs.close()
    finally:
        lock.release()

if __name__ == "__main__":

    f = "file.txt"
  
    lock = multiprocessing.Lock()
    w = multiprocessing.Process(target=worker_with, args=(lock, f))
    nw = multiprocessing.Process(target=worker_no_with, args=(lock, f))

    w.start()
    nw.start()

    w.join()
    nw.join()

在上面的例子中，如果两个进程没有使用lock来同步,则他们对同一个文件的写操作可能会出现混乱。

Semaphore

Semaphore用来控制对共享资源的访问数量，例如池的最大连接数。

import multiprocessing
import time 

def worker(s,i):
    s.acquire()
    print(multiprocessing.current_process().name + " acquire")
    time.sleep(i)
    print(multiprocessing.current_process().name + " release")
    s.release()

if __name__ == "__main__":
  
    s = multiprocessing.Semaphore(2)
    for i in range(5):
        p = multiprocessing.Process(target=worker, args=(s,i*2))
        p.start()

上面的实例中使用semaphore限制了最多有2个进程同时执行。

Event

Event用来实现进程间同步通信。

import multiprocessing
import time

def wait_for_event(e):
    """Wait for the event to be set before doing anything"""
    print ('wait_for_event: starting')
    e.wait()
    print ('wait_for_event: e.is_set()->' + str(e.is_set()))

def wait_for_event_timeout(e, t):
    """Wait t seconds and then timeout"""
    print ('wait_for_event_timeout: starting')
    e.wait(t)
    print ('wait_for_event_timeout: e.is_set()->' + str(e.is_set()))


if __name__ == '__main__':
    e = multiprocessing.Event()
    w1 = multiprocessing.Process(name='block', 
                                 target=wait_for_event,
                                 args=(e,))
    w1.start()

    w2 = multiprocessing.Process(name='non-block', 
                                 target=wait_for_event_timeout, 
                                 args=(e, 2))
    w2.start()

    time.sleep(3)
    e.set()
    print ('main: event is set')

#the output is:
#wait_for_event_timeout: starting

#wait_for_event: starting

#wait_for_event_timeout: e.is_set()->False

#main: event is set

#wait_for_event: e.is_set()->True