The big idea: A processor core can run one process at a time.
Scheduling is the rule the OS uses to decide which process runs next, and for how long. Switching fast enough makes them all look simultaneous.
What the rule is trying to balance: Finish everything quickly, keep the machine feeling responsive, and make sure nothing waits forever.
No single rule is best at all three, which is why there is more than one.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
The same three processes under each rule. Watch how the finish times move.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
First come, first served
Each process runs to completion in the order it arrived. Nothing interrupts it.
Good — the simplest rule there is, and nothing is ever skipped.
Bad — one long process at the front holds up everything behind it, however short those are.
Round robin
Each process gets a fixed time slice, then goes to the back of the queue.
Good — nobody waits long for a first turn, which is what makes a machine feel responsive.
Bad — every switch costs time, so too short a slice wastes the CPU on switching.
Priority scheduling
Each process carries a priority, and the highest one waiting runs next.
Good — urgent work goes first, which is essential in real-time systems.
Bad — a low-priority process can be pushed back indefinitely. That is called starvation.
Multilevel queue
Processes are sorted into separate queues by what they are — interactive in one, background batch work in another — and each queue gets its own rule and share of CPU time.
Good — interactive work stays responsive while batch work still progresses.
Bad — more to configure, and a badly set share can starve the lower queue.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
Match the rule to the machine: The right choice depends on what the machine is for.
A desktop must feel responsive. A control system must never miss a deadline. A batch server just needs everything finished.
Needs responsiveness
- A desktop, phone or anything with a person waiting
- Round robin or a multilevel queue
- A first turn quickly matters more than finishing first
- Switching overhead is a price worth paying
Needs deadlines met
- Control systems, medical equipment, industrial machinery
- Priority scheduling
- Some work must run before anything else, always
- Starvation must be handled separately, or low-priority work never runs
Starvation, and the usual fix: Under priority scheduling, a steady supply of urgent work means a low-priority process may never run at all.
The usual answer is ageing: raise a process's priority the longer it has waited, so everything eventually reaches the front.
How this is tested — you must compare approaches by their EFFECT on the processes, not just define them. It comes up two ways:
Paper 1 Section A
- Compare two scheduling approaches, 3-6 marks
- State an advantage and a disadvantage of one
- Choose an approach for a described system
Paper 1 Section B — case study
- Recommend scheduling for the case study system
- Justify it from what that system is used for
The classic trap: Defining both approaches and stopping. A comparison needs a sentence that mentions both: "round robin gives a short process an early turn, whereas first come first served makes it wait for everything ahead of it".
A hospital monitoring system runs patient alarm checks alongside routine record-keeping. Compare priority scheduling and round robin for this system, and recommend one.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.