[FYI - this is resent out of order due to a bounce. Some replies to this have already been posted].
I don't think the thread model is the core of the Coordinated Omission problem. Unless we consider the only solution to be sending no more than one request per 20 minutes from any given thread a threading model fix. It's more of a configuration choice the way I see it, but a pretty impossible one. The thread model may need work for other reasons, but CO is not one of them. In JMeter, as with all other synchronous testers, Coordinated Omission is a per-thread issue. It's easy to demonstrate CO with JMeter with a single client thread testing an application that has only a single client connection in the real world, or with 15 client threads testing an application that has exactly 15 real-world clients communicating at high rates (common with muxed environments, messaging, ESBs, trading systems, etc.). No amount of threading or concurrency will help get a better test results capturing for these very real system. Any occurrence of CO will make the JMeter results seriously bogus. When any one thread misses a planned request sending time, CO has already occurred, and there is no way to avoid it at that point. You certainly detect that CO has happened. The question is what to do about it in JMeter once you detect it. The major options are: 1. Ignore it and keep working with the data as if it actually meant anything. This amount to http://tinyurl.com/o46doqf . 2. You can try to change the tester behavior to avoid CO going forward. E.g. you can try to adjust the number of threads up AND at the same time the frequency of requests that each thread sends requests at, which will amount to drastically changing the test plan in reaction to system behavior. In my opinion, changing behavior dynamically will have very limited effectiveness for two reasons: The first is that the problem had already occurred, so all the data up to and including the observed CO is already bogus and has to be thrown away unless it can be corrected somehow. Only after you auto-adjust enough times to not see CO for a long time, your results during that time may be valid. The second is that changing the test scenario is valid (and possible) for very few real world systems. 3. You can try to correct for CO when you observe it. There are various ways this can be done, and most of them will amount to re-creating missing test sample results by projecting from past results. This can help correct the results data set so that it would better approximate what a tester that was not synchronous, and would have kept issuing requests per the actual test plan, would have experienced in the test. 4. Something else we hadn't yet thought about. Some correction and detection example work can be found at: https://github.com/OutlierCorrector/jmeter/commit/34c34cae673fd0871a423035a9f262d049f3d9e9 , which uses code at https://github.com/OutlierCorrector/OutlierCorrector . Michael Chmiel worked at Azul Systems over the summer on this problem, and the OutlierCorrector package and the small patch to JMeter (under the docs-2.9 branch) are some of the results of that work. This fix approach appears to work well as long as no explicitly random behavior is stated in the test scenarios (the outlier detector detects a test pattern and repeats it in repairing the data. Expressly random scenarios will not exhibit a detectable pattern.). -- Gil. On Oct 17, 2013, at 11:47 PM, Kirk Pepperdine <[email protected]<mailto:[email protected]>> wrote: 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 <[email protected]<mailto:[email protected]>> 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? --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected]<mailto:[email protected]> For additional commands, e-mail: [email protected]<mailto:[email protected]>
