进程锁 (互斥锁)
进程锁的引入:
模拟抢票程序:
from multiprocessing import Process
import json
import time
def show_ticket(i):with open("./tickets.txt",mode="r",encoding="utf-8") as file:ticket = json.load(file)print("{}:当前的票数是:{}".format(i,ticket['count']))
def buy_ticket(i):with open("./tickets.txt",mode="r",encoding="utf-8") as file:ticket = json.load(file)if ticket['count'] > 0:ticket['count'] -= 1print("{}买到票了".format(i))else:print("票卖完了")time.sleep(0.1)with open("./tickets.txt",mode="w",encoding="utf-8") as new_file:json.dump(ticket,new_file)if __name__ == '__main__':for i in range(10):Process(target=show_ticket,args=(i,)).start()Process(target=buy_ticket,args=(i,)).start()
运行结果:
实际仅有一张票,但是由于系统的并发执行速度较快,导致系统出现错误
此时,就需要使用锁来进行数据保护,防止出现数据上的错误
进程锁的使用:
from multiprocessing import Process,Lock
import json
import time
def show_ticket(i,lock):lock.acquire()with open("./tickets.txt",mode="r",encoding="utf-8") as file:ticket = json.load(file)print("{}:当前的票数是:{}".format(i,ticket['count']))lock.release()
def buy_ticket(i,lock):lock.acquire()with open("./tickets.txt",mode="r",encoding="utf-8") as file:ticket = json.load(file)if ticket['count'] > 0:ticket['count'] -= 1print("{}买到票了".format(i))else:print("票卖完了")time.sleep(0.1)with open("./tickets.txt",mode="w",encoding="utf-8") as new_file:json.dump(ticket,new_file)lock.release()
if __name__ == '__main__':lock = Lock()for i in range(10):Process(target=show_ticket,args=(i,lock)).start()Process(target=buy_ticket,args=(i,lock)).start()
运行结果:
使用进程锁可以保证一次进运行一个进程,防止进程之间数据的错误
进程锁再次使用:
不使用进程锁:
from multiprocessing import Process
import time
from multiprocessing import Lock
def func1(i,lock):# lock.acquire()print("函数第{}次执行".format(i))time.sleep(1)print("函数执行完毕")# lock.release()
if __name__ == '__main__':lock = Lock()for i in range(10):Process(target=func1,args=(i,lock)).start()
执行结果:
使用进程锁:
from multiprocessing import Process
import time
from multiprocessing import Lock
def func1(i,lock):lock.acquire()print("函数第{}次执行".format(i))time.sleep(1)print("函数执行完毕")if __name__ == '__main__':lock = Lock()for i in range(10):Process(target=func1,args=(i,lock)).start()
执行结果:
注意:由于前者未使用进程锁,因此十个进程并发执行,总执行时间1s
后者使用进程锁,需要一个进程一个进程进行执行,因此总执行时间为10s
部分代码解释:
lock = Lock() 创建一个锁对象
lock.acquire() 表示该进程拿走锁,然后执行,阻塞其他进程
lock.release() 表示该进程执行完毕,归还锁,使得其他进程得以继续执行
此时,若果进程中只有lock.acquire()方法,而没有lock.release()方法,会使得程序阻塞,无法继续向下进行
from multiprocessing import Process
import time
from multiprocessing import Lock
def func1(i,lock):lock.acquire()print("函数第{}次执行".format(i))time.sleep(1)print("函数执行完毕")if __name__ == '__main__':lock = Lock()for i in range(10):Process(target=func1,args=(i,lock)).start()
执行结果:
另一种写法:
with lock:
代替lock.acquire()方法与lock.release()方法
线程锁
线程锁的引入:
from threading import Thread
n = 0
def add():for i in range(2200000):global nn += 1
def sub():for i in range(2200000):global nn -= 1p1 = Thread(target=add)
p1.start()
p2 = Thread(target=sub)
p2.start()
p1.join()
p2.join()
print(n)
运行结果不确定,由于线程的执行速度过快,在同一时间,可能会有多个线程在进行加操作,而在加操作完成后,赋值操作仅进行了一次,因此出现数值的错误,因为错误次数不确定,因此运行的结果也不确定
部分运行结果:
线程中锁的使用:
from threading import Thread,Lock
n = 0
def add(lock):for i in range(2200000):global nlock.acquire()n += 1lock.release()
def sub(lock):for i in range(2200000):global nlock.acquire()n -= 1lock.release()
if __name__ == '__main__':lock = Lock()p1 = Thread(target=add,args=(lock,))p1.start()p2 = Thread(target=sub,args=(lock,))p2.start()p1.join()p2.join()print(n)
此时,无论程序执行多少次,数值的大小如何,由于锁的使用,每次的加运算与赋值运算都是单次进行,因此,数值不会出现错误
执行结果:
递归锁
在一个线程中可以进行多次acquire()操作
但同时,只有进行多次release()操作,才能解锁其他进程
递归锁的使用:
from threading import Thread,Lock,RLock
import time
n = 0
def add(lock):for i in range(2):global nlock.acquire()lock.acquire()n += 1time.sleep(1)print("函数add的执行次数{}".format(n))lock.release()
def sub(lock):for i in range(2):global nlock.acquire()n -= 1print("函数sub的执行次数{}".format(n))lock.release()
if __name__ == '__main__':lock = RLock()p1 = Thread(target=add,args=(lock,))p1.start()p2 = Thread(target=sub,args=(lock,))p2.start()p1.join()p2.join()print(n)
执行结果:
此时可以看到,由于函数add中上锁了两次,而只进行了一次解锁,因此在函数add执行完毕之后函数sub并不会进行执行
死锁现象
由于进程的执行顺序不当,或者资源的分配不当而导致整个进程不能进行执行,而陷入卡死的状态
死锁示例:
rom threading import Thread,Lock
import time
noodle_lock = Lock()
fork_lock = Lock()def eat(name):noodle_lock.acquire()print("{}抢到面了".format(name))fork_lock.acquire()print("{}抢到叉子了".format(name))print("{}吃面".format(name))time.sleep(0.1)fork_lock.release()print("{}放下了叉子".format(name))noodle_lock.release()print("{}放下面了".format(name))
def eat2(name):fork_lock.acquire()print("{}抢到了叉子".format(name))noodle_lock.acquire()print("{}抢到面了".format(name))print("{}吃面".format(name))time.sleep(0.1)noodle_lock.release()print("{}放下面了".format(name))fork_lock.release()print("{}放下叉子了".format(name))Thread(target=eat,args=(1,)).start()
Thread(target=eat2,args=(2,)).start()
Thread(target=eat,args=(3,)).start()
Thread(target=eat2,args=(4,)).start()
部分执行结果:
注:死锁现象只发生在多个进程且存在多个互斥锁的情况在,单个互斥锁的合理使用不会导致死锁现象的发生