Round robin scheduling algorithm

Question

What happens to average Turn Around Time if time quantum increased?

Comments

Increasing time quantam for one process may affect completion time of other processes as waiting time of other processes may increase.

We cannot conclude that increasing time quantam decreases turnaround time.

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waiting time also increase

Answer 1

Turn around time (TAT) = Completion Time - Arrival Time

Round Robin = Process Burst Time / Time Quantum = Number of times a process is scheduled.

Which means, more the Time Quantum, lesser number of times the process would be scheduled

and less the Time Quantum, more number of times the process would be scheduled.

More the number of times the process is scheduled, more will be its Completion Time.

Keeping Arrival Time constant, more the Completion Time, more will be the Turn Around Time (TAT)

So, if a time Quantum is increased, 

• Number of times a process is scheduled will increase
• So, it Completion Time will increase
• Thus its Turn Around Time would increase

** Please note: Keeping Arrival Time constant

Comments

final conclusion is wrong..
it should be:
more the Time Quantum, lesser number of times the process would be scheduled
and hence completion time will decrease resulting in decrement of TAT.
therefore avg. TAT will decrease on inc of time quantum

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Yes, I agree the final conclusion is wrong. To quote "too_simple"

it should be:
more the Time Quantum, lesser number of times the process would be scheduled
and hence completion time will decrease resulting in decrement of TAT.
therefore avg. TAT will decrease on inc of time quantum

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Well, your conclusion is also wrong.

On increasing the time quantum, TAT may increase or may decrease.

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Well now I come to think of it,

The number of times the process wil scheduled will either remain same or it will decrease, it will NEVER increase. However, the turn around time may increase of decrease given the scheduling pattern.

Case where it might increase

Initially time quanta 2

P1 needs 7 time quanta to finish and P2 and P3 needs 10. All arrives at 0

P1(2) P2(2) P3(2), P1(2) P2(2) P3(2), P1(2) P2(2) P3(2), P1(1), P2(2), P3(2), P2(2) P3(2)

TAT for P1= 19, P2 = 25, P3 = 27

Avg TAT = 23.6667

Increase the time quanta to 3

P1(3) P2(3) P3(3), P1(3) P2(3) P3(3), P1(1) P2(3) P3(3), P2(1), P3(1)

TAT for P1: 19, P2 = 26, P3 = 27

Avg TAT = 24

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Exactly !

Answer 2

Rewriting my comment as answer:

If we increase the time quantum alloted to each process, the number of times the process will be scheduled will either remain same or it will decrease, it will NEVER increase. However, the turn around time may increase of decrease given the scheduling pattern.

Its easy to see where this will decrease. Case where it will increase:
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Initially time quanta 2

P1 needs 7 time quanta to finish and P2 and P3 needs 10. All arrives at 0

P1(2) P2(2) P3(2), P1(2) P2(2) P3(2), P1(2) P2(2) P3(2), P1(1), P2(2), P3(2), P2(2) P3(2)

TAT for P1= 19, P2 = 25, P3 = 27

Avg TAT = 23.6667
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Increase the time quanta to 3

P1(3) P2(3) P3(3), P1(3) P2(3) P3(3), P1(1) P2(3) P3(3), P2(1), P3(1)

TAT for P1: 19, P2 = 26, P3 = 27

Avg TAT = 24

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