Load generator capacity calculation

Resources requirement for a load test infrastructure vary from applications to applications due to the technology stack being used and complexity of scenarios and scripts Saying my load generator would support x number of users is a very risky statement unless we have done some analysis and math to prove the statement. Following steps are suggested by HP to figure out the load generator capacity with respect to the protocol and test script

1. Run the single user test using controller. Keep a delay of few minutes in starting the script. Once script executions starts, observe the decrease in memory. Amount of memory decreased is our "First Vuser Memory"
2. Modify the test to run for 5-10 Vuser. Keep a delay of few minutes in starting the script and for each vuser. Notice the decrease in memory when each new user ramp up. This decrease in memory is our "Each Additional Vuser memory"
3. Now, for getting the Load generator capacity
4. Find out the total RAM available on the load generator. This will be "Total RAM"
5. Subtract 700-750 MB RAM for OS activities
6. Find out what is the 75% of the remaining RAM
7. Subtract "First Vuser Memory" from the remaining RAM in step 5
8. Divide the figure by "Each Additional Vuser memory+1" to get number of vuser supported by LG

So, we can have following formula to arrive at load generator capacity based on RAM

((Total LG RAM - ~750 MB) - First Vuser Memory)/(Each Additional Vuser memory + 1)

This formula will provide the good result for all protocols except protocols involving GUI interactions like citrix, truclient, RDP as these protocols have GDI interactions which is not taken into account in above calculations

Above steps can be tweaked for getting result based on other system resources as well.

The result obtained by the above can be treated as a conservative figure but it is good to play safe when you don't want to affect your test due to test infrastructure