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GATE CSE 2022 Q42: Round robin burst times inconsistent with given context switches

Question

Consider four processes P, Q, R, and S scheduled on a CPU as per round robin algorithm with a time quantum of 4 units. The processes arrive in the order P, Q, R, S, all at time t = 0. There is exactly one context switch from S to Q, exactly one context switch from R to Q, and exactly two context switches from Q to R. There is no context switch from S to P. Switching to a ready process after the termination of another process is also considered a context switch. Which one of the following is NOT possible as CPU burst time (in time units) of these processes?

Try it: multiple choice: exactly one correct option

Select one option
Wrong answer costs 2/3 mark in GATE.
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Answer: D (official GATE 2022 final answer key)

Concept used: Simulate round robin for each option and list the sequence of context switches.

First step: For each option draw the Gantt chart with quantum 4 and check for exactly one R→Q switch, one S→Q switch and two Q→R switches.

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Source: official GATE 2022 CS question paper and final answer key, organised by IIT Kharagpur. Question paper · Answer key

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