First Come First Serve (FCFS) CPU Scheduling Algorithm - Operating Systems

Introduction

This tutorial will guide you through the First Come First Serve (FCFS) CPU Scheduling Algorithm, a fundamental concept in operating systems. FCFS is one of the simplest scheduling algorithms, where processes are executed in the order they arrive in the ready queue. Understanding FCFS is essential for grasping more complex scheduling methods.

Step 1: Understand the Concept of FCFS

• FCFS operates on a straightforward principle: the first process that arrives gets the CPU first.
• It is non-preemptive, meaning once a process starts executing, it runs to completion before the CPU is allocated to the next process.
• The algorithm can be visualized as a queue where processes wait their turn.

Practical Tip

• Use a real-world analogy, like a queue at a bank, to understand how processes line up and get served.

Step 2: Identify the Components of FCFS Scheduling

1. Arrival Time: The time at which a process enters the ready queue.
2. Burst Time: The total time required by a process for execution on the CPU.
3. Completion Time: The time at which a process completes its execution.
4. Turnaround Time: The total time taken from arrival to completion. It is calculated as:
   • Turnaround Time = Completion Time - Arrival Time
5. Waiting Time: The total time a process spends waiting in the ready queue. It is calculated as:
   • Waiting Time = Turnaround Time - Burst Time

Step 3: Calculate FCFS Scheduling Metrics

To illustrate the FCFS algorithm, follow these steps to calculate the scheduling metrics for a set of processes:

1. List the processes along with their arrival times and burst times.

   | Process | Arrival Time | Burst Time | |---------|--------------|------------| | P1 | 0 | 4 | | P2 | 1 | 3 | | P3 | 2 | 1 |

2. Calculate Completion Time:

   • P1 completes at time 4 (0 + 4).
   • P2 starts at time 4 and completes at time 7 (4 + 3).
   • P3 starts at time 7 and completes at time 8 (7 + 1).

3. Calculate Turnaround Time:

   • P1: 4 - 0 = 4
   • P2: 7 - 1 = 6
   • P3: 8 - 2 = 6

4. Calculate Waiting Time:

   • P1: 4 - 4 = 0
   • P2: 6 - 3 = 3
   • P3: 6 - 1 = 5

Practical Advice

• Create a table to help visualize the calculations and ensure accuracy.

Step 4: Analyze FCFS Performance

• Pros:

  • Simple and easy to implement.
  • Fair in terms of process order.

• Cons:

  • Can lead to the "convoy effect," where shorter processes wait for longer ones, leading to increased waiting times.
  • Not optimal for time-sharing systems.

Common Pitfalls

• Failing to account for the arrival times can skew your results.
• Ignoring the potential drawbacks of FCFS can lead to inefficiencies in system performance.

Conclusion

The First Come First Serve CPU Scheduling Algorithm is a foundational concept in operating systems that prioritizes simplicity and fairness. By understanding its mechanics, including how to calculate key metrics like turnaround and waiting times, you can better appreciate its role and limitations in scheduling processes. As a next step, consider exploring more advanced scheduling algorithms, such as Shortest Job First (SJF) or Round Robin, to compare their efficiencies and applications in different scenarios.