How does this affect the fairness and average wait time guarantees of C-SCAN, and what modification could mitigate this issue?
hard🎤 Interviewer Follow-up Q15 of Q15
Operating Systems - Disk Scheduling - SSTF, SCAN, C-SCAN
Suppose a disk scheduler uses C-SCAN but the disk head speed varies dynamically, sometimes moving faster or slower between tracks. How does this affect the fairness and average wait time guarantees of C-SCAN, and what modification could mitigate this issue?
AVariable head speed causes starvation in C-SCAN; switching to SSTF is the best mitigation.
BVariable head speed does not affect C-SCAN fairness since it always services requests in one direction; no modification is needed.
CVariable head speed improves average wait time by allowing faster servicing of distant requests; no modification is necessary.
DVariable head speed breaks C-SCAN's uniform wait time guarantee; adding a dynamic priority queue based on estimated seek time can mitigate this.
Step-by-Step Solution
Step 1: Understand C-SCAN fairness assumptions
C-SCAN assumes uniform head movement speed to provide uniform wait times.
Step 2: Impact of variable head speed
Variable speed causes some requests to wait longer, breaking fairness and uniform wait time guarantees.
Step 3: Mitigation strategies
Introducing a dynamic priority queue that accounts for estimated seek time can help balance servicing order and restore fairness.
Step 4: Evaluate other options
Variable head speed does not affect C-SCAN fairness since it always services requests in one direction; no modification is needed. ignores the impact of speed variation; Variable head speed causes starvation in C-SCAN; switching to SSTF is the best mitigation. incorrectly claims starvation occurs and suggests SSTF, which can worsen starvation; Variable head speed improves average wait time by allowing faster servicing of distant requests; no modification is necessary. incorrectly claims variable speed improves wait times.
Final Answer:
Option D -> Option D
Quick Check:
Variable head speed breaks C-SCAN's uniform wait time guarantee; adding a dynamic priority queue based on estimated seek time can mitigate this. correctly identifies the problem and a plausible mitigation.
Quick Trick:C-SCAN fairness depends on constant head speed; variable speed needs dynamic adjustments
Option B is tempting because C-SCAN's fixed direction suggests fairness; Option C confuses starvation with fairness loss; Option D incorrectly assumes variable speed is beneficial without trade-offs.
Interviewer Note:
CONTEXT
Tests deep understanding of assumptions behind disk scheduling guarantees and ability to propose advanced mitigations.
Master "Disk Scheduling - SSTF, SCAN, C-SCAN" in Operating Systems
2 interactive learning modes - each teaches the same concept differently
