first-come, first-served scheduling

This is the fairness of an ordinary queue at a bank: whoever gets in line first is served first, all the way through, no jumping ahead. First-come, first-served (FCFS) scheduling does exactly this for processes — the ready queue is a simple FIFO list, and the CPU is given to processes strictly in the order they became ready.

How it works: when a process becomes ready it joins the tail of the queue; the scheduler always picks the process at the head and lets it run to the end of its CPU burst before moving on. FCFS is non-preemptive — once a process starts, it keeps the CPU until it blocks or finishes. It is the simplest scheduler imaginable: trivial to implement, and obviously free of starvation, since every job eventually reaches the front.

Its flaw is serious, though. Because a job runs to completion once started, a long job at the front makes every short job behind it wait the full long duration — this is the convoy effect, and it can wreck average waiting time and response time. FCFS also behaves badly for interactive use, since a CPU-bound job can hold the processor while quick I/O-bound jobs pile up behind it. It is rarely used alone in general-purpose systems, but it often serves as the tie-breaking rule inside more sophisticated schedulers.