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Chapter 5: CPU Scheduling

Chapter 5: CPU Scheduling. Outline. Why scheduling? Scheduling Criteria Scheduling Algorithms Real-Time Scheduling Thread Scheduling Example: Solaris Algorithm Evaluation. Process Execution. Process execution consists of alternating cycles of CPU execution and I/O wait

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Chapter 5: CPU Scheduling

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  1. Chapter 5: CPU Scheduling

  2. Outline • Why scheduling? • Scheduling Criteria • Scheduling Algorithms • Real-Time Scheduling • Thread Scheduling • Example: Solaris • Algorithm Evaluation

  3. Process Execution • Process execution consists of alternating cycles of CPU execution and I/O wait • Long-term scheduler gets a good mix of CPU-bound & I/O-bound jobs • Given that, how to scheudle ready processes? – Shor-term scheduling: focus of this chapter

  4. Why scheduling is necessary? • Maximize CPU utilization via multiprogramming, while minimizing response time and/or maximizing throughput → Efficient CPU scheduling required

  5. CPU Scheduler • Decide which process to execute next among the ready processes in memory • Order ready processes according to the specified policy such as FCFS (First Come First Serve)

  6. Preemptive vs. nonpreemptive scheduling • Preemptive scheduling: • A hihger priority process can preempt a currently running low priority process to avoid priority inversion • Most real-time scheduling algorithms, e.g., EDF & RM • More details about RT scheduling will be discussed later • Nonpreemptive scheduling: • A higher priority process waits a currently running process to finish regardless of the priority • e.g., FCFS • Others such as I/O & networking devices are not preemptive

  7. Dispatcher • Gives control of the CPU to the process selected by the short-term scheduler • Context switch between the two user processes • Context switch to user mode • Jump to the proper location in the user program to restart it → Dispatch latency : Time for dispatcher to stop one process and start another

  8. Scheduling Criteria • CPU utilization: #CPU cycles used for computation/#total CPU cycles per unit time • Throughput: # of processes that complete the execution per unit time • Response time: Amount of time it takes from when a request was submitted until the first response is produced

  9. Scheduling Criteria • Waiting time • Amount of time a process has been waiting in the ready queue • Response time = waiting time + execution time

  10. Optimization Criteria • Maximize CPU utilization • Maximize throughput • Minimize response time • Minimize waiting time • Sometimes they may conflict with each other • e.g., system can be overloaded while trying to maximize CPU utilization; response time may significantly increase as a result

  11. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling ProcessBurst Time (Execution 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

  12. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont.) • Suppose that processes arrive in the order P2 , P3 , P1 • Gantt chart: • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case • Convoy effect: A short process behind a long process suffer long wating/response time

  13. Shortest-Job-First (SJF) • Schedule the process with the shortest execution time • Two schemes: • Nonpreemptive • Once the CPU is given to a process it cannot be preempted until completes its execution • Preemptive • Shortest-Remaining-Time-First (SRTF) • If a new process arrives with exec time shorter than the remaining time of the currently running process executing process, preempt

  14. Shortest-Job-First (SJF) • SJF is optimal: It minimizes average waiting time for a given set of processes

  15. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF ProcessArrival TimeExec Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • Average waiting time = (0 + 6 + 3 + 7)/4 = 4

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

  17. Hard to implement SJF • Don’t know the exact exec time when a process arrives • Can only estimate it by using the length of previous exec time (Take moving average) • Doesn’t always work

  18. Prediction of the Length of the Next CPU Burst

  19. 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

  20. Priority Scheduling • SJF is a priority scheduling where priority is determined based on the exec time • Problem:Starvation • Low priority processes may never execute or suffer very long waiting time • Solution: Aging • As time progresses increase the priority of the process • How to support fairness while preferring high priority processes? • Hard problem

  21. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds • After the time quantum, preempt the process and add to the end of the ready queue

  22. Round Robin • 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

  23. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Example of RR with Time Quantum = 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • Gantt chart:

  24. Time Quantum and Context Switch Time

  25. Multilevel Queue • Ready queue is partitioned into separate queues, e.g., foreground (interactive) &background (batch) • Each queue can have its own scheduling algorithm • foreground – RR • background – FCFS

  26. Multilevel Queue • Scheduling must be done between the queues • Fixed priority scheduling • Serve all foreground process first • Serve background process only if the foreground queue is empty • Possibility of starvation • Time slice • Each queue gets a certain amount of CPU time which it can schedule amongst its processes, e.g., • 80% to foreground in RR • 20% to background in FCFS

  27. Multilevel Queue Scheduling

  28. Multilevel Feedback Queue • A process can move between queues • Aging can be implemented this way • Defined by the following parameters: • number of queues • scheduling algorithms for each queue • method used to determine when to demote/upgrade a process • method used to determine which queue a process will enter when that process needs service

  29. Multilevel Feedback Queues

  30. Example of Multilevel Feedback Queue • Example: • Q0– RR with time quantum 8 milliseconds • Q1– RR time quantum 16 milliseconds • Q2– FCFS • A new job enters queue Q0 . When it gets CPU, it receives 8 ms. If it does not finish in 8 milliseconds, it is moved to Q1. • At Q1 job receives 16 ms. If it does not complete within 16ms, it is preempted and moved to queue Q2.

  31. Real-Time Scheduling • Hard real-time systems • A deadline miss may cause catastrophic results, e.g., air traffic control, flight control, engine control, etc. • Soft real-time systems • A deadline miss does not cause catastrophic results • Meet as many deadlines as possible, e.g., video streaming

  32. Earliest Deadline First • Optimal Dynamic scheduling algorithm • Schedule the task with the earliest absolute deadline • Advantage: 100% utilization bound • Disadvantage: Domino effect under overload

  33. Rate Monotonic Scheduling • Optimal fixed priority scheduling algorithm • Assign the highest priority to the task with the shortest period • Advantage: Easy to implement • Disadvantage • Low utilization bound, i.e., 69% • Blind to deadlines RMS schedules T1 first T1 T2

  34. Multiple Processor Scheduling • Asymmetric multiprocessing • One processor, called master, makes all scheduling decisions • Symmetric multiprocessing (SMP) • Each processor is self-scheduling • Need to avoid for two processors to schedule one process at the same time • Supported by most OS – Linux, Solaris, Mac OS X, Windows XP, Windows 2000 … • Load balancing: Push vs. pull migration

  35. Thread Scheduling • Local Scheduling (process contention scope) • How the threads library decides which thread to put onto an available LWP • Threads belonging to one process compete for a LWP • Global Scheduling (system contention scope) • How the kernel decides which kernel thread to run next • Assign priority • POSIX Thread API • Select scheduling algorithm • Assign priority to threads

  36. Example: Solaris 2 Scheduling • Assign the highest priority to RT threads • Apply fixed priority among thread groups

  37. Algorithm Evaluation • Deterministic model • Queuing model • Little’s Formula: Queue length n = average arrival rate * average waiting time • Simulation • Implementation

  38. 5.15

  39. End of Chapter 5: Any Questions?

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