Aptitude | GATE CS 1998 | Question 52

Consider n processes sharing the CPU in a round-robin fashion. Assuming that each process switch takes s seconds, what must be the quantum size q such that the overhead resulting from process switching is minimized but, at the same time, each process is guaranteed to get its turn at the CPU at least every t seconds ?

aq

 
(A) a
(B) b
(C) c
(D) d


Answer: (A)

Explanation: Each process will get CPU for q seconds and each process wants CPU again after t seconds.
Thus, there will be (n-1) processes once after current process gets CPU again. Each process takes s seconds for context switch.



(Qp1)(s)(Qp2)(s)(Qp3)(s)(Qp1) 

It can be seen that since P1 left and arrived again, there have been n context switches and (n-1) processes. Thus, equation will be:

q*(n-1) + n*s <= t
q*(n-1) <= t - n*s
q <= (t-n.s) / (n-1) 

So, option (A) is correct.

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