jmeter-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Peter Lin <wool...@gmail.com>
Subject Re: Performance OpenJDK compared with SunJDK
Date Wed, 14 Dec 2011 15:40:51 GMT
Great discussion by the way.

Before I talked to henrik, I had no clue how complex timers are in the
JVM. It was too much info to fit in my brain and got GC-ed. I can only
imagine what Azul is seeing with their hardware.

On Wed, Dec 14, 2011 at 10:33 AM, Kirk <kirk.pepperdine@gmail.com> wrote:
> Well, I'm not sure why he's seeing a difference in performance. I've not spoken to Henrik
about timers so I can't comment on what he's telling you. I can only say that there is currently
an open CR w.r.t monotonic timers and there is some discussion on the subject in the CR. I've
also spoken to Cliff Click about Azul timers and to point to the confusion here.. he was certain
that the JDK was using the TSC, something that I was told by Charlie Hunt wasn't the case.
So I went digging to find out which of these two was right. Turns out the answer is pretty
complicated as it really depends on the OS and what the hardware has. RedHat was the last
to touch the OpenJDK timer code and they did so only in the Linux build and only to have the
high res timer used if it was available.
>
> Not a very clear picture.. and all of these timers come with their own set of faults..
which could be worked around if only we knew which one was being used  ;-)
>
> And to add to the confusion is thread scheduler efficiency which comes into play....
and I'm very suspicious of hypervisor interference with thread scheduling.Azul has found some
very very weird (almost harmonic) interference patterns that can adversely  affect the ability
to utilize cores. Without access to proper counters it's almost impossible to say for sure
whats going on.
>
> Regards,
> Kirk
>
> On Dec 14, 2011, at 4:09 PM, Peter Lin wrote:
>
>> thanks kirk for digging up those details.
>>
>> My memory is faulty, but that is basically what henrik told me back in
>> 2006/2007. I had noticed a difference between System.currentTimeMillis
>> and System.nanoTime on all JVM (sun, jrockit, ibm). Digging deeper,
>> henrik explained to me that windows uses the high performance timer,
>> which is considerably faster than linux timer.
>>
>> I agree that more threads results in more pressure on the thread scheduler.
>>
>> what got me curious is the user sees a difference between openJDK and
>> sunJDK. perhaps there is some other process running on that linux
>> system contributing to the extra CPU load.
>>
>>
>> On Wed, Dec 14, 2011 at 10:00 AM, Kirk <kirk.pepperdine@gmail.com> wrote:
>>>>
>>>>
>>>> Unfortunately it turns out that nanoTime drifts quite badly relative
>>>> to currentTimeMillis, so currentTimeMillis has to be used to anchor
>>>> that drift.
>>>> Otherwise the times just don't add up properly.
>>>>
>>>> The JMeter property "sampleresult.useNanoTime" can be set to "false"
>>>> to disable the use of nanoTime.
>>>
>>> Right, if nanoTime in Windows uses a TSC I would expect drift especially on newer
processors. Not all cores receive all clocks and since the value in the TSC * CPu frequency
is the current time, not having an accurate count of clocks would cause drift. currentTimeMillis
relying on a RTC would have smaller amounts of drift which would only be noticed between machines
(which would be correct with NTP). So your correction of drift would be consistent with this.
On Solaris, the drift between the TOD clock and the TSC is correct after a 1ms drift 125us
at a time. I think MS has attempted something similar (due to complaints from gamers) but
I'm not aware of the details.
>>>
>>> Regards,
>>> Kirk
>>>
>>>
>>> ---------------------------------------------------------------------
>>> To unsubscribe, e-mail: user-unsubscribe@jmeter.apache.org
>>> For additional commands, e-mail: user-help@jmeter.apache.org
>>>
>>
>> ---------------------------------------------------------------------
>> To unsubscribe, e-mail: user-unsubscribe@jmeter.apache.org
>> For additional commands, e-mail: user-help@jmeter.apache.org
>>
>
>
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: user-unsubscribe@jmeter.apache.org
> For additional commands, e-mail: user-help@jmeter.apache.org
>

---------------------------------------------------------------------
To unsubscribe, e-mail: user-unsubscribe@jmeter.apache.org
For additional commands, e-mail: user-help@jmeter.apache.org


Mime
View raw message