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.
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The same three processes under each rule. Watch how the finish times move.
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First come, first served
Round robin
Priority scheduling
Multilevel queue
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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.
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