Why FCFS (First Come First Served) in Operating Systems? - Purpose & Use Cases

The Big Idea

What if the first person in line never got served first? Discover how FCFS fixes this!

The Scenario

Imagine a busy bank where customers line up to be served one by one. Without a clear order, customers might get confused or argue about who should be next.

The Problem

Trying to serve customers randomly or without a clear rule can cause delays, unfairness, and frustration. It's hard to keep track manually who arrived first, leading to mistakes and unhappy customers.

The Solution

FCFS (First Come First Served) is a simple rule that solves this by always serving the customer who arrived first. This clear order makes the process fair, easy to manage, and predictable.

Before vs After

✗ Before

Serve customers as they shout or appear, no order kept

✓ After

Serve customers in the exact order they arrived

What It Enables

It enables fair and organized handling of tasks or requests by respecting their arrival order.

Real Life Example

In an operating system, FCFS schedules processes by running the one that requested CPU time first, ensuring no process is skipped or unfairly delayed.

Key Takeaways

FCFS is a simple, fair scheduling method.

It processes tasks in the order they arrive.

This prevents confusion and ensures fairness.

Practice

1. What does FCFS (First Come First Served) scheduling mean in operating systems?

easy

A. Processes with the shortest time are handled first.

B. Processes are handled in the order they arrive.

C. Processes are handled randomly.

D. Processes with the highest priority are handled first.

Solution

1. Step 1: Understand FCFS concept

   FCFS means tasks are processed in the order they come, like a queue.

2. Step 2: Compare options

   Only Processes are handled in the order they arrive. describes this order-based processing correctly.

3. Final Answer:

   Processes are handled in the order they arrive. -> Option B

4. Quick Check:

   FCFS = Order of arrival [OK]

Hint: Remember: FCFS is like waiting in line [OK]

Common Mistakes:

• Confusing FCFS with priority scheduling
• Thinking shortest tasks go first
• Assuming random order

2. Which of the following is the correct way to describe FCFS scheduling?

easy

A. Processes are scheduled in the order they arrive without preemption.

B. Processes are scheduled based on their priority levels.

C. Processes are scheduled by shortest remaining time first.

D. Processes are scheduled randomly to balance load.

Solution

1. Step 1: Identify FCFS scheduling traits

   FCFS schedules tasks in arrival order and does not interrupt running tasks.

2. Step 2: Match options to traits

   Processes are scheduled in the order they arrive without preemption. correctly states no preemption and order of arrival.

3. Final Answer:

   Processes are scheduled in the order they arrive without preemption. -> Option A

4. Quick Check:

   FCFS = Arrival order + no preemption [OK]

Hint: FCFS runs tasks fully in arrival order [OK]

Common Mistakes:

• Mixing FCFS with priority or shortest job scheduling
• Assuming tasks can be interrupted
• Thinking scheduling is random

3. Given three processes arriving at times 0, 2, and 4 with burst times 5, 3, and 1 respectively, what is the completion time of the second process using FCFS?

medium

A. 10

B. 5

C. 8

D. 9

Solution

1. Step 1: Calculate completion of first process

   Process 1 arrives at 0 and runs for 5 units, finishing at time 5.

2. Step 2: Calculate completion of second process

   Process 2 arrives at 2 but waits until process 1 finishes at 5, then runs for 3 units, finishing at 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Process 2 finishes at 5+3=8 [OK]

Hint: Add burst times in arrival order [OK]

Common Mistakes:

• Starting second process at its arrival time instead of after first finishes
• Adding arrival times incorrectly
• Ignoring waiting time

4. A student wrote this FCFS scheduling code but it gives wrong completion times. What is the likely error?

processes = [(0, 4), (1, 3), (2, 1)]  # (arrival, burst)
completion = []
current_time = 0
for arrival, burst in processes:
    if arrival > current_time:
        current_time = arrival
    completion.append(current_time)
    current_time += burst
print(completion)

medium

A. Not updating current_time after appending completion time.

B. Using a list instead of a queue for processes.

C. Not considering arrival time when updating current_time.

D. Appending completion time before updating current_time.

Solution

1. Step 1: Analyze code logic

   Completion time is appended before current_time is updated with burst time, causing wrong values.

2. Step 2: Identify correct order

   We must update current_time by adding burst before appending completion time to reflect actual finish time.

3. Final Answer:

   Appending completion time before updating current_time. -> Option D

4. Quick Check:

   Update time before recording completion [OK]

Hint: Update current time before saving completion [OK]

Common Mistakes:

• Appending completion time too early
• Ignoring arrival time adjustments
• Confusing process order

5. In an FCFS system, three processes arrive at times 0, 1, and 2 with burst times 4, 2, and 6. If the first process takes longer than expected and runs for 8 units instead of 4, how does this affect the waiting time of the third process?

hard

A. The third process waits longer because the first process delays the queue.

B. The third process waiting time remains the same.

C. The third process starts earlier due to the delay.

D. The third process is skipped and runs immediately.

Solution

1. Step 1: Understand FCFS impact of longer burst

   FCFS runs processes fully in arrival order, so a longer first process delays all others.

2. Step 2: Analyze waiting time effect on third process

   The third process must wait until the first and second finish, so longer first process increases its waiting time.

3. Final Answer:

   The third process waits longer because the first process delays the queue. -> Option A

4. Quick Check:

   Longer first task = longer wait for later tasks [OK]

Hint: Long first task delays all later tasks [OK]

Common Mistakes:

• Assuming later tasks start earlier
• Ignoring impact of burst time changes
• Thinking FCFS skips tasks