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.