Short Answer
FCFS (First Come First Serve) scheduling is a CPU scheduling algorithm in which the process that arrives first in the ready queue is executed first. It follows a simple queue rule where processes are handled in the order they come.
In this method, once a process starts execution, it runs until completion without interruption. FCFS is easy to understand and implement but may lead to long waiting time for some processes.
Detailed Explanation:
FCFS Scheduling Overview
Definition:
FCFS (First Come First Serve) scheduling is one of the simplest CPU scheduling algorithms used in operating systems. In this method, the CPU is allocated to processes based on their arrival time. The process that comes first in the ready queue gets executed first, just like a normal queue in real life.
Importance of FCFS:
FCFS is important because it is very easy to implement and understand. It is often used as a basic scheduling method in operating system concepts. It helps students understand how process scheduling works in a simple way before learning advanced algorithms.
Working of FCFS Scheduling
Process Arrival
Meaning:
Processes enter the ready queue in the order they arrive.
Explanation:
When multiple processes are ready for execution, they are placed in a queue. The first process that arrives is placed at the front of the queue and is selected first by the CPU.
CPU Allocation
Meaning:
The CPU is assigned to the first process in the queue.
Explanation:
The process at the front of the queue gets the CPU first. It executes completely without any interruption until it finishes its task. After that, the next process in the queue is selected.
Non-Preemptive Nature
Meaning:
Once a process starts execution, it cannot be stopped.
Explanation:
FCFS is a non-preemptive scheduling algorithm. This means the CPU will not switch to another process until the current process completes its execution. This makes the system simple but sometimes inefficient.
Characteristics of FCFS Scheduling
Simple Structure
Explanation:
FCFS uses a straightforward queue system. There are no complex rules for selecting processes.
Fairness
Explanation:
Every process gets a chance to execute in the order it arrives. No process is ignored.
No Priority System
Explanation:
FCFS does not consider priority or execution time. Only arrival time matters.
Advantages of FCFS Scheduling
Easy to Understand
FCFS is very simple and easy for beginners to learn and implement in operating systems.
Fair System
All processes are treated equally based on arrival time, so it is fair in execution order.
Simple Implementation
It requires minimal overhead and simple queue management.
Disadvantages of FCFS Scheduling
Long Waiting Time
If a long process arrives first, all other processes must wait, causing delay.
Poor Performance
It may lead to inefficient CPU utilization in some cases.
Convoy Effect
Small processes get stuck behind large processes, increasing overall waiting time.
Example of FCFS Scheduling
If three processes P1, P2, and P3 arrive in that order:
- P1 arrives first → executes first
- P2 arrives second → executes after P1
- P3 arrives last → executes after P2
Each process must wait for the previous one to finish before starting execution.
Importance in Operating System
Basic Learning Model:
FCFS is used to teach basic scheduling concepts.
Foundation for Advanced Algorithms:
It helps understand more complex scheduling methods like SJF and Round Robin.
Simple System Usage:
It can be used in simple systems where process timing is not critical.
Conclusion
FCFS scheduling is a simple and basic CPU scheduling algorithm where processes are executed in the order they arrive. It is easy to implement and fair but may cause long waiting time and poor performance in some cases. It is mainly used for understanding basic scheduling concepts in operating systems.