6.12 Summary

The process abstraction is forced on us by the need to satisfy complex timing requirements, particularly for multirate systems. Writing a single program that simultaneously satisfies deadlines at multiple rates is too difficult because the control structure of the program becomes unintelligible. The process encapsulates the state of a computation, allowing us to easily switch among different computations.

The operating system encapsulates the complex control to coordinate the process. The scheme used to determine the transfer of control among processes is known as a scheduling policy. A good scheduling policy is useful across many different applications while also providing efficient utilization of the available CPU cycles.

It is difficult, however, to achieve 100% utilization of the CPU for complex applications. Because of variations in data arrivals and computation times, reserving some cycles to meet worst-case conditions is necessary. Some scheduling policies achieve higher utilizations than others, but often at the cost of unpredictability—they may not guarantee that all deadlines are met. Knowledge of the characteristics of an application can be used to increase CPU utilization while also complying with deadlines.