Understanding First-Come, First-Serve (FCFS) Scheduling Algorithm

Introduction to First-Come, First-Serve Scheduling

The First-Come, First-Serve (FCFS) scheduling algorithm is the simplest form of CPU scheduling mechanism, which operates on a very straightforward principle: the first process to request the CPU is the first to be allocated the CPU. This method mimics everyday queue scenarios, such as waiting in line to pay bills, where the person arriving first gets served first.

How FCFS Works

FCFS scheduling is easily managed with a FIFO (First-In, First-Out) queue. In a FIFO queue, the first element to arrive is the first to leave after completing its task. This is a natural method followed in many real-life queues and is simple to implement in CPU scheduling.

Implementation Details

• When a process enters the ready queue, its Process Control Block (PCB) is linked at the tail of the queue.
• The CPU is allocated to the process at the head of the queue when the CPU becomes available.
• Once a process receives the CPU and completes its execution, it is removed from the queue, making way for the next process in line.

Efficiency and Limitations

While FCFS is straightforward and easy to implement, it may not always be the most efficient. The average waiting time for processes can be long, especially if a process with a lengthy CPU burst time arrives first. This can lead to increased wait times for subsequent processes, demonstrating a significant disadvantage in time-sharing systems where equitable CPU time distribution is crucial.

Examples and Analysis

Consider three processes with varying burst times arriving at the same time. If they are served in FCFS order, the process arriving first (with the longest burst time) will execute first, causing substantial wait times for the others. This scenario shows how FCFS can lead to inefficiencies, particularly when the process sequence and burst times vary.

Another example with the same processes arriving in a different order illustrates how the average wait time can significantly decrease, emphasizing how FCFS efficiency heavily depends on the order and burst times of the arriving processes.

FCFS as a Non-Preemptive Algorithm

FCFS is a non-preemptive scheduling algorithm, meaning once a process is allocated the CPU, it retains it until completion or it requests I/O, regardless of the arrival of other processes. This policy reinforces the FCFS principle but also highlights its limitation in dynamic, time-sharing environments where process needs frequently change.

Conclusion

The First-Come, First-Serve scheduling algorithm offers a simple and straightforward approach to CPU scheduling but comes with its set of limitations, particularly in efficiency and fairness in time-sharing systems. As we explore more CPU scheduling algorithms, it's essential to consider these aspects to understand which method best suits different operating environments.

For those interested in diving deeper into CPU scheduling algorithms and understanding their complexities and benefits, stay tuned for more discussions on alternative methods that may offer improved efficiency and fairness.

Watch the full lecture here: First-Come, First-Serve Scheduling Algorithm Lecture