[DrScump]

"Since mainframe computers are almost always running at 100% utilization..."

That's essentially impossible, as either CPU or I/O will bottleneck first and be the limiting factor.

As an operator, I used to make note of how different jobs used resources so I could keep overall utilization as high as possible, blending I/O bound jobs with CPU heavy jobs. (This was graveyard shift, where real-time users were few)

There was a great Boole and Babbage product called Resolve that allowed the operator (or a TSO user) to dynamically change job priorities as desired.