I see this question pop up from time to time: why doesn’t Softimage use all my CPUs/cores when I render?
mental ray is going to use all the cores it can (or should I say “all the cores it needs”?). There’s no flag to set or switch to activate. In fact, the only related option is the -threads command line flag, which will restrict the number of threads.
Here’s a screenshot of a render region of the ICE_Candle example scene. I didn’t have to do anything for Softimage to use all cores:
If Softimage doesn’t seem to be using all your cores/CPUs, I guess the first thing I would check is the task priority and affinity in the Task Manager.