In the realm of operating systems, efficient CPU scheduling is paramount to ensure smooth multitasking and optimal resource utilization. Among the various CPU scheduling algorithms, First-Come, First-Served (FCFS) stands out as the simplest and most intuitive. This article delves into the intricacies of FCFS scheduling, providing a comprehensive understanding of its workings, advantages, and disadvantages in modern computing.
What is First-Come, First-Served (FCFS) Scheduling?
First-Come, First-Served (FCFS) scheduling is a non-preemptive CPU scheduling algorithm where processes are executed in the exact order they arrive in the ready queue. Imagine it as a queue at a bank or a grocery store – the first person to join the line is the first one to be served. Similarly, in FCFS, the process that requests the CPU first is allocated the CPU first.
FCFS is categorized as a non-preemptive algorithm. This means that once a process begins its execution, it runs uninterrupted until it completes its task or voluntarily releases the CPU, typically for I/O operations. No external mechanism can halt a running process to give way to another process in the queue.
How FCFS Scheduling Works: A Step-by-Step Breakdown
The operation of FCFS scheduling is straightforward and follows these simple steps:
- Arrival: When processes enter the system, they are placed into a ready queue. This queue maintains the order of arrival – the process that arrives earliest is positioned at the front of the queue, while later arrivals are added to the back.
- Execution: The CPU scheduler selects the process at the head of the ready queue. This process is then allocated the CPU and begins execution.
- Completion or Relinquishment: The selected process continues to run until it either completes its execution cycle or needs to perform an I/O operation. In either case, the process voluntarily relinquishes the CPU.
- Repeat: Once the current process finishes or releases the CPU, the scheduler moves to the next process at the front of the ready queue and repeats the execution process.
This cycle continues until the ready queue is empty, indicating that all processes have been served.
Understanding FCFS with Practical Examples
To solidify your understanding of First-Come, First-Served (FCFS) CPU scheduling, let’s examine two illustrative scenarios. These examples will showcase how FCFS operates in situations with varying process arrival times.
Scenario 1: Processes Arriving at the Same Time
Consider a scenario where three processes – P1, P2, and P3 – all arrive at time 0. Their burst times (the CPU time required for execution) are as follows:
| Process | Arrival Time | Burst Time |
|---|---|---|
| P1 | 0 | 5 |
| P2 | 0 | 3 |
| P3 | 0 | 8 |
Step-by-Step Execution:
- P1 starts first: Since all processes arrive at the same time, and we assume they are enqueued in the order P1, P2, P3, process P1 is picked first. It runs for its burst time of 5 units, from time 0 to 5.
- P2 executes next: After P1 completes, P2 is next in the queue. It runs for 3 units, from time 5 to 8.
- P3 runs last: Finally, P3 gets its turn and executes for 8 units, from time 8 to 16.
Gantt Chart Representation:
| P1 | P2 | P3 |
0 5 8 16Calculating Turnaround Time and Waiting Time:
To evaluate the performance, we calculate the turnaround time (TAT) and waiting time (WT) for each process.
- Turnaround Time (TAT): The total time taken from process arrival to completion.
TAT = Completion Time - Arrival Time - Waiting Time (WT): The time a process spends waiting in the ready queue before execution.
WT = Turnaround Time - Burst Time
| Processes | AT | BT | CT | TAT | WT |
|---|---|---|---|---|---|
| P1 | 0 | 5 | 5 | 5 | 0 |
| P2 | 0 | 3 | 8 | 8 | 5 |
| P3 | 0 | 8 | 16 | 16 | 8 |
- Average Turnaround Time: (5 + 8 + 16) / 3 = 9.67 ms
- Average Waiting Time: (0 + 5 + 8) / 3 = 4.33 ms
Alt text: Gantt chart illustrating FCFS CPU scheduling for three processes (P1, P2, P3) arriving at the same time, showing their execution order and completion times.
Scenario 2: Processes Arriving at Different Times
Now, let’s consider a more realistic scenario where processes arrive at different times:
| Process | Burst Time (BT) | Arrival Time (AT) |
|---|---|---|
| P1 | 5 | 2 |
| P2 | 3 | 0 |
| P3 | 4 | 4 |
Step-by-Step Execution:
- P2 starts at time 0: P2 arrives first at time 0 and starts execution immediately. It runs for 3 units, completing at time 3. Completion Time of P2 = 0 + 3 = 3.
- P1 starts after P2: P1 arrives at time 2, but the CPU is busy with P2. P1 has to wait until P2 finishes. P1 starts execution at time 3 and runs for 5 units, completing at time 8. Completion Time of P1 = 3 + 5 = 8.
- P3 starts after P1: P3 arrives at time 4, but again, it has to wait for the currently running process, P1, to complete. P3 starts at time 8 and runs for 4 units, completing at time 12. Completion Time of P3 = 8 + 4 = 12.
Gantt Chart Representation:
| P2 | P1 | P3 |
0 3 8 12Calculating Turnaround Time and Waiting Time:
| Process | CT | TAT (CT – AT) | WT (TAT – BT) |
|---|---|---|---|
| P2 | 3 | 3 | 0 |
| P1 | 8 | 6 | 1 |
| P3 | 12 | 8 | 4 |
- Average Turnaround Time: (3 + 6 + 8) / 3 = 5.67 ms
- Average Waiting Time: (0 + 1 + 4) / 3 = 1.67 ms
Alt text: Gantt chart depicting FCFS CPU scheduling for three processes (P1, P2, P3) with different arrival times, illustrating the waiting and execution periods based on arrival order.
Advantages of FCFS Scheduling
FCFS scheduling, despite its simplicity, offers several notable advantages:
- Simplicity: FCFS is exceptionally easy to understand and implement. Its logic is straightforward, making it a basic yet functional scheduling algorithm.
- Fairness: Every process gets a fair chance to be executed in the order of its arrival. This eliminates the possibility of any process being unfairly prioritized or perpetually delayed in favor of others.
- Ease of Implementation: FCFS requires minimal overhead and doesn’t necessitate complex data structures for its implementation. A simple queue is sufficient to manage the ready processes.
- No Starvation: Due to its inherent fairness, FCFS prevents process starvation. Every process is guaranteed to eventually get its turn to execute, as long as it arrives in the ready queue.
- Suitable for Batch Systems: FCFS can be effective in batch processing systems where the throughput is more important than response time, and longer processing times for individual jobs are acceptable.
Disadvantages of FCFS Scheduling
Despite its benefits, FCFS scheduling also has significant drawbacks that limit its suitability for many modern operating systems:
- Convoy Effect: FCFS is susceptible to the “convoy effect”. This phenomenon occurs when a long-burst process arrives before shorter processes. The shorter processes get stuck behind the long process, leading to increased average waiting times and lower CPU utilization.
- High Average Waiting Time: In scenarios where processes have varying burst times, FCFS can result in significantly higher average waiting times compared to more sophisticated scheduling algorithms. This is especially pronounced when long processes precede short ones.
- Not Ideal for Time-Sharing Systems: FCFS is not well-suited for interactive, time-sharing operating systems. Users expect quick responses, and the potentially long waiting times in FCFS can lead to poor user experience.
- Poor Performance with Mixed Bursts: Systems with a mix of long and short burst processes perform poorly under FCFS. Short jobs may experience unacceptable delays if they arrive after long jobs.
- Unpredictable Response Times: The response time in FCFS can be highly variable and unpredictable, especially if the process queue contains processes with widely differing burst times.
Conclusion
First-Come, First-Served (FCFS) CPU scheduling is a foundational algorithm in operating system concepts. Its simplicity and inherent fairness make it easy to grasp and implement. However, its non-preemptive nature and susceptibility to the convoy effect result in significant limitations, particularly in scenarios demanding low latency and responsiveness. While FCFS may find application in specific niche areas like batch processing, more advanced scheduling algorithms are generally preferred for modern, interactive computing environments to overcome the shortcomings of FCFS and provide better overall system performance.
