详解c# SpinWait

/// <summary>
/// 循环一次
/// </summary>
/// <remarks>
/// This is typically called in a loop, and may change in behavior based on the number of times a
/// <see cref="SpinOnce"/> has been called thus far on this instance.
/// </remarks>
public void SpinOnce()
{
  if (NextSpinWillYield)
  {
    int yieldsSoFar = (m_count >= YIELD_THRESHOLD ? m_count - YIELD_THRESHOLD : m_count);
    //③循环到20次时，执行Thread.Sleep(01)
    if ((yieldsSoFar % SLEEP_1_EVERY_HOW_MANY_TIMES) == (SLEEP_1_EVERY_HOW_MANY_TIMES - 1))
    {
      //当前线程挂起，让出cpu
      //所有挂起的线程都有机会竞争当前时间片段，不限制线程优先级
      Thread.Sleep(1);
    }
    //②执行Thread.Yield()5次后，执行Thread.Sleep(0)  
    else if ((yieldsSoFar % SLEEP_0_EVERY_HOW_MANY_TIMES) == (SLEEP_0_EVERY_HOW_MANY_TIMES - 1))
    {
      //当前线程挂起，让出cpu
      //(只允许那些优先级相等或更高的线程使用当前的CPU。
      //如果没有，那当前线程会重新使用CPU时间片)
      //（上面已说明，后续补充实现）
      Thread.Sleep(0);
    }
    else
    {
      //当前线程挂起（执行状态->就绪状态）， 让出cpu，
      //（后续补充实现逻辑）
      Thread.Yield();
    }
  }
  else
  {
    //线程等待
    //4,8,16,32,64...位运算，2的n次方
    //①循环10次
    Thread.SpinWait(4 << m_count);
  }
  // m_count 递增; m_count 达到最大值后回滚Count =10
  m_count = (m_count == int.MaxValue ? YIELD_THRESHOLD : m_count + 1);
}

/// <summary>
/// 重置循环计数器
/// </summary>
public void Reset()
{
  m_count = 0;
}
#region Static Methods
/// <summary>
/// 循环.直到condition返回True
/// </summary>
public static void SpinUntil(Func<bool> condition)
{
  SpinUntil(condition, Timeout.Infinite);
}
/// <summary>
/// 循环，直到condition返回True或者时间达到timeout
/// </summary>
public static bool SpinUntil(Func<bool> condition, TimeSpan timeout)
{
  //校验时间格式是否正确
  Int64 totalMilliseconds = (Int64)timeout.TotalMilliseconds;
  if (totalMilliseconds < -1 || totalMilliseconds > Int32.MaxValue)
  {
    throw new System.ArgumentOutOfRangeException(
      "timeout", timeout, "SpinWait_SpinUntil_TimeoutWrong");
  }
  return SpinUntil(condition, (int)timeout.TotalMilliseconds);
}
/// <summary>
/// 直到condition返回True或者时间达到timeout.
/// </summary>
public static bool SpinUntil(Func<bool> condition, int millisecondsTimeout)
{
  //校验时间格式
  if (millisecondsTimeout < Timeout.Infinite)
  {
    throw new ArgumentOutOfRangeException(
      "millisecondsTimeout", millisecondsTimeout, "SpinWait_SpinUntil_TimeoutWrong");
  }
  //空值校验
  if (condition == null)
  {
    throw new ArgumentNullException("condition", "SpinWait_SpinUntil_ArgumentNull");
  }
  uint startTime = 0;
  if (millisecondsTimeout != 0 && millisecondsTimeout != Timeout.Infinite)
  {
    //自上次启动计算机以来所经过的时间（以毫秒为单位）。
    startTime = TimeoutHelper.GetTime();
  }
  SpinWait spinner = new SpinWait();
  while (!condition())
  {
    if (millisecondsTimeout == 0)
    {
      return false;
    }
    spinner.SpinOnce();
    //计时
    if (millisecondsTimeout != Timeout.Infinite && spinner.NextSpinWillYield)
    {
      if (millisecondsTimeout <= (TimeoutHelper.GetTime() - startTime))
      {
        return false;
      }
    }
  }
  return true;
}

#endregion