First-Come, First-Served (FCFS) scheduling is the most basic and straightforward CPU scheduling algorithm. Imagine a single queue at a bank or a grocery store – the first person to join the line is the first one to be served. FCFS operates on the same principle: processes are executed by the CPU in the exact order they arrive in the ready queue. This simplicity makes the First Come First Serve Scheduling Algorithm easy to understand and implement.
What is the First Come First Serve Scheduling Algorithm?
The first come first serve scheduling algorithm is a non-preemptive scheduling algorithm. This means that once a process is allocated to the CPU, it will run until it completes its execution or voluntarily releases the CPU by performing an I/O operation. No external factors, like priority or shorter burst times of other processes, can interrupt a running process under FCFS. It’s purely based on the arrival sequence of processes.
This algorithm is conceptually simple as it mirrors real-world queuing systems. Processes line up, and the CPU attends to them in that order. This approach ensures fairness in a basic sense – every process will eventually get its turn to be executed, preventing any process from being indefinitely postponed if it arrives early enough.
How Does the FCFS Algorithm Work?
The operation of the first come first serve scheduling algorithm is quite simple and follows these steps:
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Arrival: When a process needs to be executed, it enters the ready queue. The ready queue is maintained in a First-In, First-Out (FIFO) manner. Processes are added to the back of the queue as they arrive.
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Execution: The CPU scheduler selects the process at the front of the ready queue. This is the process that has been waiting the longest. The CPU is then allocated to this process.
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Process Run: The selected process runs on the CPU until one of the following occurs:
- Completion: The process finishes its execution and terminates.
- I/O Request: The process needs to perform an Input/Output (I/O) operation. In a simple FCFS implementation for CPU scheduling, the process will typically relinquish the CPU and move to an I/O waiting queue (though in basic FCFS discussion, this I/O aspect is often simplified by assuming processes run to completion).
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Dequeue and Repeat: Once the current process has finished its CPU burst (either by completing or requesting I/O in more complex scenarios), it is removed from the ready queue. The scheduler then checks if there are any more processes in the ready queue. If yes, it repeats from step 2, selecting the next process at the front of the queue. If the ready queue is empty, the CPU may become idle or execute idle processes until a new process arrives.
This cycle continues until all processes have been executed. The key characteristic is the strict adherence to the arrival order, making it predictable and easy to implement.
FCFS Scheduling Examples
To illustrate the first come first serve scheduling algorithm clearly, let’s consider two scenarios: one where all processes arrive at the same time and another where they arrive at different times. We will use Gantt charts to visualize the process execution and calculate important metrics like turnaround time and waiting time.
Scenario 1: Processes Arriving at the Same Time
Assume we have three processes, P1, P2, and P3, all arriving at time 0 with the following burst times:
| Process | Arrival Time | Burst Time |
|---|---|---|
| P1 | 0 | 5 |
| P2 | 0 | 3 |
| P3 | 0 | 8 |
Step-by-Step Execution:
- Since all processes arrive at time 0, the order of arrival is based on their process ID (P1, then P2, then P3).
- P1 starts first at time 0 and runs for its burst time of 5 units. Completion time for P1 is 5.
- P2 starts immediately after P1 finishes, at time 5, and runs for 3 units. Completion time for P2 is 8.
- P3 starts after P2, at time 8, and runs for 8 units. Completion time for P3 is 16.
Gantt Chart:
0 5 8 16
| P1| P2| P3|
----------------Calculating Turnaround Time and Waiting Time:
- Turnaround Time (TAT) = Completion Time – Arrival Time
- Waiting Time (WT) = Turnaround Time – Burst Time
| Process | Arrival Time (AT) | Burst Time (BT) | Completion Time (CT) | Turnaround Time (TAT) | Waiting Time (WT) |
|---|---|---|---|---|---|
| P1 | 0 | 5 | 5 | 5 – 0 = 5 | 5 – 5 = 0 |
| P2 | 0 | 3 | 8 | 8 – 0 = 8 | 8 – 3 = 5 |
| P3 | 0 | 8 | 16 | 16 – 0 = 16 | 16 – 8 = 8 |
- Average Turnaround Time = (5 + 8 + 16) / 3 = 29 / 3 = 9.67 ms
- Average Waiting Time = (0 + 5 + 8) / 3 = 13 / 3 = 4.33 ms
Scenario 2: Processes Arriving at Different Times
Consider another set of three processes with different arrival times and burst times:
| Process | Burst Time (BT) | Arrival Time (AT) |
|---|---|---|
| P1 | 5 | 2 |
| P2 | 3 | 0 |
| P3 | 4 | 4 |
Step-by-Step Execution:
- P2 arrives at time 0 and is the first process to arrive. It starts execution immediately.
- P2 runs for 3 units, from time 0 to 3. Completion time of P2 is 3.
- P1 arrives at time 2. While P2 is running, P1 waits in the ready queue. P1 starts execution after P2 completes, at time 3, and runs for 5 units. Completion time of P1 is 8.
- P3 arrives at time 4. It waits while P1 is running. P3 starts execution after P1 completes, at time 8, and runs for 4 units. Completion time of P3 is 12.
Gantt Chart:
0 3 8 12
| P2| P1| P3 |
------------------Calculating Turnaround Time and Waiting Time:
| Process | Burst Time (BT) | Arrival Time (AT) | Completion Time (CT) | Turnaround Time (TAT) | Waiting Time (WT) |
|---|---|---|---|---|---|
| P2 | 3 | 0 | 3 | 3 – 0 = 3 | 3 – 3 = 0 |
| P1 | 5 | 2 | 8 | 8 – 2 = 6 | 6 – 5 = 1 |
| P3 | 4 | 4 | 12 | 12 – 4 = 8 | 8 – 4 = 4 |
- Average Turnaround Time = (3 + 6 + 8) / 3 = 17 / 3 = 5.67 ms
- Average Waiting Time = (0 + 1 + 4) / 3 = 5 / 3 = 1.67 ms
Alt text: Gantt chart visually representing the First-Come, First-Served (FCFS) CPU scheduling algorithm for three processes (P2, P1, P3) with varying arrival times, showing process execution timeline from time 0 to 12.
Advantages of FCFS Scheduling
The first come first serve scheduling algorithm boasts several advantages, particularly in its simplicity:
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Simplicity: FCFS is exceptionally easy to understand and implement. It requires minimal overhead as it only needs to maintain a ready queue. This simplicity reduces the complexity of the operating system scheduler.
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Fairness (Basic): Every process gets a chance to run in the order of its arrival. This prevents starvation in a basic sense, ensuring that no process is indefinitely denied CPU access if it arrives early.
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No Starvation: As processes are served in the order they arrive, there is no risk of starvation. Each process will eventually be executed, provided that new processes keep arriving.
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Suitable for Batch Systems: FCFS can be adequate for batch processing systems where throughput is more important than response time, and longer turnaround times for individual processes are acceptable.
Disadvantages of FCFS Scheduling
Despite its simplicity, the first come first serve scheduling algorithm has significant drawbacks that make it unsuitable for many modern interactive systems:
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Convoy Effect: This is the most significant disadvantage. If a long-burst process arrives first, it blocks all shorter processes that arrive after it. These shorter processes have to wait for the long process to complete, leading to inefficient CPU and device utilization. This phenomenon is known as the convoy effect.
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High Average Waiting Time: Especially when processes have varying burst times, FCFS can lead to high average waiting times. Short processes arriving after long processes experience considerable delays.
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Not Suitable for Time-Sharing Systems: FCFS is not well-suited for time-sharing operating systems or interactive systems. In these systems, quick response times are crucial. FCFS can result in poor response times because a long-running process can monopolize the CPU, making the system feel sluggish for interactive users.
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Inefficient for Priority Systems: FCFS does not consider process priorities. A high-priority process arriving after a low-priority process will have to wait, which is undesirable in systems that need to prioritize important tasks.
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Poor Performance Metric: FCFS typically results in poor performance in terms of average waiting time and average turnaround time, especially when compared to more sophisticated scheduling algorithms like Shortest Job First (SJF) or Priority Scheduling.
In conclusion, the first come first serve scheduling algorithm is a foundational concept in operating systems, valued for its simplicity and ease of implementation. However, its significant performance limitations, particularly the convoy effect and high waiting times, make it impractical for most modern computing environments, especially those requiring responsiveness and efficiency. It serves as a good starting point for understanding CPU scheduling but is rarely used in its pure form in advanced operating systems.
