SSTF (Shortest Seek Time First) in Operating Systems - Mini Project: Build & Apply

SSTF (Shortest Seek Time First) Disk Scheduling

📖 Scenario: You are managing a disk drive that receives multiple read/write requests at different track positions. To reduce the time the disk head moves, you want to schedule these requests using the SSTF (Shortest Seek Time First) algorithm.

🎯 Goal: Build a step-by-step SSTF scheduling plan that selects the next disk request closest to the current head position until all requests are served.

📋 What You'll Learn

Create a list of disk requests with exact track numbers

Set the initial position of the disk head

Implement the SSTF logic to pick the closest request next

Complete the schedule showing the order of servicing requests

💡 Why This Matters

🌍 Real World

Disk scheduling algorithms like SSTF are used in operating systems to reduce the time it takes for the disk head to move between requests, improving overall system performance.

💼 Career

Understanding SSTF is important for roles in system administration, operating system development, and performance optimization where managing hardware resources efficiently is key.

1

Create the list of disk requests

Create a list called requests with these exact track numbers: 55, 58, 60, 70, 18, 90, 150, 38, 184.

Operating Systems

# Create the list of disk requests
# Your code here

Hint

Use a Python list with the exact numbers in the order given.

2

Set the initial disk head position

Create a variable called head and set it to 50 to represent the starting position of the disk head.

Operating Systems

requests = [55, 58, 60, 70, 18, 90, 150, 38, 184]
# Set the initial disk head position
# Your code here

Hint

Assign the number 50 to the variable named head.

3

Implement SSTF scheduling logic

Create an empty list called schedule. Then use a while loop that runs while requests is not empty. Inside the loop, find the request closest to head by calculating the minimum absolute difference. Append this closest request to schedule, update head to this request, and remove it from requests.

Operating Systems

requests = [55, 58, 60, 70, 18, 90, 150, 38, 184]
head = 50
# Implement SSTF scheduling logic
# Your code here

Hint

Use min() with a key function to find the closest request to head.

4

Complete the SSTF schedule

After the loop, create a variable called final_schedule and assign it the value of schedule. This variable will hold the order in which disk requests are serviced using SSTF.

Operating Systems

requests = [55, 58, 60, 70, 18, 90, 150, 38, 184]
head = 50
schedule = []
while requests:
    closest = min(requests, key=lambda x: abs(x - head))
    schedule.append(closest)
    head = closest
    requests.remove(closest)
# Assign the final schedule
# Your code here

Hint

Simply assign schedule to final_schedule to complete the project.

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