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Chapter 6: CPU Scheduling

Chapter 6: CPU Scheduling. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation. Basic Concepts. Maximum CPU utilization obtained with multiprogramming

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Chapter 6: CPU Scheduling

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  1. Chapter 6: CPU Scheduling • Basic Concepts • Scheduling Criteria • Scheduling Algorithms • Multiple-Processor Scheduling • Real-Time Scheduling • Algorithm Evaluation Operating System Concepts

  2. Basic Concepts • Maximum CPU utilization obtained with multiprogramming • CPU–I/O Burst(猝发)Cycle – Process execution consists of a cycle of CPU execution and I/O wait. • CPU burst distribution Operating System Concepts

  3. Alternating Sequence of CPU And I/O Bursts Operating System Concepts

  4. Histogram of CPU-burst Times Statistically:Many short CPU bursts and a few long CPU bursts Operating System Concepts

  5. CPU Scheduler(调度) • 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, e. g., I/O request, wait for an event to occur(child completion,system object: semaphore,message queue, socket …) 2. Switches from running to ready state,e.g., interrupt 3. Switches from waiting to ready. 4. Terminates. • Scheduling under 1 and 4 is nonpreemptive.(非抢占) • All other scheduling is preemptive.(抢占) • Preemptive scheduling is troublesome. Operating System Concepts

  6. Dispatcher(指派) • 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. Operating System Concepts

  7. 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(from submission to completion) • Waiting time – amount of time a process has been waiting in theready 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) Operating System Concepts

  8. Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time Operating System Concepts

  9. First-Come, First-Served (FCFS) Scheduling P1 P2 P3 0 24 27 30 ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3The Gantt Chart (甘特图, A chart that depicts progress in relation to time, often used in planning and tracking a project.)for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts

  10. FCFS Scheduling (Cont.) P2 P3 P1 0 3 6 30 Suppose that the processes arrive in the order P2 , P3 , P1 . • The Gantt chart for the schedule is: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case. • Convoy effect(传递效应)：if shorter processes are allowed to go first, the CPU and device utilization may improve. Operating System Concepts

  11. 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. • 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, the former preempts the latter. This scheme is know as the Shortest-Remaining-Time-First (SRTF). • SJF is optimal – gives minimum average waiting time for a given set of processes. Operating System Concepts

  12. Example of Non-Preemptive SJF P1 P3 P2 P4 0 3 7 8 12 16 Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) • Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Operating System Concepts

  13. Example of Preemptive SJF P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time = (9 + 1 + 0 +2)/4 = 3 Operating System Concepts

  14. Determining Length of Next CPU Burst • Can only estimate the length. • Can be done by using the length of previous CPU bursts, using exponential averaging(指数平均法). Operating System Concepts

  15. Prediction of the Length of the Next CPU Burst =1/2, 0=10 Operating System Concepts

  16. Examples of Exponential Averaging •  =0 • n+1 = n • Recent history does not count. •  =1 • n+1 = tn • Only the actual last CPU burst counts. • If we expand the formula, we get: • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor. Operating System Concepts

  17. Priority Scheduling • A priority number (integer) is associated with each process • The CPU is allocated to the process with the highest priority (in our examples, smallest integer is the highest priority). • Preemptive • nonpreemptive • 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 the priority of the processes change, e.g. increase.(dynamic priority) Operating System Concepts

  18. P2 P5 P1 0 1 6 16 Example of Nonpreemptive priority-based scheduling ProcessBurst Time Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 • All processes arrived at time 0 • Average waiting time = (6+0 + 16 + 18 +1)/5 = 8.2 P3 P4 19 18 Operating System Concepts

  19. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum,time slice), usually 10-100 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. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high. Operating System Concepts

  20. Example of RR with Time Quantum = 1 Service Time Arrival Time Process 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2 Operating System Concepts

  21. Example of RR with Time Quantum = 20 P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response. Operating System Concepts

  22. Time Quantum and Context Switch Time Operating System Concepts

  23. Turnaround Time Varies With The Time Quantum Operating System Concepts

  24. Multilevel Queue • Ready queue is partitioned into separate queues,e.g.:foreground (interactive)background (batch) • A process is permanently assigned to one queue • Each queue has its own scheduling algorithm,e.g.: foreground – RRbackground – FCFS • 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 RR, 20% to background in FCFS Operating System Concepts

  25. Multilevel Queue Scheduling Operating System Concepts

  26. 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 Operating System Concepts

  27. Example of Multilevel Feedback Queue • Three queues: • Q0 – time quantum 8 milliseconds • Q1 – time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. Operating System Concepts

  28. Multilevel Feedback Queues Operating System Concepts

  29. Multiple-Processor Scheduling • CPU scheduling more complex when multiple CPUs are available. • Homogeneous(identical) processors within a multiprocessor. • Load sharing • Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing. Operating System Concepts

  30. 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. • Soft real-time scheduling • Real-time processes must gain priority to common processes • Dispatch latency must be small enough • Nonpreemptible execution of kernel codes prevents the system from being real-time Operating System Concepts

  31. Dispatch Latency Operating System Concepts

  32. Algorithm Evaluation • Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload. FCFS, SJF, RR • Queueing models: queueing theory(排队论) • Implementation Queue length: n Arrival rate: Operating System Concepts

  33. Evaluation of CPU Schedulers by Simulation Operating System Concepts

  34. Scheduling models • Kernel level thread scheduling – system global scheduling • User level thread scheduling – process local scheduling • Scheduling in Operating system design is always referred to system global scheduling Operating System Concepts

  35. Linux Process Scheduling • Linux uses two process-scheduling algorithms: • A time-sharing algorithm for fair preemptive scheduling between multiple processes • A real-time algorithm for tasks where absolute priorities are more important than fairness • A process’s scheduling class defines which algorithm to apply. • For time-sharing processes, Linux uses a prioritized, credit based algorithm. • The crediting rulefactors in both the process’s history and its priority. • This crediting system automatically prioritizes interactive or I/O-bound processes. Operating System Concepts

  36. Linux Process Scheduling (Cont.) • Linux implements the FIFO and round-robin real-time scheduling classes; in both cases, each process has a priority in addition to its scheduling class. • The scheduler runs the process with the highest priority; for equal-priority processes, it runs the longest-waiting one • FIFO processes continue to run until they either exit or block • A round-robin process will be preempted after a while and moved to the end of the scheduling queue, so that round-robing processes of equal priority automatically time-share between themselves. Operating System Concepts

  37. Solaris 2 Scheduling Operating System Concepts

  38. Solaris 2 Scheduling • Real-time threads: fixed priority,no time slice • System service threads: fixed priority,no time slice • Time-sharing threads:default, dynamic priority and time slice(fair share scheduling), RR. • Interactive threads:windowing application threads, dynamic priority, RR • A class includes a set of priorities • class-specific priority vs global priority: Operating System Concepts

  39. Windows 2000 Priorities Operating System Concepts

  40. exercises • 6.1 • 6.3 a. FCFS:0-P1-10-P2-11-P3-13-P4-14-P5-19 SJF:0-P2-1-P4-2-P3-4-P5-9-P1-19 PRI:0-P2-1-P5-6-P1-16-P3-18-P4-19 RR:0-P1-1-P2-2-P3-3-P4-4-P5-5-P1-6-P3-7-P5-8-P1-9-P5-10-P1-11-P5-12-P1-13-P5-14-P1-19 • 6.4 a. FCFS: 0.0-P1-8-P2-12-P3-13 (8+12-0.4+13-1.0)/3=31.6/3 b. SJF: 0.0-P1-8-P3-9-P2-13 (8+9-1.0+13-0.4)/3 = 28.6/3 c. 0-1-P3-2-P2-6-P1-14 (14+6-0.4+2-1)/ 3 = 20.6 /3 Operating System Concepts

  41. exercises 6.5 在ready queue中的每个节点存放指向PCB的指针 a. 两个节点指针指向同一个PCB:该进程获得两倍的时间片 b. 好处：可以获得更多的优先级 坏处：不公平 c. 改进原始RR算法：每个进程的时间片不等，或一个进程进入就绪队列时重复添加 6.8 各调度算法之间的关系 a. Priority and SJF：最短作业时间作为优先级 b. Multilevel feedback queue and FCFS：level数为1即为FCFS c. Priority and FCFS：作业到达时间作为优先级 d. RR and SJF：时间片无穷大，作业到达就绪队列时不放在末尾，而是按作业剩余时间升序插入相应位置 Operating System Concepts

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