How to calculate no. of load generators required for a load test ?

Calculating the no of load generators required for a performance test is not straight forward task because the capacity of a load generator depends upon different criterias like workflow, script complexity, protocol used etc.

A load generator can generate hundreds of users for HTTP/HTML protocol and same load generator can limit to 20 users for a trueclient script.

One of the best way to find out the load generator capacity with respect to a test is to run a small test (may be 5 min) with 5 or 10 users and capture the load generator memory and CPU utilisation details and use that information to calculate the no of load generator required. 

How ? For example, You ran a test with 10 users for 5 min and observed that LG's memory utilisation is 200 MB (memory utilisation during test minus memory utilisation before starting the test which is memory reserved by OS). It means your 10 users are consuming 200 MB of your RAM.

Assuming memory utilisation before starting the test was 800 MB (Reserved by OS) and total memory of your LG machine is 8 GB. 

No of users(N) = (75 % of (Total memory of LG - Memory reserved by OS) )/ Memory utilised by one user

Total memory of machine = 1024*8 = 8192 MB

Memory reserved by OS = 800 MB

Memory utilised by one user = 20 MB (200 MB/10 users) 

Put these values in the formula,

N=(75% of (8192-800))/20

N=(75% of (7392))/20

N=5544/20

N=277

This means you can run 277 virtual users from 1 load generator.

No of load generators = Total no of users/ users per load generator.

So,  if you want to run 1000 users load test, you would need 4 load generator machines with same configuration.

CALCULATOR

RAM Size of LG (MB)*


Memory Reserved By OS (MB)*


Memory Consumed By One User (MB)*


Number Of Users Per LG :