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MLQ & MLFQ (Multi-Level Queue & Multi-Level Feedback Queue)

Multi-level queue scheduling (MLQ)

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  • Ready queue is divided into multiple queues depending upon priority.
  • A process is permanently assigned to one of the queues (inflexible) based on some property of process, memory, size, process priority or process type.
  • Each queue has its own scheduling algorithm. E.g., SP -> RR, IP -> RR & BP -> FCFS.
    • (SP: System process, IP: Interactive process, BP: Batch process)
    • (RR: Round Robin, FCFS: First Come First Serve)
more info
  • System process: Created by OS (Highest priority)
  • Interactive process (Foreground process): Needs user input (I/O).
  • Batch process (Background process): Runs silently, no user input required
  • Scheduling among different sub-queues is implemented as fixed priority preemptive scheduling. E.g., foreground queue has absolute priority over background queue.
  • If an interactive process comes & batch process is currently executing. Then, batch process will be preempted.

Problems with multi-level queue scheduling:

  • Only after completion of all the processes from the top-level ready queue, the further level ready queues will be scheduled.
  • Starvation for lower priority process.
  • Convoy effect is present.

Multi-level feedback queue scheduling (MLFQ) 🥷🏻

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  • The one which is actually used in practice in most of the systems.

  • Multiple sub-queues are present.

what's new?

  • Process can move between queues.
  • If a process uses too much CPU time, it will be moved to lower priority queue (to account for other processes).
  • If a process uses too much I/O, it will be moved to higher priority queue. (so that user can interact with it, else he will feel like system is hanging).
  • Aging is used to prevent starvation. (Aging: If a process waits too much in a lower-priority queue, it may be moved to a higher priority queue).

Advantages of MLFQ:

  • Less starvation then MLQ. (doesn't mean, it's completely free from starvation).
  • It is flexible.
  • Can be configured to match a specific system design requirement.

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