FCFS

FCFS (First-Come, First-Served) is a scheduling algorithm that serves tasks in the order they arrive. It is typically non-preemptive: once a task begins execution, it runs until it completes or blocks. It is widely used in simple operating systems and in queueing systems.

In CPU scheduling, FCFS is simple to implement and has low overhead, providing a form of fairness by processing processes in arrival order. However, it can lead to long average waiting times, especially if a long task arrives early and short tasks accumulate behind it (the convoy effect). It does not consider task length or priority.

In networking and disk scheduling, FCFS applies to packet or request queues, delivering service in arrival order. In networks it can produce high latency for time-sensitive traffic; in disks it can cause large seek times, reducing throughput.

Related concepts and alternatives include Shortest Job First (SJF) and Round Robin, which can offer lower or more predictable waiting times at higher complexity. FCFS remains a baseline due to its simplicity and predictability, though variants and extensions exist in theoretical queueing models and real-world systems that combine FCFS with priority or aging policies to mitigate some of its drawbacks.