FCFS disk scheduling in Operating Systems - Mini Project: Build & Apply

FCFS Disk Scheduling Simulation

📖 Scenario: You are managing a computer's disk drive. The disk head moves to read data from different tracks on the disk. The requests come in a certain order, and you want to simulate how the disk head moves using the First-Come, First-Served (FCFS) scheduling method.

🎯 Goal: Build a simple step-by-step simulation of the FCFS disk scheduling algorithm. You will create the list of disk requests, set the initial head position, calculate the total head movement as the disk head moves to each requested track in order, and finally complete the simulation by showing the total movement.

📋 What You'll Learn

Create a list called requests with the exact disk track numbers: 98, 183, 37, 122, 14, 124, 65, 67

Create a variable called head with the initial disk head position set to 53

Use a variable called total_movement to keep track of the total distance the disk head moves

Use a for loop with variables request to iterate over requests in order

Calculate the absolute distance between the current head position and the request, add it to total_movement, then update head to the request

At the end, have a variable called result that stores the total head movement as an integer

💡 Why This Matters

🌍 Real World

Disk scheduling algorithms help operating systems decide the order to access data on a hard drive, improving speed and efficiency.

💼 Career

Understanding disk scheduling is important for roles in system administration, operating system development, and performance optimization.

1

Create the list of disk requests

Create a list called requests with these exact disk track numbers in order: 98, 183, 37, 122, 14, 124, 65, 67

Operating Systems

# Create the list of disk requests
# Your code here

Hint

Use square brackets [] to create a list and separate the numbers with commas.

2

Set the initial disk head position

Create a variable called head and set it to the initial disk head position 53

Operating Systems

requests = [98, 183, 37, 122, 14, 124, 65, 67]
# Set the initial disk head position
# Your code here

Hint

Use a simple assignment to create the variable head with the value 53.

3

Calculate total head movement using FCFS

Create a variable called total_movement and set it to 0. Then use a for loop with variable request to iterate over requests. Inside the loop, calculate the absolute distance between head and request, add it to total_movement, and update head to request

Operating Systems

requests = [98, 183, 37, 122, 14, 124, 65, 67]
head = 53
# Calculate total head movement
# Your code here

Hint

Use abs() to get the absolute difference between two numbers.

4

Store the final total head movement

Create a variable called result and set it equal to total_movement to store the total distance the disk head moved

Operating Systems

requests = [98, 183, 37, 122, 14, 124, 65, 67]
head = 53
total_movement = 0
for request in requests:
    total_movement += abs(head - request)
    head = request
# Store the final total head movement
# Your code here

Hint

Simply assign result to the value of total_movement.

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