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From Oliver Lloyd <oliver_ll...@hotmail.com>
Subject Re: Odd problem with performance of JMeter
Date Wed, 22 Jun 2011 18:57:34 GMT
Hi John,  When running tests like this you need to think in terms of
throughput rather than total transactions achieved. So you mention you are
aiming to get 800,000 requests but actually you want to define a throughput
rate and have this as your target. Then, the volume of requests is simply a
matter of time, the longer you run the test the greater the volume.

So, you should setup your test plan to achieve a rate of requests per
second. You can do this using the Constant Throughput Timer. Try spreading
the requests over more threads, this will increase concurrency. The actual
concurrency is also something you may want to think about - what is the
expected concurrency that your need to prove is possible?

Keep in mind when you are doing this that the response time for each request
will be a factor in how many requests each thread can deliver, if it takes 1
second to process each request then the maximum throughput per thread is 60
per second. It's basic math.

Now, once this is place you can begin to run some tests. Try starting at a
lower value and evaluating your system at this steady rate - do you see the
same drop off? The key here is to hold the request rate at the desired level
and not let the test run as fast as it can. If you let things run wild
without any control it makes debugging issues much harder.

If, you are able to maintain a steady rate without degradation at, say, 50
requests per second, then you could try a higher rate, or you could even try
a gradual ramp up. Keep this slow, there's no point hammering your system
hard right from the start as this will produce unrealistic data.

When running tests you should keep an eye not just on CPU usage but also
lots of other things. Memory usage on the JMeter box is something that cause
issues but there are also a wide range of metrics from your system that can
also cause the behvior you mention.

Make sure you use an Assertion to verify that the response you are getting
back is in fact what you expect it to be. You might find that you are
getting errors but JMeter is displaying success because it sees a 200
response - this is a very common error.

Finally, as Deepak mentions, using listeners are high request rates can
create a bottleneck in the test rig and typically it is better to run the
actual load tests from the command line but you first need to prove that
this is your issue and you can do this by running a few simple experimental
tests at differing rates. If you can show an issue on the test rig then the
best option might be to distribute the load but before doing this you need
to establish that a steady rate - over time - is possible from one machine.
If this is not possible even at lower rates then you've very likely got a
application issue (although it is odd that you mention the response times
remain constant.)

Think of it like a scientific experiment, put a white coat on and use the
word 'logical' a lot.

--
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