How to control cpu usage of PVRTexToolCLI

I am integrating PVRTexTool to my distributed texture compression server and I would like to control cpu usage of PVRTexToolCLI. But I found that no argument could control that.
I guessed that the number of compression thread of PVRTexToolCLI depend on the number of logic cpu core, so I hooked the windows api GetSystemInfo, and changed SYSTEM_INFO::dwNumberOfProcessors to 1. Unfortunately, it didn’t work, it created 11 threads:(

Could any one tell me how PVRTexToolCLI determine the number of thread to create and what OS api that would be called?
Thank you so much!