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Subject: Re: [OT] RE: 80ms delay switching between worker threads
To: Tomcat Users List <users@tomcat.apache.org>
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From: David kerber <dckerber@verizon.net>
Message-id: <5637C648.4000104@verizon.net>
Date: Mon, 02 Nov 2015 15:23:36 -0500
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On 11/2/2015 3:09 PM, Farzad Panahi wrote:
> Quoting from David Holme's blog:
>
>> The nanoTime method uses the highest resolution clock available on the platform, and while its return value is in nanoseconds, the update resolution is typically only microseconds.
> https://blogs.oracle.com/dholmes/entry/inside_the_hotspot_vm_clocks
>
> I think we can rely on nanoTime as a clock with microsecond
> resolution. Having said that can't we say printing out nanoTime in
> websocket message handler will give us a fair number (with microsecond
> accuracy) to measure how quickly the message handler is being called?
>
> All I am saying is that I see an obvious hiccup in order of
> milliseconds when threads are switching which I have no explanation
> for.
>
> Please advise if you think the way I am measuring is wrong.

I'm with Chris on this one:  I think it's due to running on a VM rather 
than on real hardware.


>
> Cheers
>
> Farzad
>
> On Mon, Nov 2, 2015 at 4:56 AM, David kerber <dckerber@verizon.net> wrote:
>> On 10/31/2015 10:51 AM, David Balažic wrote:
>>>
>>> Just a note: When most of you say "resolution" what you think about is
>>> actually called "accuracy".
>>> (also see "precision" , here is a good roundup:
>>> http://www.tutelman.com/golf/measure/precision.php )
>>
>>
>> I'm not sure about the others, but as an Electrical Engineer, I know the
>> difference between resolution, precision, and accuracy.  In the post I made
>> earlier, I said and meant "resolution".
>>
>>
>>
>>
>>>
>>> David Balažic
>>> Software Engineer
>>> www.comtrade.com
>>>
>>>> -----Original Message-----
>>>> From: Konstantin Preißer [mailto:kpreisser@apache.org]
>>>> Sent: 31. October 2015 10:27
>>>> To: Tomcat Users List
>>>> Subject: [OT] RE: 80ms delay switching between worker threads
>>>> Importance: Low
>>>>
>>>> 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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