Consider the following CPU processes with arrival times

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Consider the following CPU processes with arrival times (in milliseconds) and length of CPU bursts (in milliseconds) as given below:

If the pre-emptive shortest remaining time first scheduling algorithm is used to schedule the processes, then the average waiting time across all processes is _______ milliseconds.

1. 1
2. 3
3. 5
4. 11

Correct Option: B

So, turn around time is for P₁ ,P₂ ,P₃ ,P₄ are :
P₁ ⇒ (12 - 0) , P₂ ⇒ (6 - 3), P₃ ⇒ (17 - 5) , P₄ ⇒ (8 - 6),
⇒ P₁ = 12 , P₂ = 3 , P₃ = 12 , P₄ = 2
As we know that Waiting time = Turn around time - Burst time
So, waiting time for P₁ ,P₂ ,P₃ ,P₄ are :
P₁ ⇒ (12 - 7) = 5
P₂ ⇒ (3 - 3) = 0
P₃ ⇒ (12 - 5) = 7
P₄ ⇒ (2 - 2) = 0

Hence, avg. waiting time =	P₁ + P₂ + P₃ + P₄	=	5 + 7	= 3.0
	4		4