What exactly are you after? Boosting the jvm process scheduler priority?
 You can use System.execute to run the renice command, but this won't
set a fixed cpu utilization, if the process is given low priority it'll
still consume 100% cpu if there aren't other processes in need of
cpu-cycles. Likewise, your process may be slowed down if it's
io-intensive, and won't be able to consume all cpu-cycles available.

Maybe you should look into a platform with realtime features, like
RT-Linux. And if you describe exactly what you need to do I'm sure
someone will be able to come up with much better answers than this :-)

That's not the problem. The problem is that the scheduler can choose whether
or not to give you the CPU. The operating system controls the CPU, and can
simply not choose to run your code (too low priority, for example), so the
system won't get the load you want. If you're running in kernel mode you can
prevent yourself from being pre-empted.

Alun Harford