1. Chapter 5: CPU Scheduling

2. 5.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling Operating Systems Examples Java Thread Scheduling Algorithm Evaluation

3. 5.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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. 5.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Alternating Sequence of CPU And I/O Bursts

5. 5.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Histogram of CPU-burst Times

6. 5.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Bursts of CPU usage alternate with periods of waiting for I/O Bursts of CPU usage alternate with periods of waiting for I/O (a) A CPU-bound process. (b) An I/O-bound process.

7. 5.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 nonpreemptive (non harming scheduling) All other scheduling is preemptive (harming scheduling)

8. 5.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

9. 5.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 (t exec +t wait = t turnaraound ) 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)

10. 5.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Optimization Criteria Max CPU utilization : 100% cpu utilization is perfect Max throughput : Max number of processes completed / time unit Min turnaround time : min average time elapsed from when process is submitted to when it has completed. Min waiting time : min average time a process spends in the run queue. Min response time : min Average time elapsed from when process is submitted until useful output is obtained.

11. 5.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Types of Schedulers First-Come, First-Served (FCFS) Round-Robin (RR) Shortest-Job-First (SJF) Priority Scheduling (PS)

12. 5.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts First-Come, First-Served (FCFS) Scheduling ProcessBurst 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: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 P1P1 P2P2 P3P3 2427300

13. 5.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts FCFS Scheduling (Cont.) 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: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect short process behind long process P1P1 P3P3 P2P2 63300

14. 5.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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, preempt. 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

15. 5.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts ProcessArrival TimeBurst Time P 1 0.07 P 2 2.04 P 3 4.01 P 4 5.04 SJF (non-preemptive) Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Example of Non-Preemptive SJF P1P1 P3P3 P2P2 73160 P4P4 812

16. 5.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Example of Preemptive SJF ProcessArrival TimeBurst Time P 1 0.07 P 2 2.04 P 3 4.01 P 4 5.04 SJF (preemptive) Average waiting time = (9 + 1 + 0 +2)/4 = 3 P1P1 P3P3 P2P2 42 11 0 P4P4 57 P2P2 P1P1 16

17. 5.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Determining Length of Next CPU Burst (Exponential Average) Can only estimate the length Can be done by using the length of previous CPU bursts, using exponential averaging

18. 5.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Prediction of the Length of the Next CPU Burst (exponential average) dg alpha 0.5

19. 5.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Examples of Exponential Averaging  =0  n+1 =  n Recent history does not count  =1  n+1 =  t n Only the actual last CPU burst counts If we expand the formula, we get:  n+1 =  t n +(1 -  )  t n -1 + … +(1 -  ) j  t n -j + … +(1 -  ) n +1  0 Since both  and (1 -  ) are less than or equal to 1, each successive term has less weight than its predecessor

20. 5.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 SJF is a priority scheduling where priority is the predicted next CPU burst time Same / static priority assignment cause problem called : starvation priority 1 priority 2 priority M

21. 5.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Priority Queuing 21

22. 5.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Starvation n Problem: l Lower-priority may suffer starvation if there is a steady supply of high priority processes. n Solution l Allow a process to change its priority based on its age or execution history (aging)

23. 5.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Round Robin (RR) Each process gets a small unit of CPU time (time quantum), 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. E.g, with five processes and a time quantum of 20 milliseconds, each process will get up to 20 milliseconds every 100 milliseconds. Performance q large  FIFO q small  q must be large with respect to context switch, otherwise overhead is too high

24. 5.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Example of RR with Time Quantum = 20 ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: Typically, higher average turnaround than SJF, but better response P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P3 02037577797117121134154162

25. 5.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Time Quantum and Context Switch Time The way in which a smaller time quantum increases context switches.

26. 5.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Turnaround Time Varies With The Time Quantum

27. 5.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground – RR background – 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

28. 5.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multilevel Queue Scheduling

29. 5.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

30. 5.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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 Scheduling A new job enters queue Q 0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

31. 5.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multilevel Feedback Queues

32. 5.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Load sharing Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing

33. 5.33 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 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

34. 5.34 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Scheduling 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

35. 5.35 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthread Scheduling API #include #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; pthread t tid[NUM THREADS]; pthread attr t attr; /* get the default attributes */ pthread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,NULL);

36. 5.36 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthread Scheduling API /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { printf("I am a thread\n"); pthread exit(0); }

37. 5.37 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Operating System Examples Solaris scheduling Windows XP scheduling Linux scheduling

38. 5.38 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Solaris 2 Scheduling

39. 5.39 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Solaris Dispatch Table

40. 5.40 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Windows XP Priorities

41. 5.41 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Linux Scheduling Two algorithms: time-sharing and real-time Time-sharing Prioritized credit-based – process with most credits is scheduled next Credit subtracted when timer interrupt occurs When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs  Based on factors including priority and history Real-time Soft real-time Posix.1b compliant – two classes  FCFS and RR  Highest priority process always runs first

42. 5.42 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts The Relationship Between Priorities and Time-slice length

43. 5.43 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts List of Tasks Indexed According to Prorities

44. 5.44 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Algorithm Evaluation Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload Queueing models Implementation

45. 5.45 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 5.15

46. 5.46 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Tugas Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum = 2) Gambar Gantt Chart dan hitung waiting time perproses dan average waiting time Process Arrival Time Burst Time P1 0.0 8 P2 2 4 P3 1.0 1 P4 2 3 P5 3 2

47. 5.47 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kerjakan dengan SJF (preeptive dan non Preemptive), FCFS, RR (gunakan time quantum = 3) Process Arrival Time Burst Tim P1 0.0 8 P2 2 4 P3 1.0 1 P4 1.0 5 Gambar Gantt Chart dan hitung waiting time perproses dan average waiting time

48. End of Chapter 5

49. 5.49 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 5.08

50. 5.50 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts In-5.7

51. 5.51 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts In-5.8

52. 5.52 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts In-5.9

53. 5.53 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Dispatch Latency

54. 5.54 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread Scheduling JVM Uses a Preemptive, Priority-Based Scheduling Algorithm FIFO Queue is Used if There Are Multiple Threads With the Same Priority

55. 5.55 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: 1. The Currently Running Thread Exits the Runnable State 2. A Higher Priority Thread Enters the Runnable State * Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not

56. 5.56 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Time-Slicing Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // perform CPU-intensive task... Thread.yield(); } This Yields Control to Another Thread of Equal Priority

57. 5.57 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Priorities PriorityComment Thread.MIN_PRIORITYMinimum Thread Priority Thread.MAX_PRIORITYMaximum Thread Priority Thread.NORM_PRIORITYDefault Thread Priority Priorities May Be Set Using setPriority() method: setPriority(Thread.NORM_PRIORITY + 2);

58. 5.58 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 5.1 A CPU scheduling algorithm determines an order for the execution of its scheduled processes. Given n processes to be scheduled on one processor, how many possible different schedules are there? Give a formula in terms of n. 5.2 Define the difference between preemptive and nonpreemptive scheduling.

59. 5.59 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 5.3 Suppose that the following processes arrive for execution at the times indicated. Each process will run the listed amount of time. In answering the questions, use nonpreemptive scheduling and base all decisions on the information you have at the time the decision must be made. Process Arrival Time Burst Time P1 0.0 8 P2 0.4 4 P3 1.0 1 a. What is the average turnaround time for these processes with the FCFS scheduling algorithm? b. What is the average turnaround time for these processes with the SJF scheduling algorithm?

60. 5.60 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Process Arrival Time Burst Time P1 0.0 8 P2 2 4 P3 1.0 1 P4 2 3 P5 3 2

61. 5.61 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts c. The SJF algorithm is supposed to improve performance, but notice that we chose to run process P1 at time 0 because we did not know that two shorter processes would arrive soon. Compute what the average turnaround timewill be if the CPU is left idle for the first 1 unit and then SJF scheduling is used. Remember that processes P1 and P2 are waiting during this idle time, so their waiting time may increase. This algorithm could be known as future-knowledge scheduling. 5.4 What advantage is there in having different time- quantum sizes on different levels of a multilevel queueing system?

62. 5.62 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts 5.5 Many CPU-scheduling algorithms are parameterized. For example, : the RR algorithm requires a parameter to indicate the time slice. Multilevel feedback queues require parameters to define the number of queues, the scheduling algorithms for each queue, the criteria used to move processes between queues, and so on. These algorithms are thus really sets of algorithms (for example, the set of RR algorithms for all time slices, and so on). One set of algorithms may include another (for example, the FCFS algorithm is the RR algorithm with an infinite time quantum).

63. 5.63 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts What (if any) relation holds between the following pairs of sets of algorithms? a. Priority and SJF b. Multilevel feedback queues and FCFS c. Priority and FCFS d. RR and SJF 5.6 Suppose that a scheduling algorithm (at the level of short-term CPU scheduling) favors those 9

64. 5.64 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSys tems/5_CPU_Scheduling.html http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSys tems/5_CPU_Scheduling.html http://siber.cankaya.edu.tr/OperatingSystems/ceng328/ node122.html http://siber.cankaya.edu.tr/OperatingSystems/ceng328/ node122.html http://groups.engin.umd.umich.edu/CIS/course.des/cis4 50/mcfadyen/chapter5.htm http://groups.engin.umd.umich.edu/CIS/course.des/cis4 50/mcfadyen/chapter5.htm http://siber.cankaya.edu.tr/OperatingSystems/week5/no de21.html