---
title: "Scheduling: Deciding Which Process Runs Next"
description: "A scheduler chooses from the ready processes, and every rule it could use favours something: throughput, response time, or fairness. Round robin, shortest job first and priority with ageing each make "
canonical: https://lightmysky.com/learn/computing/scheduling-deciding-which-process-runs-next-mt_QZYzfVIcP6
source: https://lightmysky.com/learn/computing/scheduling-deciding-which-process-runs-next-mt_QZYzfVIcP6.md
retrieved: 2026-09-12
---

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# Scheduling: Deciding Which Process Runs Next

A scheduler chooses from the ready processes, and every rule it could use favours something: throughput, response time, or fairness. Round robin, shortest job first and priority with ageing each make that trade differently.

Subject: Computing · Area: Computer Systems · Ages 19 to 21
Page: https://lightmysky.com/learn/computing/scheduling-deciding-which-process-runs-next-mt_QZYzfVIcP6

## Ready when they can

- Work out average waiting time for a job set under two different policies
- Show how a long-running job starves under strict priority, and how ageing fixes it
- Say which policy suits an interactive machine and which suits a batch one

## Lesson: Picking who runs next, and why

Four jobs arrive together needing 8, 6, 4, and 2 minutes. First-come order 8, 6, 4, 2 waits 0, 8, 14, 18, averaging 10. Shortest-first order 2, 4, 6, 8 waits 0, 2, 6, 12, averaging 5. Order alone halves the wait.

Round robin instead deals short turns to each job in turn. Nobody waits long for a first slice, so typing stays snappy, though long jobs finish later than under shortest-first. Shortest-first minimizes average wait but needs known bursts.

**Example.** Under strict priority a long low job can wait forever while short high jobs keep arriving. Ageing fixes this by raising a waiter's rank the longer it queues, so every job finally reaches the front.

**Tip.** Match the rule to the room. Favor round robin for interactive machines where response matters, and shortest-first for batch rooms where average wait matters.

**Recap.** Shortest-first shrinks average wait, round robin shrinks response, and ageing ends starvation.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Queues: First In, First Out](https://lightmysky.com/learn/computing/queues-first-in-first-out-mt_4U1bT0JW4g)
- [Processes, Context Switches and What They Cost](https://lightmysky.com/learn/computing/processes-context-switches-and-what-they-cost-mt_oiOwHKUoXG)

## Opens up

- [Virtual Memory and Address Translation](https://lightmysky.com/learn/computing/virtual-memory-and-address-translation-mt_B-PiAyderj)
- [Observability and Tail Latency Across Services](https://lightmysky.com/learn/computing/observability-and-tail-latency-across-services-mt_NT5tmumdfA)