First Come First Serve Scheduling Algorithm

First Come First Serve Scheduling Algorithm: A Fundamental Approach to CPU Management

The First Come First Serve (FCFS) scheduling algorithm is one of the simplest and most intuitive methods used in operating systems to manage process execution on a CPU. As its name suggests, FCFS allocates CPU time to processes based on their arrival order, ensuring that the first process to arrive is the first to be executed. So while its straightforward design makes it easy to implement, FCFS also has notable limitations, particularly in scenarios where process durations vary significantly. This article explores the mechanics, advantages, disadvantages, and practical applications of FCFS, providing a comprehensive understanding of its role in system scheduling Worth knowing..

Some disagree here. Fair enough.

How FCFS Works

FCFS operates by maintaining a queue of processes waiting for CPU allocation. When the CPU becomes free, the scheduler selects the process at the front of the queue—the one that arrived earliest—and assigns it to the CPU. Once the process completes its execution, the CPU moves to the next process in the queue, continuing this sequence until all processes are handled Took long enough..

Consider an example with three processes:

• P1 arrives at time 0 with a burst time of 5 units.
• P2 arrives at time 2 with a burst time of 3 units.
• P3 arrives at time 4 with a burst time of 1 unit.

Under FCFS:

1. 3. 2. P2 begins at time 5 (after P1 finishes) and runs until time 8.
         P1 starts at time 0 and runs until time 5.
         P3 starts at time 8 and completes at time 9.

The waiting times for each process would be:

• P1: 0 (starts immediately).
• P2: 5 – 2 = 3 units (waits from time 2 to 5).
• P3: 8 – 4 = 4 units (waits from time 4 to 8).

This example highlights FCFS’s reliance on arrival order, even if shorter processes are delayed behind longer ones.

Advantages of FCFS

1. Simplicity: FCFS requires minimal computational overhead, making it ideal for systems with limited resources. Its algorithm is easy to code and understand, which is critical in educational or embedded systems.
2. Fairness: Every process receives CPU time in the order it arrives, ensuring no process is starved or unfairly prioritized.
3. Non-Preemptive Nature: Once a process starts execution, it runs to completion without interruption. This predictability is beneficial for long-running tasks where abrupt termination could cause errors.

Disadvantages of FCFS

1. Convoy Effect: This is the most significant drawback. When a long process occupies the CPU, shorter processes that arrive later must wait, leading to increased waiting times. As an example, in the earlier example, P3 (which has the shortest burst time) had to wait 4 units, while P2 (with a medium burst time) waited 3 units. If P3 had arrived first, its waiting time would have been zero.
2. Inefficient Resource Utilization: The convoy effect reduces overall system throughput. Processes with short burst times are often delayed, wasting potential CPU cycles that could have been used productively.
3. Poor Performance with Variable Burst Times: FCFS struggles in environments where processes have vastly different execution times. It is less suitable for interactive systems where quick response times are critical.

Performance Metrics

To evaluate FCFS’s effectiveness, several key metrics are used:

• **Waiting