简单的无锁秒表

Simple lockless stopwatch

根据 MSDN,Stopwatch class 实例方法对于多线程访问不安全。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 "time elapsed" 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,我假设从多个线程调用是安全的?与常规 Stopwatch 的区别在于,此 class 基本上一直都是 运行,因此我不需要保留任何附加状态("running" 或 "stopped"),就像 Stopwatch 所做的那样。

(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,returns 原始 QueryPerformanceCounter 打勾。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; 
        }
    }
}

这段代码的思路,澄清一下,是:

  1. 有一种简单快速的方法来检查自某个 operation/event、
  2. 以来时间是否已经过去
  3. 如果从多个线程调用,方法不应破坏状态,
  4. 必须对 OS 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
    }
}

我建议创建 Stopwatch 的多个实例,并且只在同一个线程上读取它。

我不知道你的异步代码是什么样的,但在伪代码中我会这样做:

Stopwatch watch = Stopwatch.Startnew();
DoAsyncWork((err, result) =>
{
  Console.WriteLine("Time Elapsed:" + (watch.ElapsedMilliseconds / 1000.0));
  // process results...
});

或者:

public DoAsyncWork(callback) // called asynchronously
{
  Stopwatch watch = Stopwatch.Startnew();
  // do work
  var time = watch.ElapsedMilliseconds / 1000.0;
  callback(null, new { time: time });
}

第一个示例假定 DoAsyncWork 工作在不同的线程中完成工作,然后在完成时调用回调,编组回调用方线程。

第二个示例假定调用者正在处理线程,并且此函数自行完成所有计时,并将结果传递回调用者。

我建议的唯一更改是使用 Interlocked.Exchange(ref _lastResetTime, _stopwatch.ElapsedTicks); 而不是毫秒,因为如果您处于高性能模式,则可以从 QueryPerformanceCounter.

获得亚毫秒的结果