In CPU scheduling, what does the FIFO (First-In, First-Out) / FCFS policy run first?

Introduction / Context:
First-Come, First-Served (FCFS), also called FIFO, is one of the simplest CPU scheduling algorithms. It orders processes strictly by arrival time, providing a baseline for understanding more advanced schedulers like SJF, priority, and round-robin.

Given Data / Assumptions:

• Processes are queued as they arrive.
• Preemption is not used in basic FCFS.
• Service begins with the head of the queue.

Concept / Approach:

FCFS services requests in arrival order. The first job to enter the ready queue is the first to be executed. While simple, FCFS can lead to the convoy effect, where short jobs wait behind long ones, increasing average waiting time compared to SJF.

Step-by-Step Solution:

Verification / Alternative check:

Scheduling theory and OS curricula consistently describe FCFS as non-preemptive, arrival-order servicing.

Why Other Options Are Wrong:

• last entered: LIFO, not FCFS.
• longest in queue: equivalent to first entered, but the phrasing in option c is ambiguous and not canonical; option b is precise.
• least processor needs: describes SJF or Shortest Remaining Time First, not FCFS.

Common Pitfalls:

• Confusing FCFS with priority-based or length-based scheduling.

Final Answer:

executes first the job that first entered the queue.