1 张惠娟 副教授 实用操作系统概念

2 内容框架 概述 体系结构 进程管理 内存管理 文件管理 外设管理

3 内容 Ch4: 进程 Ch5: 线程 Ch6: CPU 调度 Ch7 ：进程同步 Ch8 ：死锁

4 Ch 6: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling Algorithm Evaluation

5 CPU 和 I/O CPU Scheduler Dispatcher Basic Concepts

6  CPU 和 I/O Maximum CPU utilization obtained with multiprogramming CPU – I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait.

7 Alternating Sequence of CPU And I/O Bursts

8  CPU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. Basic Concepts

9 CPU scheduling decisions may take place when a process:  Switches from running to waiting state.  Switches from running to ready state.  Switches from waiting to ready.  Terminates. Basic Concepts

10  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. Basic Concepts

11 General Criteria Optimization Criteria Scheduling Criteria

12  General Criteria CPU utilization keep the CPU as busy as possible Throughput processes that complete their execution per time unit Turnaround time amount of time to execute a particular process Scheduling Criteria

13 Scheduling Criteria Waiting time amount of time a process has been wiating 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  Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Scheduling Criteria

15 Scheduling Algorithms First-Come, First-Served (FCFS) Scheduling Shortest-Job-First (SJF) Scheduling Priority Scheduling Round Robin (RR) Multilevel Queue

16 FCFS Scheduling  Example ProcessBurst Time P 1 24 P 2 3 P 3 3

17 FCFS Scheduling Suppose that the processes arrive in the order: P 1,P 2, P 3 The Gantt Chart for the schedule is:  Waiting time for P 1 = 0; P 2 = 24; P 3 = 27  Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

18 FCFS Scheduling Suppose that the processes arrive in the order P 2, P 3, P 1.The Gantt chart for the schedule is:  Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3  Average waiting time: ( )/3 = 3  Much better than previous case. P1P1 P3P3 P2P

19 SJF Scheduling Definition Schemes Example

20 SJF Scheduling  Definition 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

21 SJF Scheduling  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).

22  Example of Non-Preemptive SJF Process Arrival Time Burst Time P P P P SJF Scheduling

23 SJF Scheduling 0 P1P1 P3P3 P2P P4P4 812 The Gantt chart for the schedule is Average waiting time = ( )/4 - 4

24 SJF Scheduling  Example of Preemptive SJF Process Arrival Time Burst Time P P P P

25 SJF Scheduling The Gantt chart for the schedule is Average waiting time = ( )/ P1P1 P3P3 P2P P4P4 57 P2P2 P1P1 16

26 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).  Preemptive  nonpreemptive

27  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. Priority Scheduling

28 Round Robin (RR) Definition Performance Example

29 Round Robin (RR)  Definition 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. 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.

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

31  Example ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24 Round Robin (RR)

32 The Gantt chart is: Quantum = 20 Typically, higher average turnaround than SJF, but better response. Round Robin (RR) P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

33 Multilevel Queue Definition Multilevel Queue Scheduling Multilevel Feedback Queue

34  Definition Ready queue is partitioned into separate queues:  foreground (interactive)  background (batch) Each queue has its own scheduling algorithm,  foreground – RR  background – FCFS Multilevel Queue

35 Multilevel Queue Scheduling must be done between the queues.  Fixed priority scheduling; i.e., serve all from foreground then from background. Possibility of starvation.  Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e.,80% to foreground in RR20% to background in FCFS

36 Multilevel Queue Scheduling

37 Multilevel Feedback Queues

38 Multiple-Processor Scheduling  CPU scheduling more complex when multiple CPUs are available.  Homogeneous processors within a multiprocessor.  Symmetric Multiprocessing (SMP) – each processor makes its own scheduling decisions.  Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing.

39 Real-Time Scheduling  Hard real-time systems required to complete a critical task within a guaranteed amount of time.  Soft real-time computing requires that critical processes receive priority over less fortunate ones.

40 Scheduling methods Solaris 2 Scheduling Java Thread Scheduling Thread Scheduling

41 Thread Scheduling  Scheduling methods  Local Scheduling How the threads library decides which thread to put onto an available LWP.  Global Scheduling How the kernel decides which kernel thread to run next.

42 Solaris 2 Scheduling

43 Java Thread Scheduling  JVM Uses a Preemptive, Priority-Based Scheduling Algorithm.  FIFO Queue is Used if There Are Multiple Threads With the Same Priority.  JVM Schedules a Thread to Run When:  The Currently Running Thread Exits the Runnable State.  A Higher Priority Thread Enters the Runnable State

44 Algorithm Evaluation  Deterministic modeling takes a particular predetermined workload and defines the performance of each algorithm for that workload.

45 Evaluation of CPU Schedulers by Simulation

46 小结 调度概念 几种 CPU 调度算法

47 Exercises 阅读 p135-p155 P ~6.6