SSTF (Shortest Seek Time First) in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the next disk request chosen by SSTF.

Operating Systems

next_request = min(requests, key=lambda r: abs(r - [1]))

Drag options to blanks, or click blank then click option'

Acurrent_head_position

Brequest

Crequests

Dmin

Attempts:

3 left

💡 Hint

Common Mistakes

Using the request variable inside the abs() instead of the current head position.

Confusing the list of requests with the current position.

The SSTF algorithm selects the request closest to the current head position. Here, current_head_position is used to find the minimum distance.

2fill in blank

medium

Complete the code to remove the served request from the list after SSTF selects it.

Operating Systems

requests.remove([1])

Drag options to blanks, or click blank then click option'

Amin

Bcurrent_head_position

Crequests

Dnext_request

Attempts:

3 left

💡 Hint

Common Mistakes

Removing the current head position instead of the request.

Trying to remove the entire list of requests.

After serving a request, SSTF removes the next_request from the list to avoid serving it again.

3fill in blank

hard

Fix the error in updating the current head position after serving a request.

Operating Systems

current_head_position = [1]

Drag options to blanks, or click blank then click option'

Anext_request

Bmax(requests)

Cmin(requests)

Drequests[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the head to the first or last request in the list instead of the served request.

Not updating the head position at all.

The current head position should be updated to the next_request that was just served, not the minimum or maximum of remaining requests.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each request to its distance from the current head position.

Operating Systems

distances = {r: abs(r [1] current_head_position) for r in requests if r [2] current_head_position}

Drag options to blanks, or click blank then click option'

A-

B>

C<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction for distance.

Filtering requests less than the current head position.

The distance is calculated by subtracting the current head position from each request (r - current_head_position). The filter keeps requests greater than the current head position.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each request to its distance from the current head position, considering requests both less and greater than the head.

Operating Systems

distances = {r: abs(r [1] current_head_position) for r in requests if r [2] current_head_position or r [3] current_head_position}

Drag options to blanks, or click blank then click option'

A-

B>

C<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction.

Filtering only requests greater than the head.

The distance is calculated by subtracting the current head position from each request (r - current_head_position). The filter includes requests both greater than and less than the current head position.

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