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GO Classes DPP | GATE CS | Operating System | Feedback Queue Scheduling

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1 1 vote

A system uses two feedback queues:

  • $\text{Q1}$: Round Robin with time quantum $10$ ms
     
  • $\text{Q2}$: Shortest Job First
     
  • $\text{Q1}$ has higher priority than $\text{Q2}$
     
  • New processes enter $\text{Q1}$
     
  • A process that does not finish after one quantum in $\text{Q1}$ moves to $\text{Q2}$

Initially both queues are empty.

Processes $\text{P1}$ and $\text{P2}$ are created in that order.

Their CPU burst times are:

  • $\text{P1}$: $30$ ms
     
  • $\text{P2}$: $20$ ms

What is their average waiting time?

  1. $25$ ms
     
  2. $20$ ms
     
  3. $15$ ms
     
  4. $10$ ms

1 Answer

1 1 vote

Both first enter $\text{Q1}$.

Schedule:

$0 - 10: \text{P1}$

$\text{P1}$ has $20$ ms remaining and moves to $\text{Q2}$.

$10 - 20: \text{P2}$

$\text{P2}$ has $10$ ms remaining and moves to $\text{Q2}$.

Now $\text{Q1}$ is empty.

$\text{Q2}$ uses SJF:

  • $\text{P2}$ remaining $= 10$ ms

  • $\text{P1}$ remaining $= 20$ ms

So,

$20 - 30: \text{P2}$

$30 - 50: \text{P1}$

Waiting time:

  • $\text{P1}$ waits from $10$ to $30 = 20$ ms

  • $\text{P2}$ waits from $0$ to $10 = 10$ ms


Average $:(20 + 10) / 2 = 15$ ms


Answer : C

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