Question

Consider a disk queue with requests for I/O to blocks on cylinders as 98, 183, 37, 122, 14, 124, 65, 67 and head starts at 53. What are the total head movements for FCFS and SSTF algorithms?

• A. 258 and 208
• B. 640 and 236
• C. 640 and 258
• D. 258 and 236

Hint:

SSTF (Shortest Seek Time First) always services the closest pending request and usually performs better than FCFS.

Answer 1

The Correct Option is B

Concept: Disk scheduling algorithms determine the order in which disk requests are serviced. The total head movement equals the sum of distances travelled by the disk head.

Step 1: Compute FCFS head movement.
Queue: \[ 53 \rightarrow 98 \rightarrow 183 \rightarrow 37 \rightarrow 122 \rightarrow 14 \rightarrow 124 \rightarrow 65 \rightarrow 67 \] Total movement: \[ |98-53|=45 \] \[ |183-98|=85 \] \[ |37-183|=146 \] \[ |122-37|=85 \] \[ |14-122|=108 \] \[ |124-14|=110 \] \[ |65-124|=59 \] \[ |67-65|=2 \] Total: \[ 45+85+146+85+108+110+59+2 =640 \]

Step 2: Compute SSTF movement.
SSTF always chooses the nearest request. Order: \[ 53 \rightarrow 65 \rightarrow 67 \rightarrow 37 \rightarrow 14 \rightarrow 98 \rightarrow 122 \rightarrow 124 \rightarrow 183 \] Movement: \[ 12+2+30+23+84+24+2+59 =236 \]

Step 3: Select the answer.
\[ \boxed{\text{FCFS}=640,\quad \text{SSTF}=236} \] Hence option (B) is correct.