SSTF (Shortest Seek Time First) in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding SSTF Disk Scheduling

Which of the following best describes the main goal of the SSTF (Shortest Seek Time First) disk scheduling algorithm?

A To select the request closest to the current head position to minimize seek time.

B To process requests in the order they arrive without reordering.

C To always move the disk head in one direction until the end before reversing.

D To prioritize requests based on their arrival time and size.

💡 Hint

Think about how SSTF tries to reduce the movement of the disk head.

SSTF selects the disk I/O request that is closest to the current head position, reducing the seek time compared to other algorithms like FCFS.

📋 Factual

intermediate

SSTF Disk Scheduling Behavior

Given the current head position at track 50 and pending requests at tracks 45, 70, 55, and 40, which request will SSTF service next?

A Track 70

B Track 40

C Track 55

D Track 45

💡 Hint

Calculate the distance from the current head position to each request.

The distances are: 45 (5), 70 (20), 55 (5), 40 (10). Both 45 and 55 are equally close, but SSTF typically picks the first closest request it finds, which is track 45.

🔍 Analysis

advanced

SSTF Starvation Problem

Why can the SSTF disk scheduling algorithm cause starvation for some requests?

A Because SSTF always moves the head in one direction, causing requests in the opposite direction to wait.

B Because SSTF processes requests strictly in arrival order, ignoring seek time.

C Because requests far from the current head position may be delayed indefinitely if closer requests keep arriving.

D Because SSTF prioritizes requests based on their size rather than position.

💡 Hint

Consider what happens if new requests keep appearing near the current head position.

SSTF can cause starvation because requests that are far away may never get serviced if new requests closer to the head keep arriving, delaying the farther ones indefinitely.

❓ Comparison

advanced

Comparing SSTF and SCAN Algorithms

Which statement correctly compares SSTF and SCAN disk scheduling algorithms?

A Both SSTF and SCAN always service requests in the order they arrive.

B SSTF selects the closest request regardless of direction, while SCAN moves the head in one direction servicing all requests until the end before reversing.

C SSTF moves the head in one direction only, while SCAN selects the closest request at each step.

D SCAN causes starvation, but SSTF guarantees fair servicing of all requests.

💡 Hint

Think about the movement pattern of the disk head in both algorithms.

SSTF picks the closest request regardless of direction, which can cause the head to move back and forth. SCAN moves the head in one direction servicing all requests until it reaches the end, then reverses direction.

❓ Reasoning

expert

Calculating Total Seek Time with SSTF

Given the initial head position at track 100 and pending requests at tracks 120, 90, 130, 70, and 110, what is the total seek time (sum of all head movements) if SSTF is used to service all requests?

A 90 tracks

B 80 tracks

C 100 tracks

D 110 tracks

💡 Hint

Calculate the distance to the closest request at each step and sum all movements.

Step 1: Head at 100, closest request is 110 (distance 10).
Step 2: Head at 110, closest is 120 (10).
Step 3: Head at 120, closest is 130 (10).
Step 4: Head at 130, closest is 90 (40).
Step 5: Head at 90, closest is 70 (20).
Total = 10 + 10 + 10 + 40 + 20 = 90 tracks.
However, SSTF picks the closest at each step, so after 100, the closest is 110 (10), then 120 (10), then 130 (10), then 90 (40), then 70 (20). Total is 90 tracks.

Practice

1. What does SSTF (Shortest Seek Time First) disk scheduling algorithm prioritize?

easy

A. The request closest to the current head position

B. The request that arrived first

C. The request farthest from the current head position

D. Requests in a fixed circular order

Solution

1. Step 1: Understand SSTF purpose

   SSTF aims to reduce the seek time by choosing the disk request nearest to the current head position.

2. Step 2: Compare options with SSTF behavior

   Only the request closest to the current head position matches this behavior; others describe different algorithms.

3. Final Answer:

   The request closest to the current head position -> Option A

4. Quick Check:

   SSTF = closest request [OK]

Hint: Pick the request nearest to the head position [OK]

Common Mistakes:

• Confusing SSTF with FCFS (First Come First Serve)
• Thinking SSTF picks requests in arrival order
• Assuming SSTF picks the farthest request

2. Which of the following is the correct way to describe SSTF in disk scheduling?

easy

A. It processes requests in the order they arrive.

B. It selects the request with the longest waiting time first.

C. It selects the request closest to the current head position.

D. It moves the head in one direction servicing all requests.

Solution

1. Step 1: Review SSTF definition

   SSTF selects the disk request closest to the current head position to minimize seek time.

2. Step 2: Match options with SSTF behavior

   The option "It selects the request closest to the current head position." correctly describes SSTF. Other options describe different algorithms like FCFS or SCAN.

3. Final Answer:

   It selects the request closest to the current head position. -> Option C

4. Quick Check:

   SSTF = closest request selection [OK]

Hint: Remember SSTF picks nearest request, not oldest [OK]

Common Mistakes:

• Mixing SSTF with FCFS or SCAN algorithms
• Assuming SSTF picks requests by waiting time
• Confusing direction-based scheduling with SSTF

3. Given the disk head is at position 50 and requests are at positions [45, 70, 10, 55], which request will SSTF service first?

medium

A. 45

B. 70

C. 10

D. 55

Solution

1. Step 1: Calculate distances from head position 50

   Distances: 45 (5), 70 (20), 10 (40), 55 (5)

2. Step 2: Identify closest request

   Requests at 45 and 55 are both 5 units away. SSTF can pick either, but usually picks the first closest found, here 45.

3. Final Answer:

   45 -> Option A

4. Quick Check:

   Closest request = 45 [OK]

Hint: Calculate absolute distance, pick smallest [OK]

Common Mistakes:

• Choosing the farthest request by mistake
• Ignoring equal distances and picking wrong one
• Confusing SSTF with FCFS order

4. A disk head is at position 30. Requests are at [35, 25, 40, 20]. A student incorrectly picks 40 as the next request in SSTF. What is the error?

medium

A. They ignored the current head position.

B. They chose the farthest request instead of the closest.

C. They picked the request with the longest waiting time.

D. They selected requests in arrival order.

Solution

1. Step 1: Calculate distances from head position 30

   Distances: 35 (5), 25 (5), 40 (10), 20 (10)

2. Step 2: Identify correct SSTF choice

   Closest requests are 35 and 25 (5 units away). Choosing 40 (10 units) is incorrect.

3. Final Answer:

   They chose the farthest request instead of the closest. -> Option B

4. Quick Check:

   SSTF picks closest, not farthest [OK]

Hint: Always pick request with minimum distance [OK]

Common Mistakes:

• Picking requests based on arrival time
• Ignoring actual distances from head
• Confusing SSTF with other algorithms

5. A disk head starts at position 100. The request queue is [120, 89, 130, 80, 110]. Using SSTF, what is the order in which requests will be serviced?

hard

A. [110, 120, 130, 89, 80]

B. [89, 80, 110, 120, 130]

C. [110, 120, 89, 80, 130]

D. [110, 89, 80, 120, 130]

Solution

1. Step 1: Start at head position 100 and find closest request

   Distances: 120(20), 89(11), 130(30), 80(20), 110(10). Closest is 110 (10 units).

2. Step 2: Move head to 110 and repeat

   Remaining: 120(10), 89(21), 130(20), 80(30). Closest is 120 (10).

3. Step 3: Move head to 120 and repeat

   Remaining: 89(31), 130(10), 80(40). Closest is 130 (10).

4. Step 4: Move head to 130 and repeat

   Remaining: 89(41), 80(50). Closest is 89 (41).

5. Step 5: Move head to 89 and last request is 80 (9 away)

   Order: 110, 120, 130, 89, 80.

6. Final Answer:

   [110, 120, 130, 89, 80] -> Option D

7. Quick Check:

   SSTF services closest request each step [OK]

Hint: Pick closest request step-by-step from current head [OK]

Common Mistakes:

• Not updating head position after each request
• Choosing requests by arrival order
• Ignoring equal distances and picking wrong request