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An OS contains 10 identical processes that were initiated at the same time.Each process contains 15 identical requests. Each request consume 20 msec of CPU time.A request is followed by an I/O operation which consumes 10 msec.The CPU scheduling overhead is 2 msec. The system uses Round Robin scheduling with the time quantum of 10 msec.

Q1) What is the response time of 1st request of last process ? A) 210 msec B) 140 msec c) 230 msec D) 240 msec

Q2) The subsequent request of the processes receives a response times of A) 110 msec B) 220 msec C) 230 msec D) 240 msec

Ans: Q1) D

 Q2) C

What I thought :

For 1 process, there are 15 request so 15 * ( 20 + 10) = 450 msec But all answers are so small than this approach. So no need to think about further i.e. CPU overhead then 2nd...3rd...processes.

Here my problem is I didn't get the concept behind this question properly. CPU overhead ( i.e context switching ) will take place between 10 processes or each process's 15 request. So please tell how this scenario will work.

I didn't get the meaning of 2nd question.

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Hint : Try to understand more about context switching. Context switching takes place between 10 processes. (And you can forget about those 15 requests, instead consider each process needs 15 * 20 msec of CPU time)

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  • $\begingroup$ Sorry dude but can you explain your view ? $\endgroup$ – user1745866 Nov 13 '13 at 20:19

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