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Chapter 5: CPU Scheduling

Chapter 5: CPU Scheduling. 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. Alternating Sequence of CPU And I/O Bursts. Histogram of CPU-burst Times.

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Chapter 5: CPU Scheduling

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  1. Chapter 5: CPU Scheduling

  2. 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

  3. Alternating Sequence of CPU And I/O Bursts

  4. Histogram of CPU-burst Times

  5. 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 • Terminates • Scheduling only under 1 and 4 is nonpreemptive • All other scheduling is preemptive

  6. 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

  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 • 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)

  8. Scheduling Algorithm Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time • In most cases we optimize the average measure • In some circumstances we want to optimize the min or max values rather than the average – e.g., minimize the max response time so all users get good service

  9. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling • 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 • Response time in FCFS is same as Waiting Time so Average Response time: 17

  10. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Turnaround time for P1 = 24; P2 = 27; P3 = 30 • Average turnaround time: (24 + 27+ 30)/3 = 27 • Throughput: 3/30 = 0.1

  11. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont) 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 : short process wait for one long CPU-bound process to complete a CPU burst before they get a turn

  12. 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

  13. P3 P2 P4 P1 3 9 16 24 0 Example of Non-Preemptive SJF • SJF scheduling chart • Throughput = 4/24 • Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 • Average response time = (3 + 16 + 9 + 0) / 4 = 7 • Average turnaround time = (9 + 24 + 16 + 3) / 4 = 13

  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

  15. Prediction of the Length of the Next CPU Burst

  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: n+1 =  tn+(1 - ) tn-1+ … +(1 -  )j tn-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

  17. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF • The Gantt chart for preemptive SJF is: • Throughput = 4 /16 = 0.25 • Average waiting time = (9 + 1 + 0 +2)/4 = 3 • Average response time = ( 0 + 0 + 0 + 2) / 4 = 0.5 • Average turnaround time = ( 16 + 5 + 1 + 6) / 4 = 7

  18. 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 Problem  Starvation – low priority processes may never execute Solution  Aging – as time progresses increase the priority of the process

  19. Example of Priority Scheduling (nonpreemptive) • The Gantt chart is (all processes arrive at time 0): • Average waiting time = (6 + 0 + 16 +18+1)/5 = 8.2 P2 P5 P1 P3 P4 16 18 19 0 1 6

  20. 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. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high • TA = Completion time – arrival time • Waiting Time = TA – Burst time

  21. P1 P2 P3 P1 P1 P1 P1 P1 0 10 14 18 22 26 30 4 7 Example of RR with Time Quantum = 4 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response

  22. Time Quantum and Context Switch Time

  23. Turnaround Time Varies With The Time Quantum

  24. Multilevel Queue • Ready queue is partitioned into separate queues, for example: foreground (interactive) background (batch) • Each queue has its own scheduling algorithm • foreground – RR • background – FCFS • Scheduling must be done between the queues • Commonly a fixed priority preemptive scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. • Another solution: 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

  25. Multilevel Queue Scheduling

  26. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented this way • Example, three queues: • Q0 – RR with time quantum 8 milliseconds • Q1 – RR time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedRR. 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 RR and receives 16 additional milliseconds. If it still does not complete (in 24ms) , it is preempted and moved to queue Q2. • At Q2 job is served FCFS

  27. Multilevel Feedback Queues Q0 Q1 Q2

  28. Multilevel Feedback Queue • 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

  29. Solaris Scheduling • Priority based scheduling and where, each thread belongs to a class (from 6) • Time sharing (TS) • Interactive (IA) • Real time (RT) • System (SYS) • Fair share (FSS) • Fixed priority (FP)

  30. Solaris Scheduling

  31. Solaris Dispatch Table for TS and IA Priority if:

  32. Windows XP Priorities • Priority based, preemptive scheduling algorithm • 32 priorities are divided into two classes • Variable class – 1 through 15 and Real time class – 16 through 31 • Priority is based on both the priority class it belongs to (realtime, high, etc) and the relative priority within each class • When a threads quantum runs out, the thread is interrupted. If it is in the variable priority class its priority is lowered (never below the base priority for that class) • Gives good response to interactive threads (mouse, windows) and enables I/O bound threads to keep the I/O devices busy while compute bound use spare CPU cycles in the background

  33. Linux Scheduling • Constant order O(1) scheduling time • Preemptive, priority-based with two priority ranges: time-sharing and real-time • Real-time range from 0 to 99 and time sharing value from 100 to 140 • Higher-priority tasks got longer time quanta

  34. Priorities and Time-slice length

  35. List of Tasks Indexed According to Priorities • When all tasks have exhausted their time slices, the two priority arrays are exchanged

  36. End of Chapter 5

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