同时可以执行的线程数，信号量常用于类似php,ssh等的线程池
import threading, time
def run(n):
semaphore.acquire()
print('threading in %s' %n)
time.sleep(1)
semaphore.release()
if __name__ == "__main__":
semaphore = threading.BoundedSemaphore(5)
for i in range(25):
t = threading.Thread(target=run,args=(i,))
t.start()
while threading.active_count() != 1:
pass
else:
print("All threading is done")
