I/O in MLFQ

 

What About I/O in MLFQ?

So far, we have examined how MLFQ handles CPU-bound jobs.
Now let us consider processes that perform frequent I/O, which is common for interactive jobs.

Rule Reminder: Handling Early CPU Release (Rule 4b)

Rule 4b:
If a process gives up the CPU before using up its time allotment, it stays at the same priority level.

Why Is This Rule Important?

The motivation behind Rule 4b is straightforward:

  • Interactive jobs often:

    • Run briefly on the CPU

    • Then block waiting for I/O (keyboard, mouse, network, etc.)

  • Such jobs voluntarily release the CPU

  • They are not CPU-hungry

📌 Penalizing these jobs by lowering their priority would:

  • Increase response time

  • Hurt interactivity

Therefore, MLFQ does not punish a job that frequently blocks for I/O.

Example: Interactive Job vs Batch Job

Consider the following scenario :

Job Characteristics

  • Job A (Black)

    • Long-running

    • CPU-intensive

    • Batch-oriented

    • Consumes its entire time slice

  • Job B (Gray)

    • Interactive

    • Uses the CPU for only 1 ms

    • Then performs an I/O operation

    • Repeats this behavior frequently

What Happens Under MLFQ?

Step-by-Step Behavior

  1. Job B is placed in the highest-priority queue

  2. Job B runs briefly (1 ms)

  3. Job B blocks for I/O before using up its time allotment

  4. According to Rule 4b:

    • Job B remains at the same priority level

  5. When I/O completes:

    • Job B again runs immediately

  6. Job A continues running only when Job B is blocked



Resulting Behavior

  • Job B stays at the highest priority

  • Job B receives fast CPU access

  • Job A runs in the background at lower priority

  • System responsiveness remains high

Why This Works Well

MLFQ correctly identifies Job B as:

  • Interactive

  • Latency-sensitive

  • I/O-bound

By observing that Job B does not consume full time slices, MLFQ:

  • Avoids demoting it

  • Preserves excellent response time

Key Insight

MLFQ uses I/O behavior as feedback.
Jobs that frequently block are treated as interactive and prioritized accordingly.

Summary

  • Interactive jobs often perform frequent I/O

  • They give up the CPU early

  • MLFQ keeps them at high priority

  • This improves responsiveness without harming fairness

Final Takeaway 

In MLFQ, using less CPU is rewarded—not punished.
Jobs that block early are recognized as interactive and scheduled quickly.

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