FCFS disk scheduling in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding FCFS Disk Scheduling

What is the main characteristic of the First-Come, First-Served (FCFS) disk scheduling algorithm?

A It prioritizes requests based on their priority level.

B It always selects the request closest to the current head position.

C It processes disk requests in the order they arrive without reordering.

D It reorders requests to minimize total seek time.

💡 Hint

Think about how requests are handled as they come in.

FCFS processes disk requests strictly in the order they arrive, without any reordering or optimization.

🚀 Application

intermediate

Calculating Total Head Movement in FCFS

Given the initial head position at 50 and the request queue: 82, 170, 43, 140, 24, what is the total head movement using FCFS scheduling?

A 344

B 230

C 208

D 460

💡 Hint

Calculate the absolute distance between consecutive requests starting from the initial head position.

Total head movement is the sum of distances between consecutive requests in the order they arrive.

🔍 Analysis

advanced

Identifying Drawbacks of FCFS Disk Scheduling

Which of the following is a significant drawback of the FCFS disk scheduling algorithm?

A It can cause long wait times due to lack of optimization.

B It always minimizes seek time effectively.

C It prioritizes urgent requests over others.

D It requires complex calculations to reorder requests.

💡 Hint

Consider how FCFS handles requests without reordering.

FCFS does not optimize the order of requests, which can lead to long wait times and inefficient head movement.

❓ Comparison

advanced

Comparing FCFS with Other Disk Scheduling Algorithms

Compared to algorithms like SSTF or SCAN, how does FCFS generally perform in terms of average seek time?

A FCFS always has the lowest average seek time.

B FCFS usually has higher average seek time than SSTF or SCAN.

C FCFS and SSTF have the same average seek time.

D FCFS performs better than SCAN in all cases.

💡 Hint

Think about how FCFS processes requests without optimization.

FCFS does not optimize head movement, so it usually results in higher average seek times compared to SSTF or SCAN.

❓ Reasoning

expert

Predicting FCFS Behavior with Specific Request Patterns

If the disk head starts at position 100 and the request queue is 105, 95, 110, 90, 115, what will be the total head movement using FCFS scheduling?

A 75

B 50

C 40

D 30

💡 Hint

Calculate the sum of absolute differences between consecutive requests starting from 100.

Calculate distances: |100-105|=5, |105-95|=10, |95-110|=15, |110-90|=20, |90-115|=25; total = 5+10+15+20+25 = 75

Practice

1. What does FCFS stand for in disk scheduling?

easy

A. Fastest-Cylinder First Service

B. First-Come, First-Served

C. Frequent Cylinder File System

D. First-Come, Fast-Served

Solution

1. Step 1: Understand the abbreviation FCFS

   FCFS is a common term in scheduling that means requests are handled in the order they arrive.

2. Step 2: Match the full form

   First-Come, First-Served matches the concept of serving requests in arrival order.

3. Final Answer:

   First-Come, First-Served -> Option B

4. Quick Check:

   FCFS = First-Come, First-Served [OK]

Hint: Remember FCFS means serve in arrival order [OK]

Common Mistakes:

• Confusing FCFS with other scheduling algorithms
• Thinking FCFS means fastest service
• Mixing up the abbreviation letters

2. Which of the following correctly describes the order in which FCFS disk scheduling processes requests?

easy

A. Processes requests in the order they arrive

B. Processes requests based on shortest seek time first

C. Processes requests starting from the outermost track

D. Processes requests based on priority levels

Solution

1. Step 1: Recall FCFS scheduling principle

   FCFS processes disk requests in the exact order they arrive without reordering.

2. Step 2: Compare options with FCFS behavior

   Only Processes requests in the order they arrive matches this behavior exactly.

3. Final Answer:

   Processes requests in the order they arrive -> Option A

4. Quick Check:

   FCFS = order of arrival [OK]

Hint: FCFS means no reordering, just arrival order [OK]

Common Mistakes:

• Choosing shortest seek time instead of arrival order
• Confusing with priority scheduling
• Assuming outermost track first

3. Given the disk queue requests at tracks [55, 58, 60, 70, 18] and the disk head starting at track 50, what is the total head movement using FCFS scheduling?

medium

A. 72

B. 80

C. 98

D. 85

Solution

1. Step 1: Calculate head movements in FCFS order

   Start at 50, move to 55 (5), then 58 (3), then 60 (2), then 70 (10), then 18 (52). Total = 5+3+2+10+52 = 72.

2. Step 2: Verify calculation carefully

   Recalculate: 5 + 3 + 2 + 10 + 52 = 72 confirmed.

3. Final Answer:

   72 -> Option A

4. Quick Check:

   Sum of movements = 72 [OK]

Hint: Add absolute differences in request order [OK]

Common Mistakes:

• Adding distances incorrectly
• Mixing request order
• Forgetting to include last move

4. Identify the error in this FCFS disk scheduling calculation: Starting at track 45, requests are [50, 40, 55]. The total head movement is calculated as |45-50| + |50-55| + |55-40| = 5 + 5 + 15 = 25. What is wrong?

medium

A. Starting track should be included twice

B. Absolute values should not be used

C. The total should be 30, not 25

D. The order of requests is incorrect in calculation

Solution

1. Step 1: Check the request order in calculation

   The requests are [50, 40, 55], so after 50, the head should move to 40, then 55.

2. Step 2: Verify the distances in correct order

   Calculate: |45-50|=5, |50-40|=10, |40-55|=15; total = 5+10+15=30, not 25.

3. Final Answer:

   The order of requests is incorrect in calculation -> Option D

4. Quick Check:

   Correct order matters for total movement [OK]

Hint: Follow request order exactly for distance sums [OK]

Common Mistakes:

• Mixing request order
• Ignoring absolute distance
• Miscounting starting position

5. A disk head starts at track 100. The request queue is [120, 10, 50, 90]. Using FCFS scheduling, what is the total head movement? How does this compare to SSTF (Shortest Seek Time First) scheduling for the same requests?

hard

A. FCFS: 200, SSTF: 90; SSTF reduces movement

B. FCFS: 100, SSTF: 100; Both equal

C. FCFS: 210, SSTF: 150; SSTF reduces movement

D. FCFS: 150, SSTF: 120; FCFS is better

Solution

1. Step 1: Calculate FCFS total head movement

   100->120 (20), 120->10 (110), 10->50 (40), 50->90 (40). Total = 20+110+40+40 = 210.

2. Step 2: Calculate SSTF total head movement

   From 100 closest is 90 (10)->90; closest 120 (30)->120; closest 50 (70)->50; then 10 (40). Total = 10+30+70+40 = 150.

3. Step 3: Compare

   SSTF (150) reduces movement compared to FCFS (210).

4. Final Answer:

   FCFS: 210, SSTF: 150; SSTF reduces movement -> Option C

5. Quick Check:

   SSTF reduces head movement compared to FCFS [OK]

Hint: SSTF picks closest request, reducing movement [OK]

Common Mistakes:

• Mixing request order in FCFS
• Misordering SSTF requests
• Ignoring large jumps in FCFS