C# Stopwatch shows incorrect time

Question

I have seen other user posts which show Stopwatch measuring time spent in "Thread.Sleep(5000)" to be around 5000ms.

But my program produces the following results

for (int i = 0; i < 20; ++i)
{
    Stopwatch sw = Stopwatch.StartNew();
    DateTime start = DateTime.Now;
    Thread.Sleep(5000);
    sw.Stop();
    Console.Out.WriteLine(
        "StopWatch Diff:" + 
        sw.ElapsedMilliseconds.ToString()); 
    Console.Out.WriteLine(
        "DateTime Diff:" + 
        DateTime.Now.Subtract(start).TotalMilliseconds.ToString());
}

StopWatch Diff:1684
DateTime Diff:5262.592
StopWatch Diff:1625
DateTime Diff:4997.12
StopWatch Diff:1604
DateTime Diff:4997.12
StopWatch Diff:1601
DateTime Diff:4997.12
StopWatch Diff:1690
DateTime Diff:4997.12
StopWatch Diff:1603

Is it just me who is observing this behaviour? Why stopwatch measures 1.6 seconds when 5 seconds have actually passed. It is the time that the thread is actually running?

It works just fine for me - StopWatch Diff:5000, DateTime Diff:5000.5. Is this exactly the code that you ran? — Jon B, Aug 20 '10 at 21:21
Duplicate question: http://stackoverflow.com/questions/3400254/can-the-net-stopwatch-class-be-this-terrible — Merlyn Morgan-Graham, Aug 20 '10 at 21:24
The other thread is talking about occasional drastic error in measurement. In my case the time measured is always 3 times smaller. — Alexander Demuro, Aug 20 '10 at 22:16

score 19 · Accepted Answer · answered Aug 20 '10 at 21:26

19

The Stopwatch class is not reliable.

This is unreliable on processors that do not have a constant clock speed (most processors can reduce the clock speed to conserve energy). This is explained in detail here.

answered Aug 20 '10 at 21:26

Mark Byers

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May be that's the case. Curiously, when I substitute Thread.Sleep(5000) with Thread.SpinWait(100000000) the discrepancy between time measured using Stopwatch and DateTime.Now becomes negligible. – Alexander Demuro Aug 20 '10 at 22:09
1

@user426777: When you use SpinWait it keeps the processor busy so it will probably be running at full speed at all times (constant speed). When you use Thread.Sleep, the processor doesn't have anything to do so it might automatically change to a lower clock speed to save power. You could try changing your processor to run at a constant speed and then I think it will work in both cases. – Mark Byers Aug 20 '10 at 22:12
You are right. When I start another thread which does some work and keeps processor busy, stopwatch shows the correct time. Thanks! – Alexander Demuro Aug 20 '10 at 22:24
2

Maybe Stopwatch used to be unreliable, but it appears it is fully reliable now. See https://msdn.microsoft.com/en-us/library/windows/desktop/dn553408%28v=vs.85%29.aspx#general_faq_about_qpc_and_tsc `Is QPC accuracy affected by processor frequency changes caused by power management or Turbo Boost technology? No. If the processor has an invariant TSC, the QPC is not affected by these sort of changes. If the processor doesn't have an invariant TSC, QPC will revert to a platform hardware timer that won't be affected by processor frequency changes or Turbo Boost technology.` – youen Oct 24 '15 at 13:09
1

@youen as long as your OS is Windows Vista or newer is the stipulation. Which should be fine now since those non-compliant operating systems are out of date, but important to mention nonetheless. – Jeremy Dec 29 '17 at 16:02

C# Stopwatch shows incorrect time

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