1 / 61

Operating System Concepts

Operating System Concepts. Ku-Yaw Chang canseco@mail.dyu.edu.tw Assistant Professor, Department of Computer Science and Information Engineering Da-Yeh University. Chapter 6 CPU Scheduling. CPU scheduling The basis of multiprogrammed OSs Make the computer more production

ady
Download Presentation

Operating System Concepts

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Operating System Concepts Ku-Yaw Chang canseco@mail.dyu.edu.tw Assistant Professor, Department of Computer Science and Information Engineering Da-Yeh University

  2. Chapter 6 CPU Scheduling • CPU scheduling • The basis of multiprogrammed OSs • Make the computer more production • Switch the CPU among processes • We introduce • The basic scheduling concepts • Several different CPU-scheduling algorithms • Selecting an algorithm for a particular system Chapter 6 CPU Scheduling

  3. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  4. 6.1 Basic Concepts • Several processes are kept in memory at one time • When a process has to wait, the OS takes the CPU away from that process, and gives the CPU to another process. • Almost all computer resources are scheduled before use. Chapter 6 CPU Scheduling

  5. 6.1.1 CPU-I/O Burst Cycle • Observed property of processes • Process execution consists of a cycle of • CPU execution • I/O wait • Process execution begins with a CPU burst. • That is followed by an I/O burst, then another CPU burst, then another I/O burst, and so on. Chapter 6 CPU Scheduling

  6. Alternating sequence ofCPU and I/O bursts Chapter 6 CPU Scheduling

  7. Histogram of CPU-burst times Chapter 6 CPU Scheduling

  8. 6.1.1 CPU-I/O Burst Cycle • Measure CPU bursts • An I/O bound program • Many short CPU bursts • A CPU bound program • A few very long CPU bursts • Help select an appropriate CPU-scheduling algorithm Chapter 6 CPU Scheduling

  9. 6.1.2 CPU Scheduler • Whenever the CPU becomes idle • Select one of the processes in the ready queue to be executed • Carried out by the short-term scheduler, also called CPU scheduler • A ready queue may be implemented as • A FIFO queue • A priority queue • A tree • An unordered linked list Chapter 6 CPU Scheduling

  10. 6.1.3 Preemptive Scheduling • CPU scheduling decisions take place when a process • Switches from running to waiting state • Switches from running to ready state • Switches from waiting to ready • Terminates • Scheduling only under 1 and 4 is nonpreemptive • Otherwise is preemptive • Incur a cost Chapter 6 CPU Scheduling

  11. 6.1.4 Dispatcher • Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; • 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. Chapter 6 CPU Scheduling

  12. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  13. 6.2 Scheduling Criteria • CPU utilization (max) • keep the CPU as busy as possible • Throughput (max) • number of processes that complete their execution per time unit • Turnaround time(min) • amount of time to execute a particular process • Waiting time(min) • amount of time a process has been waiting in the ready queue • Response time(min) • amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Chapter 6 CPU Scheduling

  14. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  15. 6.3 Scheduling Algorithms • Dealing the problem of deciding which of the processes in the ready queue to be allocated the CPU • First-Come, First-Served Scheduling • Shortest-Job-First Scheduling • Priority Scheduling • Round-Robin Scheduling • Multilevel Queue Scheduling • Multilevel Feedback Queue Scheduling Chapter 6 CPU Scheduling

  16. 6.3.1 First-Come, First-Served Scheduling • The process that requests the CPU first is allocated the CPU first. • The simplest CPU-scheduling algorithm • Implementation with a FIFO queue • Add to the tail of the queue • Remove from the head to the queue • The code is simple to write and understand • Average waiting time is often quite long. Chapter 6 CPU Scheduling

  17. P1 P2 P3 0 24 27 30 6.3.1 First-Come, First-Served Scheduling ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 Chapter 6 CPU Scheduling

  18. P2 P3 P1 0 3 6 30 6.3.1 First-Come, First-Served Scheduling 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. Chapter 6 CPU Scheduling

  19. 6.3.1 First-Come, First-Served Scheduling • Convoy effect • All the other processes wait for one big process to get off the CPU • Results in lower CPU and device utilization • If the shorter processes were allowed to go first • FCFS is non-preemptive • Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU. • Particular troublesome in time-sharing system • Each user needs to get a share of the CPU at regular intervals Chapter 6 CPU Scheduling

  20. 6.3.2 Shortest-Job-First Scheduling • When the CPU is available, it is assigned to the process that has the smallest next CPU burst. • Associate each process with the length of the latter’s next CPU burst • Not its total length • Another term – shortest next CPU burst • FCFS scheduling is used to break the tie • Provably optimal • Minimum average waiting time for a given set of processes • Real difficulty • Knowing the length of the next CPU burst • Used frequently in long-term scheduling Chapter 6 CPU Scheduling

  21. P4 P1 P3 P2 0 3 9 16 24 6.3.2 Shortest-Job-First Scheduling ProcessBurst Time P1 6 P2 8 P3 7 P4 3 The Gantt Chart for the schedule is: • Waiting time for P1 = 3; P2 = 16; P3 = 9; P4 = 0 • Average waiting time: (3 + 16 + 9 + 0) / 4 = 7 • Using FCFS scheme : ( 0 + 6 + 14 + 21) / 4 = 10.25 Chapter 6 CPU Scheduling

  22. 6.3.2 Shortest-Job-First Scheduling • To approximate SJF scheduling • To predict its value • Use the length of previous CPU bursts • exponential average Chapter 6 CPU Scheduling

  23. Prediction of the length of the next CPU burst Chapter 6 CPU Scheduling

  24. Examples ofExponential 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: n+1 =  tn+(1 - )  tn -1 + … +(1 -  )j  tn -1 + … +(1 -  )n+1 tn 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor. Chapter 6 CPU Scheduling

  25. 6.3.2 Shortest-Job-First Scheduling • Two schemes • Nonpreemptive • Allow the current running process to finish its CPU burst • Preemptive • If a new process arrives with CPU burst length less than remaining time of current executing process, preempt. • Called as Shortest-Remaining-Time-First (SRTF). Chapter 6 CPU Scheduling

  26. P1 P2 P4 P1 P3 0 1 5 10 17 26 Example of Preemptive SJF Process Arrival TimeBurst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 • Preemptive SJF • Average waiting time = ( (10-1) + (1-1) + (17-2) + (5-3)) / 4 = 6.5 Chapter 6 CPU Scheduling

  27. 6.3.3 Priority Scheduling • The CPU is allocated to the process with the highest priority. • A priority is associated with each process • Fixed range of number, such as 0 to 7 • We use low numbers to represent high priority • Equal-priority processes are scheduled in FCFS order • SJF is a special case of the general priority-scheduling algorithm • The priority is the inverse of the predicted next CPU burst Chapter 6 CPU Scheduling

  28. P2 P5 P1 P3 P4 0 1 6 16 18 19 Example of priority scheduling ProcessBurst TimePriority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 • Priority scheduling • Average waiting time = ( 6 + 0 + 16 + 18 + 1 ) / 5 = 8.2 Chapter 6 CPU Scheduling

  29. 6.3.3 Priority Scheduling • Priorities can be defined • Internally • Use some measurable quantity • Time limits, memory requirements… • Externally • Set by criteria external to OS • importance, political factors • Priority scheduling can be • Preemptive • Nonpreemptive • Major problem • Indefinite blocking or starvation • Solution: aging • Gradually increase the priority of processes that wait for a long time Chapter 6 CPU Scheduling

  30. 6.3.4 Round-Robin Scheduling • A small unit of time, called a time quantum ( or time slice) is defined. • Generally from 10 to 100 milliseconds • The ready queue is treated as a circular, FIFO queue. • The CPU scheduler goes around the ready queue • Allocate the CPU to each process for a time interval of up to 1 time quantum. • Designed especially for time-sharing systems • Two cases • CPU burst less than 1 time quantum • The process release the CPU voluntarily • CPU burst longer than 1 time quantum • Context switch will be executed Chapter 6 CPU Scheduling

  31. P1 P1 P2 P3 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30 Example of round-robin scheduling ProcessBurst Time P1 24 P2 3 P3 3 • Round-robin scheduling • A time-quantum of 4 milliseconds • Average waiting time = ( 6 + 4 + 7 ) / 3 = 5.66 • Often quite long • RR scheduling is preemptive. Chapter 6 CPU Scheduling

  32. 6.3.4 Round-Robin Scheduling • n processes in the ready queue and the time quantum is q • 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 • Depend heavily on the size of the time quantum • q is very large : the same as the FCFS policy • q is very small : called processor sharing • q must be large with respect to context switch, otherwise overhead is too high. Chapter 6 CPU Scheduling

  33. A smaller time quantumincreases context switches Chapter 6 CPU Scheduling

  34. 6.3.4 Round-Robin Scheduling • Turnaround time also depends on the size of the time quantum. • Rule of thumb • 80 percent of the CPU burst shouldbe shorter thanthe time quantum Chapter 6 CPU Scheduling

  35. 6.3.5 Multilevel Queue Scheduling • Processes are classified into different groups • Foreground (or interactive) processes • Background (or batch) processes • Multilevel queue-scheduling algorithm • Partition the ready queue into several separate groups • Each group has its own scheduling algorithm • Scheduling among the queues • Fixed-priority preemptive scheduling • Possibility of starvation • Time-slice between the queues • A certain portion of the CPU time Chapter 6 CPU Scheduling

  36. Multilevel Queue Scheduling Chapter 6 CPU Scheduling

  37. 6.3.6 Multilevel Feedback Queue • A process can move between the various queues • Use too much CPU time : move to a lower-priority queue • Wait too long : move to a higher-priority queue • This form of aging prevents starvation • Multilevel-feedback-queue scheduler defined by : • 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 Chapter 6 CPU Scheduling

  38. Example ofMultilevel 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. Chapter 6 CPU Scheduling

  39. Multilevel feedback queues • The most general and complex scheme Q0 Q1 Q2 Chapter 6 CPU Scheduling

  40. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  41. 6.4 Multiple-Processor Scheduling • CPU scheduling is more complex when multiple CPUs are available • Homogeneous system • Identical processors • Load sharing can occur • Separate queue • Common queue • Self-scheduling • Master-slave structure • Heterogeneous system • Different processors Chapter 6 CPU Scheduling

  42. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  43. 6.5 Real-Time Scheduling • Hard real-time systems • complete a critical task within a guaranteed amount of time. • Special-purpose software running on dedicated hardware • Lack the full functionality of modern computers and operating systems • Soft real-time computing • Critical processes receive priority over less fortunate ones • May cause unfair allocation of resources • A general-purpose system can also support special tasks Chapter 6 CPU Scheduling

  44. 6.5 Real-Time Scheduling • Soft real-time computing • Priority scheduling • Real-time process must have the highest priority • The dispatch latency must be small • OSs are forced to wait for a system call to complete or an I/O block to take place before doing a context switch • Solution: system calls must be preemptible • Insert preemption points • Make the entire kernel preemptible • Effective and complex method • Solaris 2 • Over 100 ms v.s. 2 ms Chapter 6 CPU Scheduling

  45. Dispatch Latency Chapter 6 CPU Scheduling

  46. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Process Scheduling Models Summary Exercises Chapter 6 CPU Scheduling Chapter 6 CPU Scheduling

  47. 6.6 Algorithm Evaluation • How to select a CPU-scheduling algorithm for a particular system? • Define the criteria • CPU utilization • Response time • Throughput • A criteria may include several measures • Maximize CPU utilization under the constraint that the maximum response time is 1 second. • Maximize throughput such that turnaround time is linearly proportional to total execution time. Chapter 6 CPU Scheduling

  48. 6.6.1 Deterministic Modeling • Take a particular predetermined workload and defines the performance of each algorithm for that workload • Example • Assume five processes arrive at time 0, in the order given, with the length of the CPU-burst time given in milliseconds: ProcessBurst TimePriority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 • Consider the FCFS, SJF, and RR(quantum=10 milliseconds) scheduling algorithms for this set of processes. • Which gives the minimum average waiting time? Chapter 6 CPU Scheduling

  49. 6.6.1 Deterministic Modeling • Advantage • Simple and fast • Exact numbers • Disadvantage • Too specific, and require too much exact knowledge, to be useful • Main use • Describing scheduling algorithms and providing examples Chapter 6 CPU Scheduling

  50. 6.6.2 Queueing Models • No static set of processes (and times) • What can be determined • The distribution of CPU and I/O bursts • Measured and approximated by mathematical formulas • Queueing-network analysis • CPU is a server with its ready queue • I/O system is a server with its device queue • Knowing arrival rates and service rates • Utilization, average queue length, average wait time Chapter 6 CPU Scheduling

More Related