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From Kirk Pepperdine <kirk.pepperd...@gmail.com>
Subject Re: Coordinated Omission (CO) - possible strategies
Date Fri, 18 Oct 2013 06:47:43 GMT
Hi Sebb,

In my testing, the option off creating threads on demand instead of all at once has made a
huge difference in my being able to control rate of arrivals on the server. It has convinced
me that simply using the throughput controller isn't enough and that the threading model in
JMeter *must* change. It is the threading model that is the biggest source of CO in JMeter.
Unfortunately we weren't able to come to some way of a non-disruptive change in JMeter to
make this happen.

The model I was proposing would have JMeter generate an event heap sorted by the time when
a sampler should be fired. A thread pool should be used to eat off of the heap and fire the
events as per scheduled. This would allow JMeter to break the inappropriate relationship of
a thread being a user. The solution is not perfect in that you will still have to fight with
thread schedulers and hypervisors to get things to happen on queue. However, I believe the
end result will be a far more scalable product that will require far fewer threads to produce
far higher loads on the server.

As for your idea on the using the throughput controller. IHMO triggering an assert only worsens
the CO problem. In fact, if the response times from the timeouts are not added into the results,
in other words they are omitted from the data set, you've only made the problem worse as you
are filter out bad data points from the result sets making the results better than they should
be. Peter Lawyer's (included here for the purpose of this discussion) technique for correcting
CO is to simply recognize when the event should have been triggered and then start the timer
for that event at that time. So the latency reported will include the time before event triggering.

Gil Tene's done some work with JMeter. I'll leave it up to him to post what he's done. The
interesting bit that he's created is HrdHistogram (https://github.com/giltene/HdrHistogram).
It is not only a better way to report results,it offers techniques to calculate and correct
for CO. Also Gil might be able to point you to a more recent version of his on CO talk. It
might be nice to have a new sampler that incorporates this work.

On a side note, I've got a Servlet filter that is JMX component that measures a bunch of stats
from the servers POV. It's something that could be contributed as it could be used to help
understand the source of CO.. if not just complement JMeter's view of latency.

Regards,
Kirk


On 2013-10-18, at 12:27 AM, sebb <sebbaz@gmail.com> wrote:

> It looks to be quite difficult to avoid the issue of Coordination
> Omission without a major redesign of JMeter.
> 
> However, it may be a lot easier to detect when the condition has occurred.
> This would potentially allow the test settings to be changed to reduce
> or eliminate the occurrences - e.g. by increasing the number of
> threads or spreading the load across more JMeter instances.
> 
> The Constant Throughput Controller calculates the desired wait time,
> and if this is less than zero - i.e. a sample should already have been
> generated - it could trigger the creation of a failed Assertion
> showing the time difference.
> 
> Would this be sufficient to detect all CO occurrences?
> If not, what other metric needs to be checked?
> 
> Even if it is not the only possible cause, would it be useful as a
> starting point?
> 
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