Consider n processes sharing the CPU in a round-robin fashion. Assuming that…
1998
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?
- A.
q <= (t - n*s)/(n - 1)
- B.
q >= (t - n*s)/(n - 1)
- C.
q <= (t - n*s)/(n + 1)
- D.
q >= (t - n*s)/(n + 1)
Attempted by 30 students.
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Correct answer: A
To reduce overhead, the time quantum q should be as large as possible, but it must still satisfy the response-time guarantee. After one process gets the CPU, the other n - 1 processes may each run for q seconds before it gets the CPU again. During this interval there are n process switches, each costing s seconds. Therefore the maximum gap is (n - 1)q + n*s. The guarantee requires (n - 1)q + n*s <= t. Solving gives q <= (t - n*s)/(n - 1). Hence, option A is correct.