This document discusses various CPU scheduling algorithms used in operating systems. It describes the role of the CPU scheduler and dispatcher in selecting processes to run. Common scheduling criteria like CPU utilization, throughput, turnaround time and waiting time are discussed. First Come First Served (FCFS) scheduling and its example are provided. Shortest Job First (SJF) scheduling, both preemptive and non-preemptive variants, are explained with examples. Round Robin scheduling is described where each process gets a time quantum or time slice before being preempted. Other scheduling algorithms like priority scheduling are also covered briefly.

1. CPU Scheduling Prof.PrasadSawant Lecturer MCA MACS College Pune Operating System Concepts Prasad Sawant

2. Alternating Sequence of CPU And I/O Bursts Operating System Concepts Prasad Sawant

3. CPU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state. 2. Switches from running to ready state. 3. Switches from waiting to ready. 4. Terminates. Scheduling under 1 and 4 is nonpreemptive. All other scheduling is preemptive. Operating System Concepts Prasad Sawant

4. Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler. Operating System Concepts Prasad Sawant

5. Scheduling Criteria CPU utilization – keep the CPU as busy as possible Throughput– # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular process Waiting time – amount of time a process has been waiting in the ready queue Operating System Concepts Prasad Sawant

6. Keep in mind Waiting time (WT)=start time(ST)-Arrival time (AT) Finish time (FT)=start time(ST)+Burst Time(BT) Total turn around time(TOT)=Finish time(FT)-Arrival time (AT ) Operating System Concepts Prasad Sawant

7. Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Operating System Concepts Prasad Sawant

8. First-Come, First-Served (FCFS) Scheduling Operating System Concepts Prasad Sawant Suppose that the processes arrive in the order: P1 , P2 , P3 schedule schedule 0 10 20 30 schedule Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 P3 30 P2 27 P1 24

9. First-Come, First-Served (FCFS) Scheduling Gantt Chart Operating System Concepts Prasad Sawant Suppose that the processes arrive in the order: P1 , P2 , P3 schedule schedule schedule Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 0 P1 24 P2 27 P3 30

10. Exercise Find Average waiting time for Suppose that the processes arrive in the order: P2 , P3 , P1 Operating System Concepts Prasad Sawant

11. Shortest-Job-First (SJR) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). SJF is optimal – gives minimum average waiting time for a given set of processes. Operating System Concepts Prasad Sawant

12. Example of Non-Preemptive SJF Operating System Concepts Prasad Sawant schedule schedule schedule schedule 0 P1 7 P3 8 P2 12 P4 16 Average waiting time = (0+6+3+7)/4=4

13. Example of Non-Preemptive SJF Operating System Concepts Prasad Sawant P1 7 P3 8 P2 12 P4 16 Average waiting time = (0+6+3+7)/4=4 Average TOT =(7+10+4+11)/4=8

14. Preemptive SJF Operating System Concepts Prasad Sawant 7 P schedule 10 10-1=9 schedule 1 1-1=0 schedule 17 17-2=15 schedule 5 5-3=2 0 P1 1 P2 5 P4 10 P1 17 P3 26

15. Priority Scheduling Operating System Concepts Prasad Sawant Schedule 6 6 Schedule 0 0 Schedule 16 16 Schedule 18 18 Schedule 1 1 41/5=8.2 ms 0 P2 1 P5 6 P1 16 P3 18 P4 19 0

16. SJF Preemptive Priority Scheduling Operating System Concepts Prasad Sawant P 5 scheduled 19 19-3=16 scheduled 24 24-1=23 scheduled 12 12-3=9 scheduled 3 3-3=0 48/4=12 ms 0 P1 3 P4 12 P3 19 P1 24 P2 30

17. Round Robin Scheduling Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/nof the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. Operating System Concepts Prasad Sawant

18. Time Quantum and Context Switch Time Operating System Concepts Prasad Sawant 0 0 10 1 6 10 9 2 3 4 5 6 7 8 9 10 1

19. Round Robin Time Quantum 4 Operating System Concepts Prasad Sawant 24-4=20 20-4=16 16-4=12 12-4=8 8-4=4 4-4=0 scheduled 30 scheduled 7 scheduled 10 0 P1 4 P2 7 P3 10 P1 14 P1 18 P1 22 P1 26 P1 30

20. Questions Define the difference between pre-emptive and non-pre-emptive scheduling. Explain the concept of a priority used in scheduling. Why is priority working usually chosen for real time processes? Define by difference between preemptive and non-emptive scheduling. Comment on the principle disadvantage of each of these scheduling methods: FCFS, SJF, RR Operating System Concepts Prasad Sawant

21. Bibliography Operating System Principle-Peter Galvin Galvin Prof.S.G.Lakhdive (Dept .Computer Sci ) Prof.Ramkirshna More A.C.S College Akurdi Mr. AbhishekNagar Web Administrator at Symbiois Operating System Concepts Prasad Sawant

22. Thanks you Operating System Concepts Prasad Sawant