--- title: "HRRN and RR scheduling (q=4)" subject: "Operating System" chapter: 2 key: "semester_6/operating_system:hrrn-rr-q4-awt-att" source: https://www.bctengineer.com/q/semester_6/operating_system/hrrn-rr-q4-awt-att site: BCT Engineering --- # HRRN and RR scheduling (q=4) Operating System · Chapter 2: Process Management - Asked in: 2072 Asoj, 5 marks - Video: [Highest Response Ratio Next Scheduling Algorithm | HRRN Scheduling Algorithm | Operating Systems](https://www.youtube.com/watch?v=xvRvdQ-Km8I) — Computer Science Plus - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/operating_system/hrrn-rr-q4-awt-att - Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-2 ## Question Schedule the following set of processes according to HRRN and Round Robin algorithm (Time quantum=4) and calculate average waiting time and average turnaround time. | Process | Arrival Time | CPU Time (ms) | |---|---|---| | A | 0 | 12 | | B | 2 | 8 | | C | 5 | 7 | | D | 10 | 9 | ## Short answer For HRRN, average waiting time is 10.5 ms and average turnaround time is 19.5 ms. For Round Robin (q=4), average waiting time is 15.5 ms and average turnaround time is 24.5 ms. ## Answer Waiting time = turnaround time − CPU time; turnaround = completion − arrival. For HRRN: response ratio = (waiting time + CPU time) / CPU time; after each completion the process with the highest ratio runs. #### HRRN - t = 0: only A is present, so A runs 0-12. - t = 12: B ratio = (10+8)/8 = 2.25, C = (7+7)/7 = 2.0, D = (2+9)/9 = 1.22, so **B** runs 12-20. - t = 20: C = (15+7)/7 = 3.14, D = (10+9)/9 = 2.11, so **C** runs 20-27, then **D** runs 27-36. **HRRN (non-preemptive)** ```text | A | B | C | D | 0 12 20 27 36 ``` | Process | Arrival | Burst | Completion | Turnaround | Waiting | |---|---|---|---|---|---| | A | 0 | 12 | 12 | 12 | 0 | | B | 2 | 8 | 20 | 18 | 10 | | C | 5 | 7 | 27 | 22 | 15 | | D | 10 | 9 | 36 | 26 | 17 | Average waiting time = 42/4 = **10.5**; average turnaround time = 78/4 = **19.5** #### Round Robin (q = 4) A runs first; new arrivals join the ready queue ahead of a preempted process. Queue order of execution: A, B, A, C, B, D, A, C, D. **Round Robin (q = 4)** ```text | A | B | A | C | B | D | A | C | D | 0 4 8 12 16 20 24 28 31 36 ``` | Process | Arrival | Burst | Completion | Turnaround | Waiting | |---|---|---|---|---|---| | A | 0 | 12 | 28 | 28 | 16 | | B | 2 | 8 | 20 | 18 | 10 | | C | 5 | 7 | 31 | 26 | 19 | | D | 10 | 9 | 36 | 26 | 17 | Average waiting time = 62/4 = **15.5**; average turnaround time = 98/4 = **24.5** #### Comparison | Algorithm | Average waiting time (ms) | Average turnaround time (ms) | |---|---|---| | HRRN | 10.5 | 19.5 | | Round Robin (q=4) | 15.5 | 24.5 | **Answer: HRRN: AWT = 10.5 ms, ATAT = 19.5 ms. RR: AWT = 15.5 ms, ATAT = 24.5 ms.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. At t=12, why does B run next under HRRN? A. B has the shortest burst B. B has the highest response ratio (10+8)/8 = 2.25 C. B arrived first D. B has the highest priority Answer: B. HRRN picks the highest ratio: B 2.25 beats C 2.0 and D 1.22. (Key point: HRRN selection at t=12) 2. What are the HRRN average waiting and turnaround times? A. AWT 15.5, ATAT 24.5 B. AWT 10.5, ATAT 19.5 C. AWT 12, ATAT 20 D. AWT 42, ATAT 78 Answer: B. Waiting 0, 10, 15, 17 sum to 42 (10.5); turnaround 12, 18, 22, 26 sum to 78 (19.5). (Key point: HRRN averages) 3. Under RR with q=4 (new arrivals queue ahead of preempted process), what is the execution order? A. A, B, C, D, A, B, C, D, A B. A, B, A, C, B, D, A, C, D C. A, A, A, B, B, C, C, D, D D. A, B, C, A, D, B, C, D, A Answer: B. B (arrived at 2) and C (at 5) join the queue ahead of preempted A, giving A, B, A, C, B, D, A, C, D. (Key point: RR order) 4. Which algorithm gives the lower averages here? A. RR, with AWT 15.5 B. HRRN, with AWT 10.5 and ATAT 19.5 C. Both are equal D. RR, with ATAT 19.5 Answer: B. HRRN's 10.5/19.5 beats RR's 15.5/24.5, since RR keeps splitting long jobs and delaying their completion. 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