【操作系统进程同步】自行车生产线

问题

设自行车生产线上有一只箱子, 其中有N个位置(N≥3), 每个位置可存放一个车架或一个车轮; 又设有三个工人, 其活动分别为:

伪代码

semphore maxFrameNum=N-2,maxWheelNum=N-1,frame=0,wheel=0,empty=N,mutex=1;worker1(){do{P(maxFrameNum);加工一个车架();P(emptyNum);		P(mutex);车架放入箱子中();V(mutex);V(frame);}while(true)
}worker2(){do{P(maxWheelNum);加工一个车轮();P(emptyNum);P(mutex);车轮放入箱子中();V(mutex);V(wheel);}while(true)
}worker3(){do{P(frame);P(mutex);从箱子中取一车架;V(mutex);V(maxFrameNum);V(empty);P(wheel);P(mutex);从箱子中取一车轮子;V(mutex);V(maxWheelNum);V(empty);P(wheel);P(mutex);从箱子中取一车轮子;V(mutex);V(maxWheelNum);V(empty);组装一辆车();		}while(true)
}

Python模拟

import random
import threading
import timeN = 3
maxFrameNum = N - 2
maxWheelNum = N - 1
frame = 0
wheel = 0
empty = N
mutex = 1def refresh(name):if name == 'maxFrameNum':return maxFrameNumelif name == 'maxWheelNum':return maxWheelNumelif name == 'frame':return frameelif name == 'wheel':return wheelelif name == 'empty':return emptyelif name == 'mutex':return mutexdef p(id, item, name):print('【woker%s】need a  %s(%d)' % (id, name, item))while item <= 0:print('【woker%s】is waiting for %s' % (id, name))time.sleep(random.random())item = refresh(name)global maxWheelNumglobal maxFrameNumglobal wheelglobal emptyglobal mutexglobal frameif name == 'maxFrameNum':maxFrameNum -= 1elif name == 'maxWheelNum':maxWheelNum -= 1elif name == 'frame':frame -= 1elif name == 'wheel':wheel -= 1elif name == 'empty':empty -= 1elif name == 'mutex':mutex -= 1def v(id, item, name):global maxWheelNumglobal maxFrameNumglobal wheelglobal emptyglobal mutexglobal frameif name == 'maxFrameNum':maxFrameNum += 1elif name == 'maxWheelNum':maxWheelNum += 1elif name == 'frame':frame += 1elif name == 'wheel':wheel += 1elif name == 'empty':empty += 1elif name == 'mutex':mutex += 1print('【woker%s】release  %s' % (id, name))def printMsg():print('maxFrameNum:%s,maxWheelNum:%s,frame:%s,wheel:%s,empty:%s' % (maxFrameNum, maxWheelNum, frame, wheel, empty))def woker1():while True:time.sleep(random.random() * 4)p(1, maxFrameNum, 'maxFrameNum')print('【woker1】is making a frame')p(1, empty, 'empty')p(1, mutex, 'mutex')print('【woker1】is putting a frame into the box')v(1, mutex, 'mutex')v(1, frame, 'frame')printMsg()def woker2():while True:time.sleep(random.random() * 2)p(2, maxWheelNum, 'maxWheelNum')print('【woker2】is making a wheel')p(2, empty, 'empty')p(2, mutex, 'mutex')print('【woker2】is putting a wheel into the box')v(2, mutex, 'mutex')v(2, wheel, 'wheel')printMsg()def woker3():while True:time.sleep(random.random() * 4)p(3, frame, 'frame')p(3, mutex, 'mutex')print('【woker3】is getting a frame from the box')v(3, mutex, 'mutex')v(3, maxFrameNum, 'maxFrameNum')v(3, empty, 'empty')printMsg()p(3, wheel, 'wheel')p(3, mutex, 'mutex')print('【woker3】is getting a wheel from the box')v(3, mutex, 'mutex')v(3, maxWheelNum, 'maxWheelNum')v(3, empty, 'empty')printMsg()p(3, wheel, 'wheel')p(3, mutex, 'mutex')print('【woker3】is getting a wheel from the box')v(3, mutex, 'mutex')v(3, maxWheelNum, 'maxWheelNum')v(3, empty, 'empty')printMsg()print('【woker3】is making a car')if __name__ == '__main__':w1 = threading.Thread(target=woker1)w2 = threading.Thread(target=woker2)w3 = threading.Thread(target=woker3)w1.start()w2.start()w3.start()

最终效果

 总结

1. PV操作中需要修改全局变量，而Python修改全局变量需要使用关键字global声明一下，若使用C++写会比较方便，直接使用指针就可以修改全局变量了。

2. maxFrameNum初始值设置为N-1也可以保证不产生死锁。

3. 老师的PPT中没有使用mutex变量来访问临界资源（箱子），个人感觉有必要加上mutex。（若有不同想法，欢迎留言讨论~