Benchmark a web app with Poisson process

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Hello,
I'm a computer science student and i'm looking for a tool to test a web
application.
In particular i've to benchmark the gargage collector of the jvm loaded
by tomcat.
I run my tomcat server with magnolia-cms and i want to
performs requests for a given time.
Requests (a simple url-get) are distributed according an exponential
distribution with a given rate (lambda).
When a request completed with success i take the response time and i
store it in a file.
I've wrote a python code that can do the job (i've also post the code
here: http://stackoverflow.com/q/18570619/2739328), but resulting
distribution has a slower rate than lambda.
I think this that create a new process is expensive because I checked
the ranmod generator and it is quite good.
I also wrote the same code in Java and, instead of processes, i create
threads.
In java things are better in terms of distribution, but
sometimes i measure high response time and i can't trust these measures.

So, this is my question:
with curl-loader can i simulate a Poisson Process and monitor the
response times?

Best regards,
Alberto Amadio