Real-Time Class Policies

The SunOS 5.x operating system uses a real-time scheduling policy as well as a timesharing policy. Real-time scheduling allows users to set fixed priorities on a per-process basis, so that critical processes can run in predetermined order. The real-time scheduler never moves jobs between priorities. Real-time priorities change only when a user requests a change (using the priocntl command). Contrast this fixed-priority policy with the timesharing policy, in which the system changes priorities to provide good interactive response time.

The user process with the highest real-time priority always gets the CPU as soon as it can be run, even if other processes are ready to run. An application can be written so that its real-time processes have a guaranteed response time from the operating system.

As long as there is a real-time process ready to run, no process and no timesharing process runs. Other real-time processes can run only if they have a higher priority. Real-time processes managed carelessly can have a dramatic negative effect on the performance of timesharing processes.

The real-time policy gives higher-priority processes smaller time slices, by default. The higher priorities are allocated to real-time processes that are driven by external events. The operating system must be able to respond instantly to I/O. The lower-priority real-time processes are those that need more computation time. If a process with the highest priority uses up its time slice, it runs again because there is no process with a higher priority to pre-empt it.

The scheduler manages real-time processes by using parameters in the real-time parameter table rt_dptbl. This table contains information specific to the real-time class. It is automatically loaded into core from the RT_DPTBL loadable module located in the /kernel/sched directory.