FCFS disk scheduling in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does FCFS stand for in disk scheduling?

FCFS stands for First-Come, First-Served. It is a simple disk scheduling algorithm where requests are processed in the order they arrive.

beginner

How does FCFS disk scheduling decide which request to serve next?

FCFS serves disk requests in the exact order they arrive, without reordering or prioritizing any request.

intermediate

What is a major drawback of FCFS disk scheduling?

A major drawback is that it can cause long waiting times if requests are far apart on the disk, leading to inefficient disk arm movement.

beginner

Explain with a simple example how FCFS disk scheduling works.

If the disk head is at track 50 and requests come for tracks 55, 58, and 52 in that order, FCFS will move the head to 55 first, then 58, then 52, following the arrival order.

beginner

Is FCFS disk scheduling fair? Why or why not?

Yes, FCFS is fair because it serves requests in the order they arrive, so no request is skipped or delayed unfairly.

What is the main principle of FCFS disk scheduling?

A Serve the closest request first

B Serve requests in reverse order

C Serve the request with the shortest seek time

D Serve requests in the order they arrive

FCFS serves disk requests in the order they arrive, without reordering.

Which of the following is a disadvantage of FCFS disk scheduling?

A It can cause long wait times due to inefficient head movement

B It always prioritizes the closest request

C It requires complex calculations

D It ignores the order of requests

FCFS can cause long wait times because it does not optimize the order of requests.

If the disk head is at track 30 and requests arrive for tracks 40, 10, and 50 in that order, which track will FCFS move to first?

A Track 40

B Track 30

C Track 50

D Track 10

FCFS serves requests in arrival order, so it moves to track 40 first.

Is FCFS disk scheduling considered fair to all requests?

A No, it serves requests randomly

B Yes, because it serves requests in arrival order

C No, it favors requests near the current head position

D Yes, because it serves the shortest request first

FCFS is fair since it processes requests in the order they arrive.

Which disk scheduling algorithm is simpler to implement?

A SCAN

B Shortest Seek Time First

C FCFS

D C-SCAN

FCFS is the simplest disk scheduling algorithm as it just follows request arrival order.

Describe how FCFS disk scheduling works and mention one advantage and one disadvantage.

Think about the order of requests and how the disk head moves.

Explain why FCFS disk scheduling might lead to inefficient disk arm movement.

Consider how the disk head travels between requests.

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