SilkPerformer has an internal formula which estimates the maximum number of virtual users that a single agent can run during a loadtest.  For a system running 4xPIII, 1GHz processor and 4GB RAM, SilkPerformer estimates a maximum virtual user capacity of 5900 (for Web Business Applications).

Although 4GB is available in total, a 32-Bit Operating System (OS) can only address this in total, which includes each addressable memory of the system (e.g. graphics card, internal caches, RAM, etc.).  This means that in reality each process can normally only address 2GB of RAM, i.e. 2 GB for process 1 (performer.exe), another 2GB for process 2 (perfrun.exe), and another 4GB for process 3 (2nd perfrun.exe), etc.

Since version 2007 SilkPerformer can run on a 64-Bit OS because 32-Bit applications can run inside the WoW64 process.  WoW64 is an x86 emulator and it is limited to 4GB of RAM.  This means that SilkPerformer will be able to access the full 4GB per process and hence will have greater capacity to run virtual users than the same project on a 32-Bit OS.

However, in SilkPerformer 2011 and earlier version the formula always defaults to 5900 virtual users because the formula only gives estimations based on a 32-Bit OS.

Agent - 32-Bit OS

Agent - 64-Bit OS

To determine whether an agent has the capacity to run higher levels of virtual users, run a load test and monitor the agent's "responsiveness" value during or
after a test. If it is > 95%, then the agent is ok to run the higher numbers of virtual users.

An Enhancement Request has been logged to change the internal virtual user estimation formula to take account of the additional RAM when running on a 64-Bit OS.

Note: the actual formula used can be viewed in the Online Help under the “Performance and Scalability Matrix” section.