Why FCFS disk scheduling in Operating Systems? - Purpose & Use Cases

The Big Idea

What if your computer's disk had to jump all over the place just to read your files? FCFS keeps it simple and fair!

The Scenario

Imagine you have a stack of papers on your desk, and you need to find specific pages scattered throughout. You decide to pick them up in the exact order they appear on the desk, without thinking about their location. This is like manually handling disk requests as they come, without any planning.

The Problem

Doing this manually means you might jump back and forth across the desk, wasting time moving your hand unnecessarily. Similarly, handling disk requests in the order they arrive can cause the disk head to move a lot, slowing down the system and increasing wait times.

The Solution

FCFS (First-Come, First-Served) disk scheduling processes requests in the order they arrive, just like picking papers in sequence. This simple method ensures fairness and is easy to implement, avoiding confusion about which request to handle next.

Before vs After

✗ Before

requests = [55, 58, 39, 18, 90]
for req in requests:
    move_disk_head(req)

✓ After

requests = [55, 58, 39, 18, 90]
# FCFS processes requests as they come
for req in requests:
    move_disk_head(req)

What It Enables

FCFS disk scheduling enables a straightforward and fair way to handle disk requests, ensuring every request is served in the order it arrives without complex decision-making.

Real Life Example

Think of a queue at a coffee shop where customers are served in the order they arrive. FCFS disk scheduling works the same way, serving disk requests one by one as they come.

Key Takeaways

FCFS handles disk requests in arrival order.

It is simple and fair but may cause longer wait times due to head movement.

Useful as a basic scheduling method in operating systems.

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