CLR via C# 读书笔记 34 锁条件变量模式

条件变量模式(The Condition variable Pattern)

指的是一个任务在等待一个变量的状态.(例如一个线程当变量为true的时候执行,否则就一直在休眠状态)

以下代码演示了Thread1 一直在等待m_condition的状态

Thread1在等待状态改变的过程中,间歇性的暂时释放锁的控制权

这样Thread2就有机会获得到锁,改变m_condition的状态,并且执行Pulse(All)方法, 该方法将在执行Mointor.Exit(m_lock)的时候唤醒所有等待者,此时等待者发现状态已经改变,那么就开始执行相应的业务逻辑

internal sealed class ConditionVariablePattern
{
    private readonly Object m_lock = new Object();
    private Boolean m_condition = false;
    public void Thread1()
    {
        Monitor.Enter(m_lock); // Acquire a mutual-exclusive lock
        // While under the lock, test the complex condition "atomically"
        while (!m_condition)
        {
            // If condition is not met, wait for another thread to change the condition
            Monitor.Wait(m_lock); // Temporarily release lock so other threads can get it
        }
        // The condition was met, process the data...
        Monitor.Exit(m_lock); // Permanently release lock
    }
    public void Thread2()
    {
        Monitor.Enter(m_lock); // Acquire a mutual-exclusive lock
        // Process data and modify the condition...
        m_condition = true;
        // Monitor.Pulse(m_lock); // Wakes one waiter AFTER lock is released
        Monitor.PulseAll(m_lock); // Wakes all waiters AFTER lock is released
        Monitor.Exit(m_lock); // Release lock
    }
}

这个模式的好处就是不像自旋锁(Spin Lock)一样在等待的过程中浪费cpu

自旋锁通常用于非常短时间的锁定, 如果长时间锁定将大量浪费cpu

所以稍微长的时间锁定可以采用条件变量模式

PS1:如果....如果没人吧条件置为许可的状态...那么这线程就永远等下去了..直到进程被终止的那一天