013.线程-8.同步之条件变量

阳光穿透心脏的1/2处 2022-01-15 15:45 298阅读 0赞

[为什么80%的码农都做不了架构师？>>> ][80_] ![hot3.png][]

## 8.同步之条件变量 ##

> 实际案例：

![20145858_zsPb.jpg][]

### 1.线程的锁机制，会造成调度任务一直调度、或者一直没有被调度。 ###

解决方案： 使用条件变量，请看案例2

#!/usr/bin/python
    # coding: utf-8
    import threading
    import time
    
    tmp=0
    g_lock = threading.Lock()
    
    def func():
        global tmp
        global g_lock
    
        for i in range(10, 20):
            g_lock.acquire()
    
            tmp -= 1
            print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
    
            g_lock.release()
    
    
    if __name__ == "__main__":
        p = threading.Thread(target=func, args=())
        p.setDaemon(True)
        p.start()
    
        for i in range(10):
            g_lock.acquire()
    
            tmp += 1
            print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
    
            g_lock.release()
    
        p.join()

结果：

Thread-1 : tmp = -1 
     Thread-1 : tmp = -2 
     Thread-1 : tmp = -3 
     Thread-1 : tmp = -4 
     Thread-1 : tmp = -5 
     Thread-1 : tmp = -6 
     Thread-1 : tmp = -7 
     Thread-1 : tmp = -8 
     Thread-1 : tmp = -9 
     Thread-1 : tmp = -10 
     MainThread : tmp = -9 
     MainThread : tmp = -8 
     MainThread : tmp = -7 
     MainThread : tmp = -6 
     MainThread : tmp = -5 
     MainThread : tmp = -4 
     MainThread : tmp = -3 
     MainThread : tmp = -2 
     MainThread : tmp = -1 
     MainThread : tmp = 0 
    [Finished in 0.0s]

> 条件变量

![20145900_DVSZ.jpg][]

### 2.条件变量机制： ###

（condition，有 等待wait() 和 环境notify() ， 主线程有 sleep 操作，这样 CPU 切换到子线程的几率更大。）

*  acquire(\[timeout\])/release(): 调用关联的锁的相应方法。
 *  wait(\[timeout\]): 调用这个方法将使线程进入Condition的等待池等待通知，并释放锁。使用前线程必须已获得锁定，否则将抛出异常。
 *  notify(): 调用这个方法将从等待池挑选一个线程并通知，收到通知的线程将自动调用acquire()尝试获得锁定（进入锁定池）；其他线程仍然在等待池中。调用这个方法不会释放锁定。使用前线程必须已获得锁定，否则将抛出异常。
 *  notifyAll(): 调用这个方法将通知等待池中所有的线程，这些线程都将进入锁定池尝试获得锁定。调用这个方法不会释放锁定。使用前线程必须已获得锁定，否则将抛出异常。

#!/usr/bin/python
    # coding: utf-8
    import threading
    import time
    
    tmp=0
    g_cond = threading.Condition()
    
    def func():
        global tmp
        global g_cond
    
        print '%s acquire Lock' % threading.currentThread().getName()
        g_cond.acquire()
        time.sleep(3)
    
        while True:
            if(tmp >= 3):
                tmp -= 1
                print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
            else : 
                print '%s wait singnal wait up' % threading.currentThread().getName()
                g_cond.wait()
                print '%s wait up by another thread' % threading.currentThread().getName()
    
        g_cond.release()
    
    if __name__ == "__main__":
        p = threading.Thread(target=func, args=())
        p.setDaemon(True)
        p.start()
    
        time.sleep(1)
        while True:
            g_cond.acquire()
    
            if(tmp >= 3):
                print '%s notify' % threading.currentThread().getName()
                g_cond.notify()
            else : 
                tmp += 1
                print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
    
            time.sleep(1)         
            print '%s release Lock' % threading.currentThread().getName()
            g_cond.release()
            time.sleep(1)
    
        p.join()

结果：

Thread-1 acquire Lock
    Thread-1 wait singnal wait up
     MainThread : tmp = 1 
    MainThread release Lock
     MainThread : tmp = 2 
    MainThread release Lock
     MainThread : tmp = 3 
    MainThread release Lock
    MainThread notify
    MainThread release Lock
    Thread-1 wait up by another thread
     Thread-1 : tmp = 2 
    Thread-1 wait singnal wait up
     MainThread : tmp = 3 
    MainThread release Lock
    MainThread notify
    MainThread release Lock
    Thread-1 wait up by another thread
     Thread-1 : tmp = 2 
    Thread-1 wait singnal wait up
     MainThread : tmp = 3 
    MainThread release Lock
    MainThread notify
    MainThread release Lock
    Thread-1 wait up by another thread
     Thread-1 : tmp = 2 
    Thread-1 wait singnal wait up
     MainThread : tmp = 3 
    MainThread release Lock
    MainThread notify
    [Cancelled]

如果 主线程没有 sleep 操作（因为主线程的acquire() 和 release() 操作很近）， 就会一直在主线程上，不会切换到子线程，出现以下情况：

#!/usr/bin/python
    # coding: utf-8
    import threading
    import time
    
    tmp=0
    g_cond = threading.Condition()
    
    def func():
        global tmp
        global g_cond
    
        print '%s acquire Lock' % threading.currentThread().getName()
        g_cond.acquire()
        time.sleep(3)
    
        while True:
            if(tmp >= 3):
                tmp -= 1
                print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
            else : 
                print '%s wait singnal wait up' % threading.currentThread().getName()
                g_cond.wait()
                print '%s wait up by another thread' % threading.currentThread().getName()
    
        g_cond.release()
    
    if __name__ == "__main__":
        p = threading.Thread(target=func, args=())
        p.setDaemon(True)
        p.start()
    
        time.sleep(1)
        while True:
            g_cond.acquire()
    
            if(tmp >= 3):
                print '%s notify' % threading.currentThread().getName()
                g_cond.notify()
            else : 
                tmp += 1
                print " %s : tmp = %d " % (threading.currentThread().getName(), tmp)
    
            # time.sleep(1)         
            print '%s release Lock' % threading.currentThread().getName()
            g_cond.release()
            # time.sleep(1)
    
        p.join()

结果：

转载于:https://my.oschina.net/repine/blog/644021

[80_]: https://my.oschina.net/u/2663968/blog/3051541
[hot3.png]: /images/20220115/644e86baf1f34ea8a25c7bf4688708d2.png
[20145858_zsPb.jpg]: /images/20220115/86f09cb7c3e94b9e95f9bd5218e5a812.png
[20145900_DVSZ.jpg]: /images/20220115/abbb78570024403eab6104616d597f58.png