An Operating Systems Vade Mecum by Raphael A. Finkel

By Raphael A. Finkel

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These characteristics might be determined either before the process starts or while it is running. They may even change while the process is running. For example, in spooling and batch systems, service-time requirements may be explicitly declared as part of the job description. For example, a user might indicate that this process needs 45 seconds to complete. If it takes more, the operating system is justified in terminating it. Even rough guesses can be useful to the operating system. Experience shows that less than 10 percent of processes actually exceed estimates.

In fact, PS is only theoretically interesting, because as q approaches 0, process switching happens more frequently, and kernel time rises toward 100 percent. The trick is to set q small enough so that RR is fair but high enough so that kernel time is reasonable. 8 shows how RR schedules our sample processes for both q = 1 and q = 4. If a process finishes during its quantum, another process is started immediately and is given a full quantum. Newly arrived processes are put at the end of the ready list.

One alternative is to have the user characterize the process before it starts. For example, a process could be characterized as transput-bound or compute-bound. A highly interactive process, such as a text editor, is transput-bound. A long simulation that uses internally generated random data is compute-bound. More precisely, the average service time needed (averaged over each entry into the short-term scheduler) could be specified. It seems an unreasonable burden to have the user characterize each process.

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