Introduction

In computer science, scheduling algorithms decide the order in which processes are executed by the CPU. One of the most basic and widely explained scheduling methods is first come first serve scheduling. This technique works exactly as the name suggests: whichever process enters the ready queue first will be the one executed first. It is similar to how people stand in a queue at a grocery store or a bank counter. The person who arrives earliest gets served before those who arrive later.

This method is simple, easy to understand, and forms the foundation for learning more advanced CPU scheduling algorithms. However, it also comes with certain drawbacks, especially when dealing with processes that require widely different execution times. Let’s dive deep into the concept, working, advantages, and limitations of first come first serve scheduling.

What is First Come First Serve Scheduling?

First come first serve scheduling, often abbreviated as FCFS, is a non-preemptive scheduling technique. Non-preemptive means that once the CPU starts executing a process, it will not switch to another process until the current one has completely finished. This makes FCFS straightforward to implement but less flexible in certain scenarios.

In simple words, processes are arranged in the ready queue based on their arrival time. The operating system then selects the process at the front of the queue and runs it until it finishes. After that, the CPU moves on to the next process in line.

Key Characteristics

1. Order Based on Arrival
   Processes are executed in the same order in which they arrive.
   
2. Non-preemptive Nature
   Once a process is running, it cannot be stopped until its burst time (execution time) is completed.
   
3. Queue Representation
   The ready queue is often imagined as a simple First In, First Out (FIFO) structure, like a line of people waiting for service.
   
4. Simplicity
   The algorithm is easy to implement and understand, which is why it is taught as the first scheduling concept to computer science students.
   

How First Come First Serve Scheduling Works

To better understand, let’s consider an example. Suppose we have three processes:

• Process P1 with burst time 5
  
• Process P2 with burst time 3
  
• Process P3 with burst time 8
  

And assume they all arrive in the order P1 → P2 → P3.

Step 1: Arrange in the Queue

The ready queue looks like: P1 → P2 → P3.

Step 2: Start Execution

• The CPU first executes P1 for 5 units of time.
  
• Once P1 finishes, P2 gets the CPU and runs for 3 units.
  
• Finally, P3 runs for 8 units.
  

Step 3: Completion Order

The order of completion is exactly the same as the order of arrival.

This example shows how FCFS ensures fairness in terms of process arrival but may lead to delays if one long process blocks shorter ones waiting behind it.

Important Terms in FCFS

To evaluate the performance of any scheduling algorithm, we need to understand a few key terms:

1. Burst Time (BT): The total time a process requires to complete execution.
   
2. Arrival Time (AT): The time at which a process enters the ready queue.
   
3. Completion Time (CT): The time at which a process finishes execution.
   
4. Turnaround Time (TAT): The total time spent by a process in the system, calculated as:
   TAT = CT – AT
   
5. Waiting Time (WT): The amount of time a process spends waiting in the ready queue before execution.
   WT = TAT – BT
   

Example Calculation

Let’s solve a small problem to see how FCFS works in numbers.

Given:

• P1: AT = 0, BT = 4
  
• P2: AT = 1, BT = 3
  
• P3: AT = 2, BT = 1
  

Step 1: Execution Order

Since processes arrive in order P1 → P2 → P3, they are executed in the same sequence.

Step 2: Completion Time

• P1 completes at time 4.
  
• P2 starts at 4 and finishes at 7.
  
• P3 starts at 7 and finishes at 8.
  

Step 3: Turnaround Time (TAT)

• P1: CT – AT = 4 – 0 = 4
  
• P2: 7 – 1 = 6
  
• P3: 8 – 2 = 6
  

Step 4: Waiting Time (WT)

• P1: TAT – BT = 4 – 4 = 0
  
• P2: 6 – 3 = 3
  
• P3: 6 – 1 = 5
  

This shows how some processes end up waiting longer even though their burst time is short. This is one of the main criticisms of FCFS.

Advantages of First Come First Serve Scheduling

1. Fair and Simple
   Every process is treated equally in terms of arrival order. No process gets unfair priority.
   
2. Easy Implementation
   The algorithm only needs a queue structure, making it simple for operating systems to apply.
   
3. Predictable Execution
   Since execution order is fixed, it is easy to predict the order in which processes will complete.
   

Limitations of First Come First Serve Scheduling

1. Convoy Effect
   If a long process arrives first, it delays all the shorter processes waiting behind it. This leads to increased waiting times.
   
2. Poor Response Time
   In interactive systems where users expect quick responses, FCFS may cause noticeable delays.
   
3. Not Efficient for Mixed Workloads
   When processes have widely different burst times, performance becomes unbalanced. Short processes may suffer while waiting for longer ones to finish.
   

Real-Life Analogy

Imagine standing in line at a coffee shop. If the first person in line orders 10 complicated drinks, everyone behind them has to wait until the barista finishes. Even though some people only want a quick espresso, they cannot be served until the large order is completed. This mirrors how FCFS can sometimes create inefficiencies.

Use Cases of FCFS

Despite its drawbacks, FCFS is still useful in certain scenarios:

• Batch Processing Systems: Where jobs are executed in the order they are submitted without interruption.
  
• Simple Systems: Environments where fairness and predictability matter more than performance.
  
• Non-Interactive Workloads: Where processes do not require immediate responses.
  

Comparison with Other Scheduling Algorithms

• Shortest Job Next (SJN): Focuses on reducing waiting time by running the shortest process first, unlike FCFS which simply follows order.
  
• Round Robin (RR): Uses time slices to prevent long processes from blocking others, whereas FCFS does not allow interruptions.
  
• Priority Scheduling: Executes processes based on priority levels rather than arrival time.
  

When to Choose FCFS

First come first serve scheduling is best suited for systems where:

• All processes have similar burst times.
  
• The workload is not interactive.
  
• Simplicity of implementation is more important than speed optimization.
  

Conclusion

First come first serve scheduling is one of the most fundamental CPU scheduling techniques. Its non-preemptive nature ensures fairness and makes it easy to implement. However, the convoy effect and inefficiency with mixed workloads limit its real-world usage in complex systems.

Even with its limitations, FCFS is important because it introduces the concepts of process scheduling and lays the foundation for understanding more advanced algorithms. While modern operating systems often rely on sophisticated scheduling strategies, FCFS remains a key concept in computer science education and is still applicable in certain controlled environments.