Describe various CPU-scheduling algorithms. Describe various multi- processor CPU-scheduling algorithms

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1 CPU Scheduling

2 Objectives Describe various CPU-scheduling algorithms Describe various multi- processor CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPUscheduling algorithm for a particular system CPU scheduling 2

3 Basic Concepts Almost all computer resources are scheduled before use. The CPU is one of the primary computer resources. CPU scheduling is the basis of multi-programmed operating systems. What are multi-programmed operating systems? What are the benefits of multiprogramming? CPU scheduling 3

4 Basic Concepts In a single-processor system, only one process can run at a time; any others must wait until the CPU is free and can be rescheduled. The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization. How is multiprogramming achieved? CPU scheduling 4

5 Cycle of CPU execution and I/O wait The success of CPU scheduling depends on an observed property of processes:» Process execution consists of a cycle of CPU execution and I/O wait. Processes alternate between these two states. CPU scheduling 5

6 Cycle of CPU execution and I/O wait CPU scheduling 6

7 Cycle of CPU execution and I/O wait Process execution begins with a CPU burst. That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on. Eventually, the final CPU burst ends with a system request to terminate execution. CPU scheduling 7

8 Short term Scheduler Selects from among the processes in ready queue, and allocates the CPU to one of them Queue may be ordered in various ways. What are this ways? (think real life queues) CPU scheduling 8

9 CPU scheduling 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 CPU scheduling 9

10 Non-preemptive & preemptive CPU Scheduling algorithms can be categorized into two;» Non-Preemptive» Preemptive CPU scheduling 10

11 Non-preemptive & preemptive Under non preemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state The process cannot be interrupted With Preemptive a process can be interrupted during its course of execution CPU scheduling 11

12 Scheduling Algorithms

13 CPU scheduling deals with the problem of deciding which of the processes in the ready queue is to be allocated the processor. There are many different CPU scheduling algorithms. These are; CPU scheduling 13

14 First-Come, First-Served Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling CPU scheduling 14

15 First-Come, First-Served (FCFS) Scheduling The simplest CPU-scheduling algorithm With this scheme, the process that requests the CPU first is allocated the CPU first. The implementation of the FCFS policy is easily managed with a FIFO queue. When a process enters the ready queue, its PCB is linked onto the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue. The running process is then removed from the queue. CPU scheduling 15

16 First come First Served (FCFS)

17 First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 CPU scheduling 17

18 First-Come, First-Served (FCFS) Scheduling The Gantt Chart for the schedule is: P 1 P 2 P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17miliseconds CPU scheduling 18

19 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order: P 2, P 3, P 1 Will there be a difference in the average waiting time? CPU scheduling 19

20 FCFS Scheduling (summary) The FCFS scheduling algorithm is non-preemptive. What does this mean? FCFS algorithm is troublesome for time-sharing systems because Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O. CPU scheduling 20

21 Class Exercise 1 Process l Time Burst Time P P P P Determine the average waiting time if FCFS scheduling algorithm was in use CPU scheduling 21

22 Shortest Job First (SJF)

23 Shortest-Job-First (SJF) Scheduling This algorithm associates with each process the length of the process's next CPU burst. When the CPU is available, it is assigned to the process that has the smallest next CPU burst. What if two processes have the same CPU burst? If the next CPU bursts of two processes are the same, FCFS scheduling is used to break the tie. CPU scheduling 23

24 Example of SJF Process l Time Burst Time P P P P P 4 P 3 P 1 P Average waiting time = ( ) / 4 = 7 miliseconds CPU scheduling 24

25 Class Exercise 2 Process Burst Time P 1 24 P 2 3 P 3 3 Assume STF scheduling algorithm is used, what would the order of the processes be? Compute average waiting time if STF scheduling algorithm was used CPU scheduling 25

26 Difficulties with SJF The real difficulty with the SJF algorithm is knowing the length of the next CPU request. However For long-term (job) scheduling, we can use the length of the process specified when the process is submited. SJF is much difficult to implement at the level of short-term CPU scheduling. There is no way to know the exact length of the next CPU burst. What are the possible solutions to address this issue? CPU scheduling 26

27 SJF cont The SJF algorithm can be either preemptive or nonpreemptive. In Preemtive, The choice arises when a new process arrives at the ready queue while a previous process is still executing. The next CPU burst of the newly arrived process may be shorter than what is left of the currently executing process CPU scheduling 27

28 SJF cont A preemptive SJF algorithm will preempt the currently executing process, whereas a non-preemptive SJF algorithm will allow the currently running process to finish its CPU burst. CPU scheduling 28

29 SJF cont Preemptive SJF scheduling is sometimes called shortestremaining-time-first scheduling. CPU scheduling 29

30 Shortest Remaining time First (SRTF)

31 Example of Shortest-remaining-time-first ProcessAarrArrival TimeT Burst Time P P P P Class Discussion Questions What will be the order of process execution? Compute the average waiting time Compute the average waiting time if the algorithm is non preemptive CPU scheduling 31

32 Example of Shortest-remaining-time-first ProcessAarri Arrival TimeTBurst Time P P P P Preemptive SJF Gantt Chart P 2 P 1 P 1 P 4 P Average waiting time = 6.5 msec CPU scheduling 32

33 Non-preemptive ProcessAarri Arrival TimeTBurst Time P P P P Order of execution will be P1-P2-P4-P3 P1 0 P2 7 P3 9 P4 15 Average waiting time = 7.75 msec CPU scheduling 33

34 Class Exercise ProcessAarri Arrival TimeTBurst Time P P P P i. Draw a gantt chart to represent the following processes ii. Compute the average waiting time of the following set of processes using» SJF (preemptive)-3m/s» SJF (non-preemptive)-4m/s CPU scheduling 34

35 Priority Scheduling

36 Priority Scheduling A priority is associated with each process, and the CPU is allocated to the process with the highest priority. Would we be right to say that the SJF algorithm is a special case of the general priority scheduling algorithm? What would happen if two processes have the same priority? When two processes have the same priority, First come first served algorithm is used to schedule CPU scheduling 36

37 Priority Scheduling Note that we discuss scheduling in terms of high priority and low priority. Priorities are generally indicated by some fixed range of numbers, such as 0 to 7 or 0 to 4,095. However, there is no general agreement on whether 0 is the highest or lowest priority. Some systems use low numbers to represent low priority; others use low numbers for high priority. This difference can lead to confusion. In our class, we assume that low numbers represent high priority. CPU scheduling 37

38 Example of Priority Scheduling assumption is that all processes are in the ready queue ProcessAarriBurst TimeT Priority P P P P P Class discussion questions Draw the Gantt chart for the processes above What will the average waiting time be? CPU scheduling 38

39 Example of Priority Scheduling Solution assumption is that all processes are in the ready queue P 5 P 2 P 3 P 1 P Priority scheduling Gantt Chart CPU scheduling 39

40 Priority Scheduling Priorities can be defined either internally or externally. Internally defined priorities use some measurable quantity or quantities to compute the priority of a process. For example, time limits, memory requirements, the number of open files, and the ratio of average I/O burst to average CPU burst have been used in computing priorities. CPU scheduling 40

41 Priority Scheduling External priorities are set by criteria outside the operating system, such as the importance of the process. Examples? Priority scheduling can be either preemptive or nonpreemptive. Explain CPU scheduling 41

42 Problems with Priority Scheduling A major problem with priority scheduling algorithms is indefinite blocking, or starvation. A process that is ready to run but waiting for the CPU can be considered blocked. A priority scheduling algorithm can leave some low priority processes waiting indefinitely. In a heavily loaded computer system, a steady stream of higher-priority processes can prevent a low-priority process from ever getting the CPU. CPU scheduling 42

43 Indefinite blocking solution A solution to the problem of indefinite blockage of low-priority processes is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time. For example, if priorities range from 127 (low) to 0 (high), we could increase the priority of a waiting process by 1 every 15 minutes. CPU scheduling 43

44 Class Exercise Process Duration Priority Arrival Time P P P P Draw a Gantt chart to represent the following processes Compute the average waiting time of the following set of processes using preemptive priority scheduling algorithm CPU scheduling 44

45 Round Robin (RR)

46 Round Robin (RR) The round-robin (RR) 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 a time quantum or time slice, is defined. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1 time quantum. CPU scheduling 46

47 Example of RR with Time Quantum = 4 The Gantt chart is: Process Burst Time P 1 24 P 2 3 P 3 3 P 1 P 2 P 3 P 1 P 1 P 1 P 1 P What will the average waiting time be? What will the average Turn-around time be? CPU scheduling 47

48 Class Exercise : Time Quantum = 4 Process Burst Time Arrival time P 1 8 P 2 4 P 3 9 P 4 5 Draw the Gantt chart to represent the execution for a time quantum of 4 and 5 Compute the average waiting time for a time quantum of 4 and 5 Compute the average turn-around time for a time quantum of 4 and 5 (comment on the turn around time) CPU scheduling 48

49 Turnaround Time Vs Time Quantum Would we be right to conclude that turnaround time of a set of processes improves as the time-quantum size increases? CPU scheduling 49

50 Turnaround Time Vs Time Quantum The average turnaround time of a set of processes does not necessarily improve as the time-quantum size increases. In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum. CPU scheduling 50

51 Multilevel Queue

52 Multilevel Queue Multilevel queues are used for situations where processes are easily classified into different groups. A common division is made between foreground (interactive) processes and background processes. These two types of processes have different response-time requirements and so may have different scheduling needs CPU scheduling 52

53 Multilevel Queue A multilevel queue scheduling algorithm partitions the ready queue into several separate queues. The processes are permanently assigned to one queue, generally based on some property of the process, such as memory size, process priority, or process type. Each queue has its own scheduling algorithm. CPU scheduling 53

54 Multilevel Queue Scheduling CPU scheduling 54

55 Multilevel Feedback Queue In contrast to Multilevel queue, this algorithm allows a process to move between queues. The idea is to separate processes according to the characteristics of their CPU bursts. If a process uses too much CPU time, it will be moved to a lower-priority queue. This scheme leaves I/Obound and interactive processes in the higher-priority queues. CPU scheduling 55

56 Multilevel Feedback Queues CPU scheduling 56

57 Scheduling Criteria Different CPU scheduling algorithms have different properties, and the choice of a particular algorithm may favor one class of processes over another. Many criteria have been suggested for comparing CPU scheduling algorithms. Which characteristics are used for comparison can make a substantial difference in which algorithm is judged to be best. The criteria include the following: CPU scheduling 57

58 Scheduling Criteria CPU utilization keep the CPU as busy as possible Throughput # of processes that complete their execution per time unit Turnaround time From the point of view of a particular process amount of time to execute a particular process Waiting time amount of time a process has been waiting in the ready queue Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) CPU scheduling 58

59 Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time CPU scheduling 59

60 Multilevel Queue Scheduling-Example ML queue, 2 levels 10 units FCFS RR gets priority over FCFS Process Arrival Burst Queue P FCFS P RR P FCFS P RR Non-preemptive and preemptive Draw a Gantt charts to represent this algorithm both for preemptive and non-preemptive Compute average waiting and turn around time. CPU scheduling 60

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