The scheduler determines when and for how long processes run. Therefore, the scheduler's behavior strongly affects a system's performance.
By default, all user processes are time-sharing processes. A process changes class only by a priocntl(2) call.
All real-time process priorities have a higher priority than any time-sharing process. Time-sharing processes or system processes cannot run while any real-time process is runnable. A real-time application that occasionally fails to relinquish control of the CPU can completely lock out other users and essential kernel housekeeping.
Besides controlling process class and priorities, a real-time application must also control other factors that affect its performance. The most important factors in performance are CPU power, amount of primary memory, and I/O throughput. These factors interact in complex ways. The sar(1) command has options for reporting on all performance factors.