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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
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.
Shortest-first shrinks average wait, round robin shrinks response, and ageing ends starvation.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.