无锁更新中的SpinWait

问题描述

在阅读 Albahari在C#中的线程时，我注意到锁定免费更新"模式在周期结束时使用SpinWait:

While reading Albahari's Threading in C#, I've noticed that the "lock free update" pattern uses a SpinWait at the end of the cycle:

static void LockFreeUpdate<T> (ref T field, Func <T, T> updateFunction)
  where T : class
{
    var spinWait = new SpinWait();
    while (true)
    {
        // read
        T snapshot1 = field;

        // apply transformation
        T calc = updateFunction (snapshot1);

        // compare if not preempted
        T snapshot2 = Interlocked.CompareExchange (ref field, calc, snapshot1);

        // if succeeded, we're done
        if (snapshot1 == snapshot2) return;

        // otherwise spin
        spinWait.SpinOnce();
    }
}

注意最后的spinWait.SpinOnce()调用.是仅在单线程环境中产生线程才需要此调用，还是有其他目的?

Note the spinWait.SpinOnce() call at the end. Is this call needed only to yield the thread in a single-threaded environment, or does it have an additional purpose?

推荐答案

简短答案...是. SpinWait的目的是允许将线程产生给系统，以免由于SpinLock而导致处理不足.

Short answer... Yes. The intent of SpinWait is to allow yielding the thread to the System so as not to starve processing due to SpinLock.

从MSDN

https://msdn .microsoft.com/en-us/library/system.threading.spinwait(v = vs.110).aspx

SpinWait是一种值类型，这意味着低级代码可以利用 SpinWait，无需担心不必要的分配开销. SpinWait是 通常不适用于普通应用.在大多数情况下，您 应该使用.NET Framework提供的同步类， 例如Monitor.对于大多数需要旋转等待的用途， 但是，应该优先使用SpinWait类型而不是SpinWait 方法.

SpinWait is a value type, which means that low-level code can utilize SpinWait without fear of unnecessary allocation overheads. SpinWait is not generally useful for ordinary applications. In most cases, you should use the synchronization classes provided by the .NET Framework, such as Monitor. For most purposes where spin waiting is required, however, the SpinWait type should be preferred over the SpinWait method.

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