同步的方法基本與多線程相同。
1) Lock
當多個進程需要訪問共享資源的時候，Lock可以用來避免訪問的沖突。
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，如果兩個進程沒有使用lock來同步,則他們對同一個文件的寫操作可能會出現混亂。
2）Semaphore
Semaphore用來控制對共享資源的訪問數量，例如池的最大連接數。
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()
上面的實例中使用semaphore限制了最多有2個進程同時執行。
3）Event
Event用來實現進程間同步通信。
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
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