c# - 简单的无锁秒表

Question

根据 MSDN，Stopwatch 类实例方法对于多线程访问是不安全的。这也可以通过检查各个方法来确认。

但是，由于我在代码中的几个地方只需要简单的“已用时间”计时器，我想知道它是否仍然可以无锁地完成，使用类似的东西:

public class ElapsedTimer : IElapsedTimer
{
    /// Shared (static) stopwatch instance.
    static readonly Stopwatch _stopwatch = Stopwatch.StartNew();

    /// Stopwatch offset captured at last call to Reset
    long _lastResetTime;

    /// Each instance is immediately reset when created
    public ElapsedTimer()
    { 
        Reset();
    }

    /// Resets this instance.
    public void Reset()
    {
        Interlocked.Exchange(ref _lastResetTime, _stopwatch.ElapsedMilliseconds);
    }

    /// Seconds elapsed since last reset.
    public double SecondsElapsed
    {
        get
        {
             var resetTime = Interlocked.Read(ref _lastResetTime);
             return (_stopwatch.ElapsedMilliseconds - resetTime) / 1000.0;
        }
    }
}

因为 _stopwatch.ElapsedMilliseconds 基本上是对 QueryPerformanceCounter 的调用，我假设从多线程调用是安全的？与常规 Stopwatch 的不同之处在于，此类基本上一直在运行，所以我不需要保持任何附加状态(“运行”或“停止”)，如 秒表 可以。

(更新)

在@Scott 在下面的回答中提出建议后，我意识到 Stopwatch 提供了一个简单的静态 GetTimestamp 方法，它返回原始的 QueryPerformanceCounter 蜱虫。也就是说，代码可以修改成这样，是线程安全的:

public class ElapsedTimer : IElapsedTimer
{
    static double Frequency = (double)Stopwatch.Frequency;

    /// Stopwatch offset for last reset
    long _lastResetTime;

    public ElapsedTimer()
    { 
        Reset();
    }

    /// Resets this instance.
    public void Reset()
    {
        // must keep in mind that GetTimestamp ticks are NOT DateTime ticks
        // (i.e. they must be divided by Stopwatch.Frequency to get seconds,
        // and Stopwatch.Frequency is hw dependent)
        Interlocked.Exchange(ref _lastResetTime, Stopwatch.GetTimestamp());
    }

    /// Seconds elapsed since last reset
    public double SecondsElapsed
    {
        get
        { 
            var resetTime = Interlocked.Read(ref _lastResetTime);
            return (Stopwatch.GetTimestamp() - resetTime) / Frequency; 
        }
    }
}

澄清一下，这段代码的想法是:

1. 有一种简单快速的方法来检查自某个操作/事件以来时间是否已经过去，
2. 如果从多个线程调用，方法不应破坏状态，
3. 必须对操作系统时钟变化不敏感(用户变化、NTP 同步、时区等)

我会像这样使用它:

private readonly ElapsedTimer _lastCommandReceiveTime = new ElapsedTimer();

// can be invoked by multiple threads (usually threadpool)
void Port_CommandReceived(Cmd command)
{
    _lastCommandReceiveTime.Reset();
}

// also can be run from multiple threads
void DoStuff()
{
    if (_lastCommandReceiveTime.SecondsElapsed > 10)
    {
        // must do something
    }
}

Answer 1

我建议的唯一更改是使用 Interlocked.Exchange(ref _lastResetTime, _stopwatch.ElapsedTicks); 而不是 Milliseconds 因为如果您处于高性能模式，则可以从查询性能计数器。