How to efficiently utilize the resources of Load Generators?

“How do I calibrate the total number of Load generators required to generate a load of 300 Virtual user if I have each load generators equipped with 3 GB of RAM and 3.3 GHz of CPU clock speed?”

Solution:
Memory foot print which HP as a vendor provides is not very accurate in a general sense. The impact of Memory and CPU footprint of any protocol on a Load generator also depends greatly on the length of the business flow and on the workload profile which the client expects the system to achieve. 

Example: Hypothetically, if in a SAP GUI application if you have 100 virtual users just entering employee name and number in a field and submitting it to the server and are iterating it only 2 times in a half hour period (which if is your test duration) then I would say you may not need more than 1 load generator which is sized with 3 GB of RAM and 3.3 GHz dual core Intel processor.

 Adding to the above, HP does not provide any kind of CPU footprint report. Hence I generally go about telling people to calculate the heaviness/payload of load runner process on their own. This can be done by identifying metrics of resource consumption by 1 user and theoretically determining total resource needed by multiplying the observed consumption for single virtual user with the expected number of concurrent users.

Using Windows Task manager(Process tab) while executing a single Vu will help you determine the memory consumed by a single ‘MDRV’ process which if multiplied with expected concurrent virtual user count will help you in calibrating total memory required to run a load test. Similarly to estimate the CPU resource for a certain protocol, I would recommend you to monitor and record few CPU counters of your Load Generator through ‘perfmon’ while deriving baseline result and multiply the metrics with the target Vu count which is mentioned in the workload profile.

It is ideally recommended to have as many boxes as possible to match the theoretical requirement, but as we all know, one of the many challenges a performance tester faces is lack of sufficient load generator machines. Hence for the benefit of time and to avoid further pestering from your boss to somehow resolve the issue – try utilizing your load generator boxes as efficiently as possible by:

1. Log only when error occurs.
2. Precisely calculate think-time between transactions. This can help
in exerting less load on the load generators. Strictly follow real user screen pause patterns.(note this point as
very important!).
3. Uninstall any daemon applications running on Load generators
which is not a part of AUT.
4. Avoid excessively using the “Show Vuser” option during test
execution.
5. Declaration of variables with large size should be avoided, which
directly helps in conserving overall memory. Consult the development
team if your unable to estimate the right size(This is a major concern
in Java Vuser and VB Vuser scripts).
6. Never over-iterate a script.