Lecture 5 CPU Scheduling

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1 Lecture 5 CPU Scheduling (Uniprocessor System) 1

2 Lecture Contents 1. Basic Concepts 2. Scheduling Criteria 3. Scheduling Algorithms 2

3 Switching CPU among processes in multiprogrammed OSs makes a computer more productive. We introduce basic CPU-scheduling concepts and present several CPU-scheduling algorithms. 3

4 1. Basic Concepts In multiprogramming system, several processes are kept in memory at one time. When one process has to wait, OS takes CPU away from that process and gives CPU to another process. Almost all computer resources are scheduled before use. CPU is one of primary computer resources. Thus, CPU scheduling is central to OS. 4

5 1.1 CPU-I/O Burst Cycle Process execution consists of cycle of CPU execution and I/O wait. Processes alternate between these two states. Process execution begins with CPU burst. This is followed by I/O burst, which is followed by another CPU burst, then another I/O burst, and so on. Eventually, final CPU burst ends with system request to terminate execution. 5

6 Alternating sequence of CPU and I/O bursts. 6

7 1.1 CPU-I/O Burst Cycle Frequency curve of durations of CPU bursts is shown on next slide. Curve has large number of short CPU bursts and small number of long CPU bursts. I/O-bound process has many short CPU bursts. CPU-bound process might have a few long CPU bursts. This distribution can be important in the selection of appropriate CPU-scheduling algorithm. 7

8 Histogram of CPU-burst durations. 8

9 1.2 CPU Scheduler Whenever CPU becomes idle, OS must select one of processes in ready queue to be executed. The selection process is carried out by the shortterm scheduler (CPU scheduler or dispatcher). The scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process. Records in queues are process control blocks (PCBs) of processes. 9

10 1.3 Nonpreemptive, Preemptive Scheduling Nonpreemptive scheduling scheme does not consider priorities of processes. - Once a process is in the running state, it will continue until it terminates or blocks itself for I/O - Nonpreemptive scheduling ensures that a process relinquishes control of the CPU only when it finishes with its current CPU burst. - used by Microsoft Windows 3.x 10

11 1.3 Nonpreemptive, Preemptive Scheduling Preemptive scheduling scheme considers priorities of processes. - Currently running process may be interrupted and moved to ready state by the OS (e.g., due to the execution of a process of higher priority) - Preemptive scheduling allows a process to be interrupted in the midst of its execution, taking the CPU away and allocating it to another process. 11

12 1.4 Dispatcher Another component involved in CPU-scheduling function is dispatcher. Dispatcher is OS module that gives control of CPU to the process selected by short-term scheduler (also called CPU scheduler). The function of dispatcher involves the following: 12

13 1.4 Dispatcher - Switching context - Switching to user mode - Jumping to proper location in user process to restart the process The time it takes for dispatcher to stop one process and start another running is known as dispatch latency. 13

14 Long-, Short-, and Medium-Term Schedulers. 14

15 Lecture Contents 1. Basic Concepts 2. Scheduling Criteria 3. Scheduling Algorithms 15

16 2. Scheduling Criteria Criteria used for comparing/evaluating CPUscheduling algorithms are - CPU utilization. Keeping CPU as busy as possible is good. CPU usage can range from 0% to 100%. - Throughput: is the number of processes that are completed per time unit (e.g., milliseconds). 16

17 2. Scheduling Criteria - Turnaround time: is interval from time of submission of process to time of completion. Turnaround time is sum of periods spent waiting to get into memory, waiting in ready queue, executing on CPU, and doing I/O. - Waiting time: is the sum of the periods spent waiting in ready queue. 17

18 2. Scheduling Criteria - Response time: is time from submission of request until the first response is produced. CPU utilization and throughput should be maximized. Turnaround time, waiting time, and response time should be minimized. 18

19 Lecture Contents 1. Basic Concepts 2. Scheduling Criteria 3. Scheduling Algorithms 19

20 3. Scheduling Algorithms CPU scheduling deals with problem of deciding which of processes in ready queue is to be allocated CPU. Different CPU-scheduling algorithms are firstcome, first-served (FCFS), shortest-job-first (SJF) (also called shortest process next (SPN)), priority, round-robin, multilevel queue, multilevel feedback queue. 20

21 3.1 First-Come, First-Served Scheduling Simplest CPU-scheduling algorithm is firstcome, first-served (FCFS) scheduling algorithm. Process that requests CPU first is allocated CPU first. FCFS is implemented with FIFO queue. When process enters ready queue, its PCB is linked onto tail of queue. 21

22 3.1 First-Come, First-Served Scheduling When CPU is free, it is allocated to process at head of queue. The running process is then removed from the queue. Weakness: average waiting time (AWT) is often quite long. Example: Consider following set of processes that arrive at time 0, with length of the CPU burst given in milliseconds: 22

23 3.1 First-Come, First-Served Scheduling If processes arrive in order P 1, P 2, P 3, and are served in FCFS manner, we get the result shown in the following Gantt chart. 23

24 3.1 First-Come, First-Served Scheduling Gantt chart is bar chart that illustrates particular schedule, including the start and finish times of each of participating processes. Waiting time is 0 millisecond for process P 1, 24 milliseconds for process P 2, and 27 milliseconds for process P 3. Thus, average waiting time is ( ) / 3 = 17 milliseconds. 24

25 3.1 First-Come, First-Served Scheduling If processes arrive in order P 2, P 3, P 1, results are as shown in following Gantt chart: AWT is now ( ) / 3 = 3 milliseconds. Weakness: - AWT under FCFS policy is generally not minimal. It depends on arriving order of processes. 25

26 3.1 First-Come, First-Served Scheduling - FCFS causes convoy effect. The situation in which all other shorter processes wait for one big process to get off CPU is called convoy effect. This effect results in lower CPU and device utilization. - Possible solution: allows shorter processes to run first. FCFS is nonpreemptive (i.e., no priority) so it troublesome for time-sharing systems. 26

27 3.2 Shortest-Job-First Scheduling Shortest-job-first (SJF) scheduling algorithm (also known as shortest-next-cpu-burst or shortest process next (SPN)) 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. If the next CPU bursts of two processes are the same, FCFS is used to break the tie. 27

28 3.2 Shortest-Job-First Scheduling Example: Consider the following set of processes that arrive at time 0, with the length of the CPU burst given in milliseconds: SJF schedules these processes according to the following Gantt chart: 28

29 3.2 Shortest-Job-First Scheduling Waiting time is 3 milliseconds for process P 1, 16 milliseconds for process P 2, 9 milliseconds for process P 3, and 0 millisecond for process P 4. Thus, AWT is ( )/4 = 7 milliseconds. AWT FCFS is milliseconds. // = ( ) / 4 = 41 / 4 =

30 3.2 Shortest-Job-First Scheduling Gantt chart for FCFS AWT FCFS is ( ) / 4 = 41 / 4 = milliseconds. 30

31 3.2 Shortest-Job-First Scheduling Strengths: - SJF is optimal, (i.e., it gives minimum AWT for given set of processes. Weakness: - Knowing length of next CPU burst is difficult. - Possible solution: predicted from its previous CPU bursts. SJF is used frequently in long-term scheduling, but not in short-term scheduling. 31

32 3.2 Shortest-Job-First Scheduling SJF can be either preemptive or nonpreemptive. - When new process arrives at ready queue while previous process is still executing. If next CPU burst of newly arrived process is shorter than what is left of currently executing process, preemptive SJF will preempt currently executing process. In contrast, nonpreemptive SJF will allow currently running process to finish its CPU burst. - Preemptive SJF is also called shortestremaining-time-first (SRTF) scheduling. 32

33 3.2 Shortest-Job-First Scheduling Example: Consider the following four processes, with the length of the CPU burst given in milliseconds: Resulting preemptive SJF is shown in the following Gantt chart: 33

34 3.2 Shortest-Job-First Scheduling Process P 1 is started at time 0, since it is only process in queue. Process P 2 arrives at time 1. Remaining time for process P 1 (7 milliseconds) is larger than the time required by process P 2 (4 milliseconds), so process P 1 is preempted, and process P 2 is scheduled. 34

35 3.2 Shortest-Job-First Scheduling AWT for preemptive SJF is [(10-1) + (1-1) + (17-2) + (5-3)] / 4 = ( ) / 4 = 26 / 4 = 6.5 milliseconds. 35

36 3.2 Shortest-Job-First Scheduling AWT for nonpreemptive SJF is = ((0-0) + (8-1) + (17-2) + (12-3)) / 4 = ( ) / 4 = 31 / 4 = 7.75 milliseconds. 36

37 3.3 Priority Scheduling SJF is special case of priority scheduling algorithm. In SJF algorithm, priority p is defined as inverse of next CPU burst (b) (i.e., p = 1 / b). The larger the CPU burst, the lower the priority, and vice versa. In priority scheduling, priority is associated with each process, and CPU is allocated to process with highest priority. 37

38 3.3 Priority Scheduling Equal-priority processes are scheduled in FCFS order. We uses a smaller priority number implies a higher priority. Example: Consider the following set of processes that arrive at time 0 in the order P 1, P 2,, P 5, with length of CPU burst given in milliseconds: 38

39 3.3 Priority Scheduling Result of priority scheduling following Gantt chart: is shown in 39

40 3.3 Priority Scheduling AWT is ( ) / 5 = 8.2 milliseconds. Priority definition can be based on time limit, memory requirement, number of open files, and ratio of average I/O burst to average CPU burst, importance of process, and so on. 40

41 3.3 Priority Scheduling Priority scheduling can be either preemptive or nonpreemptive. - When process arrives at ready queue, its priority is compared with priority of currently running process. Preemptive priority scheduling will preempt CPU if priority of newly arrived process is higher than priority of currently running process. In contrast, nonpreemptive priority algorithm will put new process at head of the ready queue. 41

42 3.3 Priority Scheduling Weakness: - Priority scheduling may cause starvation (or indefinite blocking). This means that priority algorithm can leave some low-priority processes waiting indefinitely. 42

43 3.3 Priority Scheduling Weakness: - Possible solution: is aging. Aging is technique of gradually increasing priority of processes that wait in system for long time. For example, we may increase priority of waiting processes by 1 every 5 seconds. Eventually, even process with an initial low priority will have high priority in system and will be executed. 43

44 3.4 Round-Robin Scheduling Round-robin (RR) scheduling algorithm is designed especially for time-sharing systems. RR is similar to FCFS, but preemption is added to enable system to switch between processes. Small unit of time (e.g., 10 to 100 milliseconds in length), called time quantum or time slice, is defined. Ready queue is treated as circular queue. 44

45 3.4 Round-Robin Scheduling CPU scheduler goes around ready queue, allocating CPU to each process for 1 time quantum. The ready queue in RR scheduling is treated as a FIFO queue of processes. New processes are added to tail of ready queue. CPU scheduler picks first process from ready queue, sets a timer to interrupt after 1 time quantum, and dispatches process. 45

46 3.4 Round-Robin Scheduling Two cases will happen: - Process may have CPU burst of less than 1 time quantum. Process itself will release CPU voluntarily. Scheduler will proceed to next process in ready queue. 46

47 3.4 Round-Robin Scheduling Two cases will happen: - If CPU burst of currently running process is longer than 1 time quantum, the timer will go off and will cause interrupt to OS. Context switch will be executed, and current process will be put at tail of ready queue. CPU scheduler will select next process in ready queue for execution. Weakness: AWT under the RR policy is often long. 47

48 3.4 Round-Robin Scheduling Example: Consider following set of processes that arrive at time 0, with length of CPU burst given in milliseconds: 48

49 3.4 Round-Robin Scheduling If time quantum of 4 milliseconds is used, process P 1 gets first 4 milliseconds. Since it requires another 20 milliseconds, it is preempted after first time quantum, and CPU is given to next process in queue, process P 2. Process P 2 does not need 4 milliseconds, so it quits before its time quantum expires. CPU is given to next process, process P 3. Once each process has received 1 time quantum, CPU is returned to process P 1 for an additional time quantum. The resulting RR schedule is as follows: 49

50 3.4 Round-Robin Scheduling Calculate AWT for above RR schedule. P 1 waits for 6 millisconds (10-4), P 2 waits for 4 millisconds, and P 3 waits for 7 millisconds. Thus, AWT is ( ) / 3 = 17 / 3 = 5.66 milliseconds. RR scheduling algorithm is preemptive. 50

51 3.4 Round-Robin Scheduling Performance of RR depends heavily on size of time quantum. - If time quantum is extremely large, RR policy is the same as FCFS policy (AWT FCFS is often quite long). - If time quantum is extremely small (say, 1 millisecond), RR approach incurs contextswitching overhead (see figure on next slide) so CPU utilization is reduced. 51

52 3.4 Round-Robin Scheduling How a smaller time quantum increases context switches. 52

53 3.4 Round-Robin Scheduling Time quantum should be large compared with context-switch time, but it should not be too large (i.e., RR degenerates to FCFS policy. 53

54 Exercises Exercise 1: Consider the following set of five processes P 1, P 2, P 3, P 4, and P 5. The priorities and the lengths of the CPU-burst time in milliseconds are given below. 54

55 Exercises The processes are assumed to have arrived in the order P 1, P 2, P 3, P 4, P 5, all at time 0. A smaller priority number implies a higher priority. Draw Gantt chart illustrating the execution of these processes and compute the average waiting time (AWT) of each of the following CPU scheduling algorithms: FCFS (First-come, firstserved), non-preemptive SJF (shortest-job-first), non-preemptive priority, and round-robin (RR) with time quantum of 1 millisecond. 55

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