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An operating system used Shortest Remaining System Time first (SRT) process scheduling algorithm. Consider the arrival times and execution times for the following processes:

 Process Execution Time Arrival time P1 20 0 P2 25 15 P3 10 30 P4 15 45

What is the total waiting time for process P2?

1. 5
2. 15
3. 40
4. 55

 P1 P2 P3 P2 P4

0                20                 30                40                55                70

Waiting time for process P2 = Completion time – Arrival time – burst time = 55 – 15 – 25 = 15

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Ans should be A)..

@Gate Keeda- why process P2 will preempt? At 30 th time both P2 and P3 have same remaining time,  CPU will not switch to P3. it continue executing P2..right?

No.At time 30th both do not have same remaining time. P3 has 10 and P2 has 15. Check that again to make sure.

Could anyone explain why the process is pre-empted ? IS SRT pre-emptive if not mentioned ?

EDIT  : SRTN is pre emptive form of SJF

http://www.moreprocess.com/process-2/shortest-job-first-sjf-shortest-remaining-time-next-srtn-scheduling-algorithm

Process is executed on the basis of their remaining time in execution  ...

For process $P2$ :
\begin{align*} \text{Waiting Time} &= \text{TAT} - \text{BT}\\ &= \left ( \text{CT-AT}\right) - BT \\ &= \left( 55 - 15 \right ) - 25 \\ &= 15 \end{align*}

+1 vote

we can solve using this way