First Come First Served (OS)

operating-systems scheduling

Definition

First Come First Served (FCFS)

First Come First Served (FCFS) is the simplest scheduling strategy, where the processor selects the process that has been in the Ready Queue for the longest time.

Characteristics

• Selection Function: The process with the longest wait time in the Ready Queue is chosen.
• Decision Mode: Non-preemptive. Once a process starts running, it continues until it terminates or blocks for I/O.

Performance and Behaviour

• Bias: FCFS favours long, CPU-intensive processes over short processes or I/O-intensive ones.
• Normalized Turnaround Time: Short processes may suffer high relative delay if they are stuck behind a single long process.
• Utilisation: Can lead to poor resource utilisation (the “convoy effect”), where I/O devices sit idle while processes wait for a long CPU burst to finish.
• Usage: Rarely used as a standalone policy in modern systems but often serves as the underlying mechanism for specific priority levels in more complex schedulers.