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From Konstantin Preißer <kpreis...@apache.org>
Subject [OT] RE: 80ms delay switching between worker threads
Date Sat, 31 Oct 2015 09:27:15 GMT
Hi Christopher,

> -----Original Message-----
> From: Christopher Schultz [mailto:chris@christopherschultz.net]
> Sent: Saturday, October 31, 2015 3:43 AM
> 
> What OS are you using? IIRC, the Windows timer has horrible resolution.
> you can call System.currentTimeNanos all you want, but you won't get
> anything meaningful lower than some threshold regardless of the actual
> least significant digits coming back from those calls.

While that may have been true in ancient versions like XP and Vista, at least starting with
Win7 QueryPerformanceCounter() uses the processor's TSC [1] (where Vista used the HPET if
available) so you should have a very high resolution here. E.g. running the following Java
program:

    int[] iterations = { 100, 120, 150, 250 };
    
    for (int i = 0; i < iterations.length; i++) {
        for (int j = 0; j < 3; j++) {
            long currentTime = System.nanoTime();
            double startValue = 1000;
            for (int z = 0; z < iterations[i]; z++) {
                startValue = Math.pow(startValue, 0.99);
            }
            long difference = System.nanoTime() - currentTime;
            System.out.println(iterations[i] + " pow iterations ms took " + (difference /
1000L) + " µs");
        }
    }

prints on my system something like:

100 pow iterations ms took 25 µs
100 pow iterations ms took 7 µs
100 pow iterations ms took 7 µs
120 pow iterations ms took 8 µs
120 pow iterations ms took 9 µs
120 pow iterations ms took 8 µs
150 pow iterations ms took 11 µs
150 pow iterations ms took 10 µs
150 pow iterations ms took 13 µs
250 pow iterations ms took 18 µs
250 pow iterations ms took 17 µs
250 pow iterations ms took 17 µs


So there should at least be a microsecond resolution. On a C# program using Stopwatch I get
similar results in the range from 5 to 12 µs.

Note, QueryPerformanceFrequency() [2] can be used to get the frequency of the timer which
is exposed in .Net through static System.Diagnostics.Stopwatch.Frequency field as ticks per
second. On my system it prints "3323580" so the resolution should be around ~0.3 microseconds.


Regards,
Konstantin Preißer

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/dn553408%28v=vs.85%29.aspx
[2] https://msdn.microsoft.com/de-de/library/windows/desktop/ms644905%28v=vs.85%29.aspx


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