FCFS (First Come First Served) in Operating Systems - Full Explanation

Introduction

Imagine many people waiting in line to buy tickets. The problem is how to decide who gets served first so everyone is treated fairly and the process runs smoothly.

Explanation

Basic Principle

FCFS means the first process that arrives is the first to be served. The operating system handles processes in the exact order they come in, without skipping or reordering.

Processes are handled strictly in the order they arrive.

Queue Mechanism

Processes wait in a queue, like a line at a store. The process at the front of the queue gets the CPU until it finishes or blocks, then the next process moves up.

A simple queue controls the order of process execution.

Non-preemptive Scheduling

Once a process starts running, it keeps the CPU until it finishes or waits for input. The system does not interrupt it to run another process.

Processes run to completion without interruption.

Advantages

FCFS is easy to understand and implement. It is fair in the sense that no process jumps ahead of others.

Simplicity and fairness in order of arrival are key benefits.

Disadvantages

Long processes can delay shorter ones, causing a problem called the "convoy effect." This can lead to inefficient CPU use and longer waiting times.

Long tasks can block shorter ones, reducing efficiency.

Real World Analogy

Imagine a bakery where customers line up to buy bread. The baker serves each customer in the order they arrived, even if some customers want just one loaf and others want many.

Basic Principle → Customers are served in the exact order they arrive at the bakery.

Queue Mechanism → The line of customers waiting at the bakery counter.

Non-preemptive Scheduling → The baker finishes serving one customer completely before moving to the next.

Advantages → Everyone knows their turn will come without skipping.

Disadvantages → A customer buying many loaves slows down the line for others.

Diagram

Diagram

┌───────────────┐
│ Process Queue │
├───────────────┤
│ P1            │
│ P2            │
│ P3            │
│ P4            │
└─────┬─────────┘
      │
      ↓
┌───────────────┐
│ CPU           │
│ (Executes P1) │
└───────────────┘

This diagram shows processes lined up in a queue and the CPU executing them one by one in order.

Key Facts

FCFS Scheduling → Processes are executed in the order they arrive without interruption.

Non-preemptive → Once a process starts, it runs until completion or waiting.

Convoy Effect → Long processes delay shorter ones, causing inefficiency.

Process Queue → A line where processes wait their turn for the CPU.

Fairness → No process skips ahead; order is strictly by arrival time.

Common Confusions

FCFS always gives the best performance.

FCFS always gives the best performance. FCFS is simple but can cause delays due to the convoy effect, so it is not always efficient.

Processes can be interrupted in FCFS.

Processes can be interrupted in FCFS. FCFS is non-preemptive, meaning a process runs fully once started without interruption.

Summary

FCFS schedules processes strictly in the order they arrive, like a line at a store.

It is simple and fair but can cause delays when long processes block shorter ones.

FCFS does not interrupt running processes until they finish or wait.

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