What is fcfs?

First-Come, First-Served (FCFS) Scheduling

First-Come, First-Served (FCFS), also known as First-In, First-Out (FIFO), is a simple and straightforward scheduling algorithm that executes processes in the order they arrive in the ready queue. It is a non-preemptive algorithm, meaning once a process starts executing, it continues until it completes, regardless of the arrival of other processes.

Key Features:

• Simplicity: FCFS is easy to understand and implement.
• Fairness: Every process gets a chance to run eventually.
• Non-Preemptive: A process runs to completion without interruption.

How it Works:

1. Processes are added to the ready queue as they arrive.
2. The process at the head of the queue is selected for execution.
3. The selected process runs until it completes or blocks (e.g., waiting for I/O).
4. When a process completes or blocks, the next process in the queue is selected.

Advantages:

• Easy to understand and implement.
• Simple to program.
• Fair in the sense that processes are served in the order they arrive.

Disadvantages:

• Convoy Effect: Long processes can block shorter processes, leading to poor CPU utilization and longer average waiting time for other processes.
• Not suitable for time-sharing systems because of its non-preemptive nature.
• May not be optimal in terms of minimizing average waiting time or turnaround time.
• The CPU Utilization can be low.

Use Cases:

FCFS is suitable for batch processing systems or situations where fairness is more important than minimizing response time. It is often used as a baseline algorithm for comparison with other scheduling algorithms.

Example:

Assume three processes arrive in the order P1, P2, P3 with burst times of 24, 3, and 3 units, respectively.

• P1 will run for 24 units.
• P2 will run for 3 units after P1 completes.
• P3 will run for 3 units after P2 completes.

The average waiting time would be (0 + 24 + 27) / 3 = 17 units. This illustrates how a long process (P1) can significantly increase the waiting time for other processes.

Alternatives:

Other scheduling algorithms like Shortest Job First (SJF) and Priority Scheduling often provide better performance in terms of average waiting time and turnaround time, but they are more complex to implement.