c# - 系统正常运行时间和 MemoryBarrier

Question

我需要一种可靠的方法来获得系统正常运行时间，并最终使用了如下方法。添加了一些评论以帮助人们阅读它。我不能使用任务，因为它必须在 .NET 3.5 应用程序上运行。

// This is a structure, can't be marked as volatile
// need to implement MemoryBarrier manually as appropriate
private static TimeSpan _uptime;

private static TimeSpan GetUptime()
{
    // Try and set the Uptime using per counters
    var uptimeThread = new Thread(GetPerformanceCounterUptime);
    uptimeThread.Start();

    // If our thread hasn't finished in 5 seconds, perf counters are broken
    if (!uptimeThread.Join(5 * 1000))
    {
        // Kill the thread and use Environment.TickCount
        uptimeThread.Abort();
        _uptime = TimeSpan.FromMilliseconds(
            Environment.TickCount & Int32.MaxValue);
    }

    Thread.MemoryBarrier();
    return _uptime;
}

// This sets the System uptime using the perf counters
// this gives the best result but on a system with corrupt perf counters
// it can freeze
private static void GetPerformanceCounterUptime()
{
    using (var uptime = new PerformanceCounter("System", "System Up Time"))
    {
        uptime.NextValue();
        _uptime = TimeSpan.FromSeconds(uptime.NextValue());
    }
}

我纠结的部分是 Thread.MemoryBarrier() 应该放在哪里？我在读取值之前放置它，但当前线程或其他线程可能已写入它。以上看起来正确吗？

编辑，根据大牛回答

这就是我最终实现的，感谢你们的洞察力。

private static TimeSpan _uptime;

private static TimeSpan GetUptime()
{
    var uptimeThread = new Thread(GetPerformanceCounterUptime);
    uptimeThread.Start();

    if (uptimeThread.Join(5*1000))
    {
        return _uptime;
    }
    else
    {
        uptimeThread.Abort();
        return TimeSpan.FromMilliseconds(
            Environment.TickCount & Int32.MaxValue);
    }
}

private static void GetPerformanceCounterUptime()
{
    using (var uptime = new PerformanceCounter("System", "System Up Time"))
    {
        uptime.NextValue();
        _uptime = TimeSpan.FromSeconds(uptime.NextValue());
    }
}

编辑2

根据 Bob 的评论更新。

private static DateTimeOffset _uptime;

private static DateTimeOffset GetUptime()
{
    var uptimeThread = new Thread(GetPerformanceCounterUptime);
    uptimeThread.Start();

    if (uptimeThread.Join(5*1000))
    {
        return _uptime;
    }
    else
    {
        uptimeThread.Abort();
        return DateTimeOffset.Now.Subtract(TimeSpan.FromMilliseconds(
            Environment.TickCount & Int32.MaxValue));
    }
}

private static void GetPerformanceCounterUptime()
{
    if (_uptime != default(DateTimeOffset))
    {
        return;
    }

    using (var uptime = new PerformanceCounter("System", "System Up Time"))
    {
        uptime.NextValue();
        _uptime = DateTimeOffset.Now.Subtract(
            TimeSpan.FromSeconds(uptime.NextValue()));
    }
}

Answer 1

Thread.Join 已经确保 uptimeThread 执行的写入在主线程上可见。您不需要任何明确的内存屏障。 (没有 Join 执行的同步，您需要在两个线程上设置屏障 - 在写入之后和读取之前)

但是，您的代码有一个潜在的问题:写入 TimeSpan 结构不是原子的，主线程和 uptimeThread 可能会同时写入它(Thread .Abort 只是表示中止，但不会等待线程完成中止)，导致写入中断。我的解决方案是在中止时根本不使用该字段。此外，对 GetUptime() 的多个并发调用可能会导致相同的问题，因此您应该改用实例字段。

private static TimeSpan GetUptime()
{
    // Try and set the Uptime using per counters
    var helper = new Helper();
    var uptimeThread = new Thread(helper.GetPerformanceCounterUptime);
    uptimeThread.Start();

    // If our thread hasn't finished in 5 seconds, perf counters are broken
    if (uptimeThread.Join(5 * 1000))
    {
        return helper._uptime;
    } else {
        // Kill the thread and use Environment.TickCount
        uptimeThread.Abort();
        return TimeSpan.FromMilliseconds(
            Environment.TickCount & Int32.MaxValue);
    }
}

class Helper
{
    internal TimeSpan _uptime;

    // This sets the System uptime using the perf counters
    // this gives the best result but on a system with corrupt perf counters
    // it can freeze
    internal void GetPerformanceCounterUptime()
    {
        using (var uptime = new PerformanceCounter("System", "System Up Time"))
        {
            uptime.NextValue();
            _uptime = TimeSpan.FromSeconds(uptime.NextValue());
        }
    }
}

但是，我不确定中止性能计数器线程是否会正常工作 - Thread.Abort() 仅中止托管代码执行。如果代码在 Windows API 调用中挂起，线程将继续运行。