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- 1. Understand Process Management •Process Scheduling Algorithms 1
- 2. HOME PREVIOUS TOPIC NEXT PREVIOUS QUESTION PAPERS FOR OS CPP TUTORIALS 2
- 3. Recap In the lastclass, you have learnt Shortest Job First scheduling algorithm and its -Advantages -Disadvantages 3
- 4. Objectives On completion ofthis class, you would be able to know • Priority Scheduling • Round Robin Scheduling 4
- 5. Priority Scheduling • Priorityscheduling algorithm is special case of SJF • A priority is associated with each process • Priorities are indicated by fixed range of numbers • In this scheduling CPU is allocated to the process with the highest priority • Equal priority process scheduled in FCFS order 5
- 6. Priority Scheduling • Considerthe following set of processes assumed to have arrived at time 0, – in order P1,P2,P3-----P5 – with the length of the CPU burst time given in milliseconds – and their associated priorities Process Burst time Priorities P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 6
- 7. Priority Scheduling • UsingPriority Scheduling we should schedule these processes according to the following Gantt chart P2 P5 P1 P3 P4 0 1 6 16 18 19 • The average waiting time is 8.2 milliseconds i.e. (0+1+6+16+18)/5 = 8.2 milliseconds Priority can be defined internally or externally 7
- 8. Priority Scheduling • Internallydefined priorities use some measurable quantity or quantities to compute the priority of a process i.e. – Time limits – Memory requirements – Open files – The ratio of average I/O burst to average CPU burst • Externally priorities are set by criteria process type, work, department sponsoring 8
- 10. Priority Scheduling • Priorityscheduling can be either be Preemptive or Non Preemptive • When a process arrives at the ready queue, its priority is compared with the priority of currently running process • Preempt the CPU if the priority of the newly arrived process is higher than the priority of the currently running processes 10
- 11. Priority Scheduling • Ifthe priority scheduling algorithm is Non – Preemptive put the new process at the head of the ready queue • A major problem with this is indefinite blocking • A solution to this problem is Aging • Aging is a technique which gradually increases the priority of processes that wait in the system for a long time 11
- 12. Round-Robin Scheduling Algorithm •The round robin scheduling algorithm is designed especially for Time Sharing Systems • It is similar to FCFS scheduling, but Preemption is added to switch between processes • A small unit of time called Time quantum or time slice is defined • A time quantum is generally from 10 to 100 mille seconds • The CPU scheduler allocates the CPU to each process for a time interval of up to 1 time quantum 12
- 13. Round-Robin Scheduling Algorithm •The average waiting time under Round Robin policy is often long • Consider the following set of processes that arrive at time 0, with the – length of the CPU burst time given in milliseconds: Process Burst Time P1 24 P2 3 P3 3 13
- 14. Round-Robin Scheduling Algorithm •Use a time quantum of 4milliseconds • Process P1 gets the first 4msec it is Preempted after the first time quantum • P2 & P3 does not require 4msec they quit before their time quantum 14
- 15. Round-Robin Scheduling Algorithm •The CPU is returned to process P1 • The average waiting time 17 / 3= 5.66milliseconds • The average turn around time for 3 processes of time quantum 10 is 29 15
- 17. Round-Robin Scheduling Algorithm •Round Robin scheduling is Preemptive • If CPU burst time of process, is larger than the process time quantum then CPU is Preempted from the process • No process is allocated CPU for more than 1 time quantum
- 18. Round-Robin Scheduling Algorithm •Performance of RR scheduling depends on the size of time quantum • If time quantum is extremely very large RR approach is same as FCFs • If it is extremely small, RR approach is called processor sharing 18
- 19. Summary In this class,you have learnt • Priority Scheduling Algorithm • Round Robin Scheduling 19
- 20. Frequently Asked Questions 1.Explain Round Robin Scheduling 2. Explain Priority Scheduling 3. What are the Internal & External priorities 20
- 21. Quiz 1. In __________scheduling CPU allocated to the process with the highest priority a) FCFS b) SJF c) RR d) Priority 21
- 22. Quiz 2. Priority schedulingcan be __________ a) Preemptive b) Non Preemptive c) Both d) None 22
- 23. Other subject materials • Web designing • Micro processors • C++ tutorials • java home
- 24. Quiz 3. RR schedulingis designed for _______ systems a) Time sharing b) Real time c) Distributed d) None 24
- 25. Quiz 4. If thetime quantum is extremely large than RR is same as a) SJF b) FCFS c) Priority d) None 25