CPU Scheduling. Rab Nawaz Jadoon. Assistant Professor DCS. Pakistan. COMSATS, Lahore. Department of Computer Science

Transcription

1 CPU Scheduling Rab Nawaz Jadoon DCS COMSATS Institute of Information Technology Assistant Professor COMSATS, Lahore Pakistan Operating System Concepts

2 Objectives To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPUscheduling algorithm for a particular system.

3 Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait CPU burst distribution

4 Scheduling Types There are two basic types of scheduling Non-Preemptive Process can t be taken away from the CPU E.g. early days of computing i.e. batch processing systems Response time of little concern Preemptive Process can be taken away from the CPU. Scheduler may preempt a process before it blocks or terminates in order to allocate the CPU to another process E.g. Interactive systems, Multiprogramming system, Time Sharing Systems

5 Alternating Sequence of CPU And I/O Bursts

6 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 non-preemptive. All other scheduling is preemptive.

7 OS scheduling Schedulars Long term Admission or High level scheduler Short term Medium term 7

8 Long Term Scheduler It decides which jobs or processes are to be admitted to the ready queue (in the Main Memory), that is, when an attempt is made to execute a program, its admission to the set of currently executing processes is either authorized or delayed by the long-term scheduler. The long term queue exists in the Hard Disk or the "Virtual Memory". Long-term scheduling is also important in large-scale systems such as batch processing systems, computer clusters, supercomputers etc. 8

9 Medium term Scheduler The medium-term scheduler temporarily removes processes from main memory and places them on secondary memory (such as a disk drive) or vice versa. This is commonly referred to as "swapping out" or "swapping in, (Swapper) The medium-term scheduler may decide to swap out a process which, has not been active for some time, or a process which has a low priority, or a process which is page faulting frequently, or a process which is taking up a large amount of memory in order to free up main memory for other processes. 9

10 Medium term scheduler Or swapping the process back in later when more memory is available, or when the process has been unblocked and is no longer waiting for a resource. 10

11 Short term Scheduler It also known as the CPU scheduler decides, which of the ready, in-memory processes are to be executed (allocated a CPU) next following a clock interrupt, an I/O interrupt, an operating system call or another form of signal. Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers, a scheduling decision will at a minimum have to be made after every time slice, and these are very short. This scheduler can be preemptive, implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process. 11

12 Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: Switching context Switching to user mode Jumping to the proper location in the user Program to restart that program Dispatch latency Time it takes for the dispatcher to stop one process and start another running.

13 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 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)

14 Scheduling Algorithm Optimization Criteria

15 Scheduling Algorithms FCFS or FIFO (First Come First Served or First in First out) SJF (Shortest Job First) SRT (Shortest Remaining Time) Priority Scheduling Round Robbin Scheduling Highest Response Ration Next (HRRN) Multilevel Feed back Queues (MFQs) etc 15

16 FCFS/FIFO Jobs are scheduled according to their job arrival time. It is a non preemptive algorithm. It tends to favor CPU bound processes. FIFO 16

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 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 = 17

18 FCFS Scheduling (Cont) Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case Convoy effect short process behind long process

19 Shortest-Job-First (SJF) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time SJF is optimal gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU request.

20 Process Arrival Time Burst Time P P P P SJF scheduling Gantt Chart Example of SJF P 4 P 1 P 3 P Average waiting time = ( ) / 4 = 7

21 SRT (Shortest Remaining Time) It is preemptive version of SJF. Any time a new process enters the pool of processes to be scheduled, the scheduler compares the expected value for its remaining processing time with that of the process currently scheduled. If the new process s time is less, the currently scheduled process is preempted. Like SJF, SRT favors short jobs, and long jobs can be victims of starvation. It is effective for those systems that received a stream of incoming jobs, but it is no longer useful in most of the today s operating systems. 21

22 SRT Simply, in SRT a newly arriving process with a shorter estimated run preempts a running process with a longer run time to completion. Process Arrival time Processing Time A B C D

23 SRT Process A starts executing: it is the only choice at time 0. It remain running when process B arrives because its remaining time is less. At time 3, Process B is the only process in the queue. At time 4.001, process C arrives and starts running because its remaining time (4) is less than process B s remaining time (4.999). At time 6.001, process C remains running because its remaining time (1.999) is less than process D s remaining time (2). When process C terminates, process D runs b/c its remaining time is less than that of process B. 23

24 Shortest Remaining time Gantt Chart Process Arrival time Processing Time A B C D A B C D B

25 Priority Scheduling A priority number (integer) is associated with each process. The CPU is allocated to the process with the highest priority (smallest integer highest priority) or Highest integer Highest priority, it varies from OS to OS, Preemptive Non-preemptive SJF is a priority scheduling where priority is the predicted next CPU burst time Problem Starvation low priority processes may never execute Solution Aging as time progresses increase the priority of the process

26 Example Draw the Gantt Chart illustrating their executing using, highest number = highest priority Preemptive Non preemptive Process Arrival time Burst Priority A B C D

27 Example Priority Scheduling Preemptive A B C D B A Non preemptive A C D B

28 Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. It is preemptive counter part of FIFO. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

29 Round Robbin Scheduling Extensively used in time shared systems. If time quantum is small then, Good Response time but more context switches If large then same behavior as in FIFO 29

30 RR Working Interrupts from the interval timer ensures that processing is suspended and control is transferred to the scheduling algorithm at the conclusion of the time quantum. 30

31 Quantum Size In Linux the default quantum size assigned to a process is 100ms, but can vary from 10 to 200ms, depending on process priority and behavior. In Window XP, this value is 20ms, but can vary depending on whether the process in running in the foreground or background of the GUI. 31

32 Performance q large FIFO Round Robin (RR) q small q must be large with respect to context switch, otherwise overhead is too high. 32

33 Example RR Scheduling Draw the Gantt Chart illustrating their executing using, Round Robbin (quantum = 2) Round Robbin (quantum = 1) Process Arrival time Processing Time A B C D A B A C D B B C

34 Round Robbin (q=1) A B A B C B C D B C D B C B Note: 13 context switches 34

35 Example of RR with Time Quantum = 4 Process Burst Time P 1 24 P 2 3 P 3 3 The Gantt chart is: P 1 P 2 P 3 P 1 P 1 P 1 P 1 P Typically, higher average turnaround than SJF, but better response

36 Time Quantum and Context Switch Time

37 Highest Response Ratio Next Brinch Hanson developed the HRRN It correct some of the weaknesses in SJF HRRN Particularly the excessive bias against longer processes and the excessive favoritism toward short processes. HRRN is a non preemptive scheduling In HRRN, process priority is a function not only of its service time but also of its time spend waiting for service. Jobs gain higher priority the longer they wait, which prevents indefinite postponement (process starvation). 37

38 HRRN HRRN calculates dynamic priorities accordingg to the following formula, Service time appears in denominator, shorter processes receive preference. However, because the waiting time appear in numerator, longer processes that have been waiting will also be given favorable treatment. This technique prevents the scheduler from indefinitely postponing processes. 38

39 HRRN Example Processes Service time Waiting time P1 5sec 20sec P2 3sec 9sec In this case process P1 has a priority 5 and Process P2 has 4, so the system executes P1 before. 39

40 Quiz With HRRN, are short processes always scheduled before long ones??? False: The longer a process waits, the more likely it will be Scheduled before shorter ones. 40

41 Multilevel Queue In multilevel queue scheduling we assign a process to a queue and it remains in that queue until the process is allowed access to the CPU. Consider processes with different CPU burst characteristics. If a process uses too much of the CPU it will be moved to a lower priority queue. This will leave I/O bound and (fast) interactive processes in the higher priority queue(s). 41

42 MLFQ Assume we have three queues (Q0, Q1 and Q2). Q0 is the highest priority queue and Q2 is the lowest priority queue. The scheduler first executes process in Q0 and only considers Q1 and Q2 when Q0 is empty. Whilst running processes in Q1, if a new process arrived in Q0, then the currently running process is preempted so that the Q0 process can be serviced. 42

43 Any job arriving is put into Q0. MLFQ When it runs, it does so with a quantum of 8ms (say). If the process does not complete, it is preempted and placed at the end of the Q1 queue. This queue (Q1) has a time quantum of 16ms associated with it. Any processes not finishing in this time are demoted to Q2, with these processes being executed on a FCFS basis. 43

44 MLFQ The above description means that any jobs that require less than 8ms of the CPU are serviced very quickly. Any processes that require between 8ms and 24ms are also serviced fairly quickly. Any jobs that need more than 24ms are executed with any spare CPU capacity once Q0 and Q1 processes have been serviced. 44

45 Multilevel Feedback Queue A process can move between the various queues; aging can be implemented this way Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when that process needs service

46 Example of Multilevel Feedback Queue Three queues: Q 0 RR with time quantum 8 milliseconds Q 1 RR time quantum 16 milliseconds Q 2 FCFS

47 Multilevel Feedback Queues

48 Overview Summary Scheduling algorithm CPU Overhead Throughput Turnaround time Response time First In First Out Shortest Job First Priority based scheduling Low Low High Low Medium High Medium Medium Medium Low High High Round-robin scheduling High Medium Medium High Multilevel Queue scheduling High High Medium Medium 48

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