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First Come First Served Scheduling

Runs tasks in the order they arrive.

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Quick Facts

Compact metadata for implementation and discovery.

Category: Scheduling
Difficulty: Beginner
Family: Queue Scheduling
Tier: Extended
Status: Verified
Review: Source Checked
Sources: 1
Formulas: 0
Relationships: 1
Code example: Ready
Visualization: Ready
Aliases: FCFS

Overview

First Come First Served Scheduling is a simple non-preemptive policy that processes the oldest ready task first.

Core Concepts

task orderingqueue frontierprocess schedulingqueue schedulingready queuetask priorityresource allocation

Quick facts

Complexity

Runtime and memory behavior at a glance.

Best

O(n)

Average

O(n log n)

Worst

O(n log n)

Space

O(n)

When to Use

Use First Come First Served Scheduling when you need to keep tasks in arrival order.
Use it for batch queues with inputs like tasks with arrival times.
Use it when tasks, priorities, deadlines, or resource limits drive execution order.
Use it when the Queue Scheduling tradeoff is acceptable for the problem size.

When Not to Use

Avoid it when fairness, latency, and throughput requirements conflict but are not prioritized.
Avoid it when the required output is not execution order.

What It Does

Keeps tasks in arrival order
Runs each task until completion

Use Cases

Batch queues
Simple service systems
Scheduling education

Inputs

Tasks with arrival times

Outputs

Execution order
Waiting and completion times

Pseudocode

1.Sort or receive tasks by arrival time
2.Place ready tasks in a queue
3.Run the front task until it finishes
4.Remove it and run the next task
5.Repeat until all tasks finish

Canonical Example

Input: tasks = A(4ms), B(2ms), C(1ms)
Output: execution order and completion times
Explanation: First Come First Served Scheduling: The scheduler chooses which task should run next according to its rule.

Implementation Notes

Keep the input contract explicit: Tasks with arrival times.
Use this step as the implementation spine: Sort or receive tasks by arrival time.
Then preserve the next state change: Place ready tasks in a queue.
Record arrival time, remaining work, and completion time so schedule metrics can be checked.

Common Pitfalls

FCFS is fair by arrival order but can suffer from long tasks blocking short tasks.
Ignoring starvation, preemption cost, or inaccurate duration estimates.
Returning execution order without handling empty, invalid, or boundary inputs.

Interactive Mechanism Walkthrough

Graph Frontier & Edge Relaxation

Show tasks entering and leaving a queue in arrival order.

Step 1 of 5: Sort or receive tasks by arrival time

arrival-queue

Notes

First Come First Served Scheduling operates in O(n log n) time and O(n) space within the Queue Scheduling family. Often compared with Shortest Job First and Round Robin Scheduling.

Sources (1)

What each reference supports for this entry.

Operating System Concepts

Book

Operating-systems reference for first-come-first-served scheduling and FIFO ready-queue dispatch.

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Alternative toShortest Job First

Scheduling · Core

First-come-first-served preserves arrival order, while shortest-job-first reorders by estimated duration.

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