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OCR H446 1.2.1 MLFQ and scheduler selection

Part 7 of 12 · H446 1.2.1 · Systems software

Multi-level feedback queues complete the five scheduling algorithms named in H446 1.2.1, and this worksheet uses them to pull the whole set together: several ready queues with different quantums, and processes moving between them according to observed behaviour. The second half is pure selection under stated constraints.

Students will:

  • describe how processes move between the queues of a multi-level feedback queue
  • explain why a process that repeatedly uses its whole quantum may be moved down
  • set out the decision rule and pre-emption behaviour of all five named algorithms
  • recommend and justify an algorithm for print, interactive and short-request workloads
  • assess the intended benefit and the possible cost of short high-priority quantums

Inside: 5 explanation cells, 4 multiple-choice questions, 3 fill-in-the-blanks cells and 4 written answers. 38 marks, about 45 to 55 minutes.

Series: H446 1.2.1 · Systems software, part 7 of 12.

Shared by Coding PathwayVerified teacher

  • 16 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

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Multi-level feedback queues and scheduler selection

No single scheduling rule is best for every workload. A multi-level feedback queue (MLFQ) uses several ready queues and changes how processes are treated according to priority or observed behaviour.

This worksheet also consolidates selection among all five OCR algorithms.

Worked model: more than one queue

Multi-level feedback queueseveral queues, different treatment, feedback between levelshigh-priority queueshort quantum, responsive jobsmiddle queuemedium quantumlower-priority queuelonger CPU-bound workuses full quantumuses full quantumpriority may risePolicies vary. Secure: multiple queues, different priorities or quantums,and movement between queues based on process behaviour.

An MLFQ has multiple queues. The queues may have different priorities, time quantums or scheduling approaches. A new or interactive process may begin in a high-priority queue. A process that repeatedly uses its full quantum may move down; a waiting process may later be promoted to reduce starvation.

Exact implementation policies vary. For OCR, secure the principles: several queues, different treatment, and movement based on behaviour or priority.

Multiple choice1 mark

Which description is essential to an MLFQ?

  • ASeveral queues with differing priorities or scheduling treatment
  • BOne queue in strict arrival order
  • CNo process may ever change position
  • DEvery process must have the same remaining time
Fill in the blanks4 marks
An MLFQ places processes into different entry 1. Each level may have a different entry 2, time quantum or scheduling approach. A process can move between levels according to its entry 3 or changing priority. This entry 4 is the feedback in the name.
Multiple choice1 mark

Why might a CPU-bound process that repeatedly uses its entire quantum move to a lower-priority queue?

  • ABecause it has finished
  • BTo reserve faster response opportunities for short or interactive work
  • CTo convert it into a device driver
  • DBecause MLFQ contains only one queue
Written answer3 marks

Describe how a multi-level feedback queue works.

Include the three principles highlighted in the model.

Students type their answer here.

Five algorithms, five decision rules

AlgorithmMain decisionPre-emption in the OCR model
FCFSearliest arrivalno
Round robinrotate after a fixed quantumyes
SJFshortest estimated complete jobno
SRTshortest time still requiredyes
MLFQqueue level and that level's policymay use pre-emption

Selection questions are not asking for a universally 'best' scheduler. Identify the workload priority, apply the algorithm's mechanism and explain the consequence.

Fill in the blanks5 marks
A print queue with equal priority commonly suits entry 1. Regular time slices identify entry 2. A non-pre-emptive length comparison identifies entry 3, while a new shorter job can displace the current process under entry 4. Several priority levels with feedback identify entry 5.
Multiple choice1 mark

A desktop must keep several interactive applications responsive without estimating their completion times. Which simple algorithm is most suitable?

  • AShortest job first
  • BFirst come first served
  • CRound robin
  • DShortest remaining time
Written answer9 marks

Recommend and justify a scheduling algorithm for each case: (i) equal-priority print jobs; (ii) interactive processes needing regular response; (iii) a service that should interrupt long work when a shorter request arrives.

Use a different named algorithm where appropriate and link mechanism to consequence.

Students type their answer here.

Multiple choice1 mark

A student writes, ‘MLFQ puts every process into one queue, then runs them in arrival order.’ What is the central error?

  • AIt uses the word process
  • BArrival order can never appear in any scheduler
  • CMLFQ cannot use priorities
  • DIt omits the multiple queues and feedback movement
Written answer4 marks

An MLFQ gives high-priority queues short quantums and lower queues longer quantums. Explain one intended benefit and one possible cost of this design.

Develop a consequence for both interactive and CPU-bound work.

Students type their answer here.

Written answer5 marks

A research server runs both short user queries and long simulations. Assess whether MLFQ is more suitable than FCFS.

Compare both mechanisms, apply them to both workload types and reach a justified judgement.

Students type their answer here.

Closed-book checkpoint

Retrieve the selection logic rather than repeating the earlier algorithm names.

Fill in the blanks4 marks
A scheduling choice should begin with the workload's main entry 1. Time slicing supports regular responsiveness, arrival order supports a simple entry 2, and a remaining-time comparison supports rapid service for newly arriving entry 3 jobs. A feedback design adapts treatment as process characteristics entry 4.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.