FCFS scheduling, step by step

0 / 6

First Come First Served (FCFS) Scheduling

The process that arrives first runs first. Processes line up in the ready queue in the order they arrive, and the one at the front runs first. It is non-preemptive, so once a process starts running, it is not stopped until it finishes.

At a bank: customers are served at the counter in ticket-number order. Even a customer with a quick errand waits until the person ahead is done.

1 / 6

0ms: P1 arrives and runs right away

P1 is the only one in line, so it runs right away without waiting. Its run time is 5ms, so it finishes at 5ms.

2 / 6

1–3ms: P2, P3 and P4 join the line in order of arrival

P2 arrives at 1ms, P3 at 2ms and P4 at 3ms, and each goes to the back of the line. P4, with the shortest run time of 2ms, is also last. FCFS ignores run time and only looks at arrival order.

3 / 6

5ms: P1 finishes and P2 starts running

P2, at the front of the line, runs. P2 waited 4ms, from 1ms to 5ms.

4 / 6

8ms: P2 finishes and P3 starts running

P3 waited 6ms, from 2ms to 8ms. P3 has the longest run time, 8ms.

5 / 6

16ms: P3 finishes and P4 starts running

P4, which needs only 2ms, waited 13ms behind the long P3, from 3ms to 16ms. When a short job lines up behind a long one, it waits this long (the convoy effect).

6 / 6

18ms: all done

Waiting times are P1 0ms, P2 4ms, P3 6ms and P4 13ms, an average of 5.75ms. The other scheduling methods use the same four processes, so you can compare them with this value.

Figure showing FCFS scheduling: four processes line up in the ready queue in the order they arrive, the process at the front of the line runs each time the one before it finishes, and how long each process waited

CPU Scheduling at a glance

Figure showing FCFS scheduling: four processes line up in the ready queue in the order they arrive, the process at the front of the line runs each time the one before it finishes, and how long each process waited