Virtual User Distribution (VUD) is the number of virtual users simulated per Load Injector Server (LI Server). VUD affects the number of LIs the test requires and influences CPU utilization of each LI. 

You can set a custom VUD in the Number of Virtual Users per Load Injector Server field or apply the calibrated value while configuring a Stress Test.

It is important that VUD will not be set too high or too low. If it’s set to high LIs will hit its CPU limit and not be able to emulate all the virtual users. If VUD is set too low, LIs are not going to be used fully while more LIs than you actually need will be used for the test. 

Based on our experience, by default, we use the average VUD that is optimal to ensure ideal CPU utilization per LI (up to 80%) and efficient virtual user distribution between LIs. Although for maximum effectiveness it is recommended to apply VUD calibrated to your test settings. 

To calibrate your Stress Test device, on the Test Scenario page click Recommend VUD in the Virtual User Distribution section. To use the value in the test scenario, point to the Recommended value field and click Apply Value