How to calculate Virtual User "footprint"

Originally authored by Mark Tomlinson in HP.com and i felt its very useful for everyone.

One of the most common questions we get about LoadRunner Virtual Users relates to the resources required to execute the scripts on the Load Generator.  One advantage of the maturity of LoadRunner is that we have supported so many different drivers and protocols and environments over the past 2 decades.  We've learned so much about how to give a more detailed response to really advise users on how much Load Generator resources will be required to be successful.  You might imagine that the answer isn't black & white or even close to a 1 sentence answer.  Here are some simple ideas that can help you determine how to configure your Load Generators.

For Memory: each protocol has different parts that affect how much memory is required, so there is no single answer across all virtual users - Web is different RDP, which is different from SAP Click & Script, which is different from RMI-Java.  Some vuser types have external drivers (like ICA, RDP or SAP) so the guidelines didn’t include the footprint for the external executable driver. The Click & Script vuser types really can confuse you, because these seem like new versions of old protocols...but that's not actually true - the C&S protocols are completely new architecture, but more than anything, every vuser’s memory foot print is GREATLY impacted by the following factors:

• the size of the driver library (this is fairly static)
• the # of lines of code included in the recording (varies greatly by customer)
• the # and size of parameters included in the recording (varies greatly by customer and script type)

For CPU: of course, each driver has slight differences in CPU overhead, but for the most part they are all very efficient (and - yes, we will continue to improve Click & Script to be better!!) the amount of CPU used on a LoadRunner load generator will vary by the following factors:

• iteration and pacing, which controls how “active” the vuser is (varies greatly by customer)
• stress testing scenarios usually use more CPU, as opposed to real-world testing which has slower vusers (but more of them)
• customized code or extra script processing (like string manipulation, or calculations) will chew up more CPU

For Disk: the main thing here is logging, the more customers increase the details and amount of logging the more disk will be consumed external parameter files (writing or reading from individual vuser threads) will really hammer local disk some vusers with external driver executables will have additional logging of their own, or caching of content.

For Network: the result of multiple virtual users running on single load generator is a concentration of all those vusers network traffic on single NIC the payload of network api calls varies greatly for each and every different application stress testing (e.g. fast iteration pacing, no think-times) could easily result in over-utilization of NIC bandwidth

When it comes to calculating your virtual user footprint, it's actually quite easy.  But first, let me tell you that not everyone should need to do extensive calculations of the virtual user resource utilization.  This is important *only* when you have a very high number of virtual users or a very limited number of Load Generators.  The basic approach is to run a preliminary test with just 1 script, while you measure the resource utilization on the Load Generator directly.  You are specifically interested in the mmdrv.exe process on the Load Generator, which is LoadRunner's primary multi-threaded driver.  Measuring the private bytes reserved by this process for 1, 2, 3, 4 then 5 then 10 virtual users will give you some clue as to how much memory is required by each additional virtual user added.  Simultaneously you should monitor CPU, Network and Disk just to determine if there are any excessive utilizations.

It is important to note that you should be gathering information about the performance of your script running on the Load Generator - using the same run-time settings that you will use during the full scenario run.  If you are stress testing with very little think time or delay, then you'll want to use those same settings.

Source:- http://h30499.www3.hp.com/t5/HP-LoadRunner-and-Performance/How-To-Understand-and-calculate-Virtual-User-quot-footprint-quot/ba-p/2407591#.VNPsedLF_ng