Definition
First Come, First Served (FCFS) is a scheduling algorithm in computing that handles tasks in the exact order they arrive. The process that requests service first is executed first, and the next one waits until it finishes. So what does FCFS mean in practice? It works much like a queue at a supermarket checkout: no priorities, no shortcuts, just strict arrival order. Because it is non-preemptive, once a process starts running it keeps the CPU until it completes.
Phonetic
The phonetic transcription of “First Come, First Served” is: /ˈfɜːrst kʌm, ˈfɜːrst ˈsɜːrvd/
Key Takeaways
- How it works: First Come, First Served (FCFS) serves tasks strictly in the order they arrive in the ready queue. It is the simplest CPU scheduling algorithm and is non-preemptive, meaning a running process is never interrupted to make way for another.
- Pros and cons: FCFS is fair and easy to implement, but it can cause the “convoy effect,” where short processes wait behind one long process, driving up average waiting time. It also ignores task priority.
- Applications: The principle appears in CPU scheduling, disk scheduling, and network packet scheduling, as well as everyday queues at banks, supermarkets, and call centers. It often serves as a baseline that more advanced schedulers improve upon.
Importance
First Come, First Served (FCFS) matters because it represents the foundational scheduling policy that many computing systems and networks build on. By treating every process in arrival order, FCFS keeps scheduling decisions simple, predictable, and free of favoritism. In operating systems and network traffic management alike, this rule reduces complexity and guarantees a baseline level of fairness. The trade-off is the well-known convoy effect: when a long-running process arrives first, shorter processes queued behind it must wait, which can hurt overall resource utilization.
Explanation
FCFS is used to manage processes across operating systems, disk scheduling, network scheduling, and web server request handling. The algorithm queues processes in the order they request service and dispatches them in that same sequence, so the process that asks for resources first receives them first. The core purpose is equal resource sharing and the prevention of starvation, the situation in which a process waits indefinitely for resources. In networking, for example, FCFS can decide the order in which data packets leave a server. The drawback is that a single large request arriving ahead of many small ones can create long waiting times, making FCFS less efficient for workloads with widely varying task sizes. For workloads that need fairer time-sharing, a round robin scheduler is a common alternative.
Examples
1. Customer service lines: In most retail stores, banks, and restaurants, customers are served in the order they arrive, regardless of what they need.
2. Event ticket sales: Many concerts and sporting events sell tickets first-come, first-served. The earliest buyers secure their spots, and once tickets sell out, none remain.
3. Public services and amenities: Clinics, government offices, and shared amenities such as picnic tables or BBQ grills are frequently allocated first-come, first-served. Emergency rooms follow this principle too, though they make exceptions for patients in critical condition.
Frequently Asked Questions (FAQ)
What does FCFS mean in operating systems?
FCFS stands for First Come, First Served. It is a CPU scheduling policy in which the process that arrives first in the ready queue is executed first. Scheduling follows arrival order, with no priorities applied.
What are the drawbacks of First Come, First Served scheduling?
FCFS can cause the convoy effect, where small processes get stuck behind a large one, raising average wait times and slowing overall throughput. Because it is non-preemptive and ignores priority, it is not always the most efficient choice.
Is First Come, First Served the same as FIFO?
In many contexts, yes. FCFS and First In, First Out (FIFO) both mean the first item to arrive is the first to be processed, and the terms are often used interchangeably in computer science.
Related Finance Terms
- Job Scheduler
- Queuing Theory
- Non-preemptive Scheduling
- Concurrency Control
- Process Scheduling