CPU Scheduling Tanenbaum Ch 2.4 Silberchatz and Galvin Ch 5.

CPU Scheduling

Tanenbaum Ch 2.4

Silberchatz and Galvin Ch 5

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Basic Concepts• CPU - I/O Burst Cycle• CPU Burst Distribution

0

20

40

60

80

100

120

140

160

0 8 16 24 32

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CPU Scheduling

• Decision points:– Process switch from running to waiting state– Process switch from running to ready state– Process switch from waiting to ready state– Process terminates

• Scheduling types– non-preemptive– preemptive

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Task Dispatcher

• Functions:– Switching context– Switching to user mode– Jumping to PC location

• The time needed to stop one process and start up another is known as dispatch latency.

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Batch Interactive Real time

Categories of Scheduling Algorithms

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 2-39. Some goals of the scheduling algorithm under different circumstances.

Scheduling Algorithm Goals


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First-come first-served Shortest job first Shortest remaining Time

next

Scheduling in Batch Systems


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Scheduling Algorithms(First-come, First-served)

• Determine avg waiting time if: – (P1, P2, P3) or (P3, P2, P1)

Process Burst Time

P1 24

P2 3 P3 3

0 10 20 30

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• Determine avg waiting time if: – (P1, P2, P3)

Process Burst Time

P1 24

P2 3 P3 3

0 10 20 30

P1 ……………………………………………..P2…..P3…….

Waiting time:P1: 0P2: 24P3: 27

avg: 17

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• Determine avg waiting time if: – (P3, P2, P1)Process Burst Time

P1 24

P2 3 P3 3

0 10 20 30

P3….P2….P1………………………………………………...

Waiting time:P1: 6P2: 3P3: 0

avg: 3

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Scheduling Algorithms(Shortest Job First)

• Schedule jobs based on their length.• May be preemptive

– If a job is queued that is shorter than the remaining time on the current job, there is a switch.

• May be non-preemptive.– Once a job has started, it works to its normal switch.

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• Non-Preemptive / preemptive SJF

Process Arr. Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

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• Preemptive SJFProcess Arr. Time Burst Time

P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..

P1|

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P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..P2..

P1 P2 | |

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P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..P2..P3

P1 P2 P3| | |

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P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..P2..P3P2..

P1 P2 P3P4| | | |

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P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..P2..P3P2..P4…….

P1 P2 P3P4| | | |

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P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1..P2..P3P2..P4…….P1……….

P1 P2 P3P4| | | |

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• Non-preemptive SJFProcess Arr. Time Burst Time

P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

0 10 20 30

P1…………..P3P2…….P4…….

P1 P2 P3P4| | | |

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• Determining the length of the jobs.• Use weighted average burst lengths

10

*)1(*1

nnn t

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2

1

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Round-robin scheduling Priority scheduling Multiple queues Shortest process next Guaranteed scheduling Lottery scheduling Fair-share scheduling

Scheduling in Interactive Systems


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Round Robin Scheduling

• Developed in response to time-sharing systems.

• Each job is given control of the CPU for a short period - a time quantum. (In the range of 10-100 milliseconds.)

• Control then passes to the next job (FIFO?)• Average waiting time dependent on job size,

quantum size.

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Figure 2-41. Round-robin scheduling. (a) The list of runnable processes. (b) The list of runnable

processes after B uses up its quantum.

Round-Robin Scheduling


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0 10 20 30

Process Burst Time

P1 6

P2 3 P3 1

P4 7

All processes arrive at T=0

Vary Quantum time from1 to 7. Calculate averageturnaround time (to job completion).

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0 10 20 30

Process Burst Time

P1 6

P2 3 P3 1

P4 7

All processes arrive at T=0Quantum Average Size Turnaround

1 11

P1P2P3P4P1P2P4P1P2P4P1P4P1P4P1P4P4

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0 10 20 30

Process Burst Time

P1 6

P2 3 P3 1

P4 7


1 112 11.5

P1..P2..P3P4..P1..P2P4..P1..P4..P4

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0 10 20 30

Process Burst Time

P1 6

P2 3 P3 1

P4 7


1 112 11.53 10.75

P1. . P2. . P3P4. . P1. . P4. . .P4

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10

11

12

9

8

13

1 2 3 4 5 6 7 Quantum

avg. turnaround

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Priority Scheduling

• Each process (or class of processes) is given a priority.

• Jobs are executed in priority order• Issue: If there are sufficient high priority

jobs, lower priority jobs may never get scheduled.

• One solution: Aging of processes. After a job has been in queue for a given period of time, raise its priority.

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Figure 2-42. A scheduling algorithm with four priority classes.

Priority Scheduling


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Multi-level queue Scheduling

• Partition the job space into distinct classes or queues– foreground / background– system / interactive / editing / batch / student– etc.

• Independently assign queueing service disciplines• Assign queue priorities

– Highest to lowest (empty one queue before starting next)– Divide time between queues

• 80 / 20 • 30 / 20 / 20 / 20 / 10

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Multilevel Feedback Queue Scheduling

• Similar to Multilevel queue scheduling, but jobs are allowed to move between queues.

• Avoids process starvation by allowing neglected jobs to move up to a higher queue.

• Differentiate queues by (for example) quantum size– queue 1 = 8 ms– queue 2 = 16 ms– queue 3 = FCFS

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Figure 2-43. (a) Possible scheduling of user-level threads with a 50-msec process quantum and threads that run 5 msec per

CPU burst.

Thread Scheduling (1)


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Figure 2-43. (b) Possible scheduling of kernel-level threads with the same characteristics as (a).

Thread Scheduling (2)


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Multiple Processor Scheduling

• CPU scheduling more complex when multiple CPUs are available

• Homogeneous processors within a multiprocessor are usual (heterogeneous processors found in distributed systems).

• Load sharing used with SMP. – Single queue, multiple servers.

• Asymmetric Multiprocessing (AMP)– Assign roles (system control, application processing)– Avoids system data sharing

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Benefits of Multiprocessors Process Scheduling

– Process inter-arrival time variation can be characterized by coefficient of variation = s / s• standard deviation of interarrival time / mean service time• ratio of 1 = exponential distribution. (your mileage may vary…)

– Differences between scheduling algorithms become much less important in multiprocessor systems

Single processor

Dual processorRR

to F

CF

Sth

rupu

t rat

io

Coefficient of Variation

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Benefits of Multiprocessors Thread Scheduling

• Allows true parallel processing within an application

• However, if there is significant interaction among threads, small differences in thread management & scheduling can have big results.

• General Approaches used– Load Sharing - (not necessarily load balancing)– Gang Scheduling - Schedule related threads together– Dynamic Scheduling - allow # of threads to vary

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Real-Time Scheduling

• System Response Classifications– Hard real-time System - requires response

guarantees– Soft real-time System – response not guaranteed

• Dispatch Latency

Interruptprocessing

conflicts dispatch

Real-timeprocessexecution

Dispatch latency

Response interval

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Summary

• Primary job of an OS is to schedule processes– Batch– Interactive– Real Time

• Selection of scheduling algorithm depends on optimization criteria

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Questions• On a system with multilevel queue scheduling, would you expect to

see the same scheduling algorithms used on all levels, or would you expect them to be different? Justify your answer.

• Consider the following set of processes, with the length of the CPU burst time given in milliseconds. The processes are assumed to have arrived in the order P1, P2, P3, P4. Process Job Size

P1 10P2 1P3 2P4 5

Of the scheduling algorithms FCFS, SJF, and RR(qt=2) which algorithm offers the best waiting time?

• In many multiprocessing systems, although there is a fixed limit to the amount of time that a job can keep control of the CPU, in practice, the jobs are released earlier. Why?

• The book talked about the multiple queueing system used by CTSS. If a process needed 30 quanta to complete, how many times would it be swapped in to complete?

• How does Lottery Scheduling work? What is its principle advantage over, say, Guaranteed Scheduling?

CPU Scheduling Tanenbaum Ch 2.4 Silberchatz and Galvin Ch 5.

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