1 Scheduling Algorithms FCFS First-Come, First-Served Round-robin SJF Multilevel Feedback Queues

2 Round-Robin Scheduling Quantum expires: move to back of ready queue Variants used in most real systems Tradeoffs: length of quantum Large: response time increases Small: throughput decreases quantum → 0 = overhead dominates context switches, cache misses

3 Example: Round-Robin waiting running

4 Example: Round-Robin waiting running

5 Example: Round-Robin waiting running

6 Example: Round-Robin waiting running

7 Example: Round-Robin waiting running

8 Example: Round-Robin waiting running

9 Example: Round-Robin waiting running

10 Example: Round-Robin waiting running

11 Example: Round-Robin waiting running

12 Example: Round-Robin waiting running

13 Example: Round-Robin waiting running + Fair - Long average wait times

14 Round-Robin vs. FCFS Example 1: 5 jobs, 100 seconds each, quantum = 1s ignore context switch time

15 Round-Robin vs. FCFS Example 1: 5 jobs, 100 seconds each, quantum = 1s ignore context switch time

16 Round-Robin vs. FCFS Example 2: 5 jobs: 50, 40, 30, 20, 10 seconds each, quantum = 1s

17 Round-Robin vs. FCFS Example 2: 5 jobs: 50, 40, 30, 20, 10 seconds each, quantum = 1s

18 This Time: Scheduling Algorithms FCFS First-Come, First-Served Round-robin SJF Multilevel Feedback Queues

19 Example: SJF Schedule job with least work until I/O or done

20 Example: SJF Schedule job with least work until I/O or done

21 Example: SJF Schedule job with least work until I/O or done

22 Example: SJF Schedule job with least work until I/O or done

23 Example: SJF Schedule job with least work until I/O or done

24 Example: SJF Schedule job with least work until I/O or done

25 Example: SJF 5 jobs, length 50, 40, 30, 20, 10 seconds, quantum = 1s

26 Example: SJF 5 jobs, length 50, 40, 30, 20, 10 seconds

27 SJF/SRTF: Shortest-Job First Advantages: Provably optimal – minimizes average waiting time Works for preemptive & non-preemptive schedulers Preemptive SJF = SRTF Shortest remaining time first Disadvantages: Impossible to predict CPU time job has left Long-running CPU-bound jobs can starve

28 This Time: Scheduling Algorithms FCFS First-Come, First-Served Round-robin SJF Multilevel Feedback Queues

29 Multilevel Feedback Queues (MLFQ) Use past behavior to predict future, assign job priorities Overcome prediction problem in SJF Assumption: I/O-bound in past, I/O-bound in future Scheduler favors jobs that used least CPU time Adaptive: Change in behavior → change in scheduling decisions

30 MLFQ: Approximating SJF Multiple queues, different priorities Round-robin scheduling at each priority level Run all jobs at highest priority first (till this queue is empty), then next, etc. Can lead to starvation Increase quantum exponentially at lower priorities

31 MLFQ Example

32 MLFQ: Assigning Priorities Job starts in highest priority queue Quantum expires → CPU-bound Drop priority one level Quantum does not expire → I/O-bound Increase priority one level  CPU-bound jobs move down, I/O-bound jobs move up

33 Summary of Scheduling Algorithms FCFS: unfair, average waiting time poor Round robin: fair, average waiting time poor SJF: unfair, minimizes average waiting time requires accurate prediction Multilevel Feedback Queuing: approximates SJF