FCFS disk scheduling in Operating Systems - Full Explanation

Introduction

Imagine a disk drive receiving many requests to read or write data. The challenge is deciding the order to handle these requests to keep the disk working efficiently and reduce waiting time.

Explanation

First-Come, First-Served Order

FCFS disk scheduling processes disk requests in the exact order they arrive. It does not rearrange or prioritize requests based on their location on the disk. This means the disk head moves to each requested position one by one, following the arrival sequence.

FCFS serves disk requests strictly in the order they come without reordering.

Disk Head Movement

The disk head moves from its current position to the location of the next request in the queue. This movement can be long or short depending on where the requests are located. FCFS does not optimize the path, so the head may move back and forth across the disk.

Disk head movement in FCFS can be inefficient because it follows request order without optimization.

Advantages of FCFS

FCFS is simple to implement and easy to understand. It is fair because every request is handled in the order it arrives, so no request waits indefinitely. This makes it predictable in behavior.

FCFS is simple and fair but may not be efficient.

Disadvantages of FCFS

Because FCFS does not optimize the order of requests, the disk head may travel long distances unnecessarily. This can increase the total time to complete all requests and cause longer waiting times for some requests, especially if they are far apart.

FCFS can cause slow performance due to unoptimized disk head movement.

Real World Analogy

Imagine a librarian who helps people in the exact order they arrive, regardless of where the books they want are located in the library. Sometimes the librarian has to walk back and forth a lot, which takes extra time.

First-Come, First-Served Order → Helping library visitors strictly in the order they arrive without skipping anyone.

Disk Head Movement → The librarian walking to different shelves to get books, sometimes moving long distances.

Advantages of FCFS → Everyone gets helped fairly and predictably, no one is skipped.

Disadvantages of FCFS → The librarian wastes time walking back and forth, making the process slower.

Diagram

Diagram

┌───────────────┐
│ Disk Requests │
└──────┬────────┘
       │ Arrive in order
       ↓
┌───────────────────────────┐
│ FCFS Scheduler            │
│ Processes requests in     │
│ arrival order             │
└──────┬────────────────────┘
       │ Moves disk head to each request
       ↓
┌───────────────────────────┐
│ Disk Head Movement        │
│ May move long distances   │
│ without optimization      │
└───────────────────────────┘

This diagram shows how FCFS disk scheduling processes requests in arrival order and moves the disk head accordingly.

Key Facts

FCFS disk scheduling → A disk scheduling method that processes requests in the order they arrive.

Disk head → The part of the disk drive that reads or writes data by moving to different disk locations.

Request queue → A list of disk read/write requests waiting to be processed.

Fairness → Each request is handled in the order it arrives, preventing starvation.

Inefficiency → FCFS can cause long disk head movements, increasing total processing time.

Common Confusions

FCFS always provides the fastest disk access.

FCFS always provides the fastest disk access. FCFS does not optimize disk head movement, so it can be slower than other methods that reorder requests to reduce travel distance.

FCFS can skip requests to improve speed.

FCFS can skip requests to improve speed. FCFS strictly processes requests in arrival order and does not skip or reorder any requests.

Summary

FCFS disk scheduling handles disk requests in the exact order they arrive, ensuring fairness.

The disk head moves to each request without optimizing the path, which can cause inefficiency.

While simple and predictable, FCFS may lead to longer waiting times compared to other scheduling methods.

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