Which disk scheduling algorithm is generally considered fair but can result in high seek times?

Question

BlackTom AI · Accepted Answer

FCFS (First-Come, First-Served) processes disk I/O requests strictly in the order they arrive, so it guarantees that every request is served in arrival order without starvation — this is the core reason it is considered “fair.” However, because FCFS ignores the current head position when choosing the next request, it can force the head to move long distances between consecutively queued requests, producing high total seek time.

Relevant concepts:
- Fairness: requests are served in arrival order; no request is skipped or indefinitely delayed.
- Seek time: distance the disk head moves between requests; large random jumps increase total seek time.
- Alternative algorithms: SSTF picks the nearest request (lower average seek but can starve distant requests); SCAN/C-SCAN sweep the head in one direction (more predictable and lower worst-case seek than FCFS).

Step-by-step reasoning with a concrete example:
1. Assume head starts at track 50. Pending requests in arrival order: 10, 90, 11, 89.
2. FCFS service order is exactly the arrival order:
   - Move 50 -> 10: distance 40
   - Move 10 -> 90: distance 80
   - Move 90 -> 11: distance 79
   - Move 11 -> 89: distance 78
   - Total FCFS seek = 40 + 80 + 79 + 78 = 277
3. Compare with SSTF (for contrast): from 50, the nearest request is 11 (dist 39), then 10 (1), then 89 (79), then 90 (1). Total SSTF seek = 39 + 1 + 79 + 1 = 120 — much lower than FCFS.
4. This shows FCFS’s fairness does not optimize head movement and can yield very high seek times when requests are scattered.

Why the correct answer is FCFS:
- FCFS is the only option that guarantees strict arrival-order service (fairness) while potentially causing large, unnecessary head movements and high seek times.
Why other options are incorrect:
- SSTF minimizes immediate seek but can starve far-away requests (not inherently fair).
- SCAN/C-SCAN reduce worst-case and average seek by sweeping and are more efficient/predictable than FCFS; they are less likely to produce the extreme seeks FCFS can.

Conclusion: The correct answer is b. FCFS, which matches the provided answer.

Which disk scheduling algorithm is generally considered fair but can result in high seek times?单项选择题

类似问题

What is a major disadvantage of the FCFS disk scheduling algorithm?

What is the primary goal of disk scheduling algorithms?

Consider a disk with tracks numbered 0 to 199. The current head position is 50 (Head Movement is Right). The request queue is: 98, 183, 37, 122, 14, 124, 65, 67. How Many Head movements are there in servicing requests using FCFS?Tip: Enter numerical data ONLY. E.g: "5" and NOT "5 Head Movements".

Consider a disk with tracks numbered 0 to 199. The current head position is 157 (Head Movement is Right). The request queue is: 17, 93, 151, 4, 128, 67, 199, 35. How Many Head movements are there in servicing requests using C-SCAN?Tip: Enter numerical data ONLY. E.g: "5" and NOT "5 Head Movements".

The goal of disk scheduling algorithms is to optimize disk I/O by reducing the impact of:

Which disk scheduling algorithm is most likely to cause the head to change direction frequently?

The SCAN disk scheduling algorithm is also known as the:

Which disk scheduling algorithm is generally preferred for general-purpose operating systems?

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单