SSTF (Shortest Seek Time First) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SSTF stand for in disk scheduling?

SSTF stands for Shortest Seek Time First. It is a disk scheduling algorithm that selects the disk I/O request closest to the current head position to minimize seek time.

beginner

How does SSTF decide which disk request to serve next?

SSTF chooses the request that requires the least movement of the disk arm from its current position, reducing the seek time for each operation.

intermediate

What is a potential drawback of the SSTF algorithm?

SSTF can cause starvation, where some requests far from the current head position may wait a long time if closer requests keep arriving.

intermediate

Compare SSTF with FCFS (First-Come, First-Served) in disk scheduling.

SSTF usually reduces average seek time by choosing the closest request next, while FCFS serves requests in arrival order without considering distance, which can lead to longer seek times.

advanced

In what scenario might SSTF perform poorly despite its goal to minimize seek time?

If requests are clustered far apart, SSTF might keep switching between nearby requests and neglect others, causing some requests to wait too long, leading to starvation.

What is the main goal of the SSTF disk scheduling algorithm?

A Serve requests from the outer edge to the inner edge

B Serve requests in the order they arrive

C Minimize the seek time by selecting the closest request

D Serve requests randomly

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

Which problem can SSTF cause when handling disk requests?

A Starvation of some requests

B Increased total seek time always

C Ignoring the closest requests

D Serving requests in random order

SSTF can cause starvation because requests far from the current head position may wait too long.

How does SSTF differ from FCFS in disk scheduling?

A Both serve requests in the same order

B SSTF serves the closest request; FCFS serves in arrival order

C SSTF serves requests randomly; FCFS serves closest first

D FCFS minimizes seek time; SSTF does not

SSTF picks the closest request to minimize seek time, while FCFS serves requests as they arrive.

If the disk head is at position 50, and requests are at 45, 55, and 60, which will SSTF serve next?

A Request at 50

B Request at 55

C Request at 60

D Request at 45

Requests at 45 and 55 are both 5 units away from 50, so SSTF can choose either. Typically, it picks the closest; if equal, either can be chosen.

Which of these is NOT a characteristic of SSTF?

A Always serves requests in arrival order

B Minimizes seek time

C Can cause starvation

D Selects the nearest request to current head

SSTF does not serve requests in arrival order; it serves the closest request next.

Explain how the SSTF disk scheduling algorithm works and one advantage and one disadvantage of using it.

Think about how the disk arm moves and what happens to far away requests.

Compare SSTF with FCFS disk scheduling in terms of seek time and fairness.

Consider both efficiency and how requests are treated over time.

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