--- title: "CPU free percentage with I/O (round robin)" subject: "Embedded System" chapter: 6 key: "semester_6/embedded_system:cpu-free-percentage-rr" source: https://www.bctengineer.com/q/semester_6/embedded_system/cpu-free-percentage-rr site: BCT Engineering --- # CPU free percentage with I/O (round robin) Embedded System · Chapter 6: RTOS - Asked in: 2079 Jestha, 4 marks - Video: [Round Robin CPU Scheduling Algo](https://www.youtube.com/watch?v=Kd_zN3P7KkU) — Easy Learning with Nisha - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/cpu-free-percentage-rr - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-6 ## Question Three processes P1, P2 and P3 arrive at time zero. Their total execution time is 10 ms, 15 ms, and 20 ms respectively. They spent first 20% of their execution time in doing I/O, next 60% in CPU processing and the last 20% again doing I/O. For what percentage of time was the CPU free? Use Round robin algorithm with time quantum 5 ms. ## Short answer With round-robin quantum 5 ms, CPU is idle during 0–2 ms and 29–33 ms, giving 6 ms free out of 33 ms total; CPU free = 18.18 %. ## Answer The CPU is free whenever no process is ready to use it. Each process does I/O first, then CPU work, then I/O again; I/O of different processes is assumed to run in parallel (each has its own device), and only CPU bursts are scheduled by round robin with quantum 5 ms. #### Phase times | Process | Total | First I/O (20%) | CPU (60%) | Last I/O (20%) | |---|---|---|---|---| | P1 | 10 | 2 | 6 | 2 | | P2 | 15 | 3 | 9 | 3 | | P3 | 20 | 4 | 12 | 4 | All three processes arrive at 0 and start with I/O, so they enter the CPU ready queue at 2, 3 and 4 ms respectively. #### Schedule ```text |idle| P1 | P2 | P3 | P1 | P2 | P3 |P3|idle| 0 2 7 12 17 18 22 27 29 33 ``` - 0-2: CPU free (everyone is doing first I/O). - 2-7: P1 runs 5 ms (1 ms of its 6 ms left). P2 and P3 join the queue at 3 and 4. - 7-12: P2 runs 5 ms (4 left). 12-17: P3 runs 5 ms (7 left). - 17-18: P1 finishes its CPU work; it then does last I/O 18-20. - 18-22: P2 finishes its CPU work (4 ms); last I/O 22-25. - 22-27: P3 runs 5 ms (2 left); 27-29: P3 runs the last 2 ms (alone, no other process is ready). - 29-33: P3 does its last I/O; the CPU is free. Everything ends at 33 ms (P1 at 20, P2 at 25, P3 at 33). #### Calculation Total CPU busy time = 6 + 9 + 12 = 27 ms Total time = 33 ms CPU free time = 33 - 27 = 6 ms (0-2 and 29-33) CPU free percentage = 6 / 33 x 100 = 18.18 % **Answer: the CPU is free for about 18.18 % of the time (CPU utilisation 81.82 %).** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How long is P2's first I/O phase? A. 5 ms B. 9 ms C. 3 ms (20% of 15) D. 6 ms Answer: C. 20% of 15 = 3. (Key point: Phase times) 2. What is total CPU busy time? A. 33 ms B. 27 ms (6 + 9 + 12) C. 45 ms D. 20 ms Answer: B. CPU is used for 60% of 10, 15 and 20. (Key point: CPU busy time) 3. When is the CPU free? A. 0-2 ms and 29-33 ms B. 2-7 ms C. Only at 0-2 D. 7-12 ms Answer: A. Initially all do I/O; at the end P3 does its last I/O. (Key point: Idle intervals) 4. CPU free percentage? A. 81.82% B. 20% C. 27% D. 18.18% Answer: D. 6/33 x 100 = 18.18%. 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