Galvin Edition 9 Exercise 10 Question 9 (Page No. 498)

Question

None of the disk-scheduling disciplines, except $FCFS$, is truly fair (starvation may occur).
a. Explain why this assertion is true.
b. Describe a way to modify algorithms such as $SCAN$ to ensure fairness.
c. Explain why fairness is an important goal in a time-sharing system.
d. Give three or more examples of circumstances in which it is important that the operating system be unfair in serving $I/O$ requests.

Answer 1

a. New requests for the track over which the head currently resides can theoretically arrive as quickly as these requests are being serviced.

b. All requests older than some predetermined age could be “forced” to the top of the queue, and an associated bit for each could be set to indicate that no new request could be moved ahead of these requests. For SSTF, the rest of the queue would have to be reorganized with respect to the last of these “old” requests.

c. To prevent unusually long response times. As in time shared OS we need everyone should get a fair share of resources they want.

d. Paging and swapping should take priority over user requests. It may be desirable for other kernel-initiated I/O, such as the writing of file system metadata, to take precedence over user I/O. If the kernel supports real-time process priorities, the I/O requests of those processes should be favored
Ref: hscc.cs.nthu.edu.tw › ~sheujp › homework › HW12_ref