The UNIX system scheduler determines when processes run. It maintains process priorities based on configuration parameters, process behavior, and user requests. It uses these priorities to assign processes to the CPU.
Scheduler functions give users varying degrees of control over the order in which certain processes run and the amount of time each process can use the CPU before another process gets a chance.
By default, the scheduler uses a time-sharing policy. A time-sharing policy adjusts process priorities dynamically in an attempt to give good response time to interactive processes and good throughput to CPU-intensive processes.
The scheduler also provides an alternate real-time scheduling policy. Real-time scheduling allows users to set fixed priorities--priorities that the system does not change. The highest priority real-time user process always gets the CPU as soon as it can be run, even if other system processes are also eligible to be run. A program can therefore specify the exact order in which processes run. You can also write a program so that its real-time processes have a guaranteed response time from the system.
For most versions of SunOS 5.0 through 5.8, the default scheduler configuration works well and no real-time processes are needed: administrators need not change configuration parameters and users need not change scheduler properties of their processes. However, for some programs with strict timing constraints, real-time processes are the only way to guarantee that the timing requirements are met.