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    Scheduling

    Basic Concepts

    Ref: Hard Real-Time Computing Systems

    Giorgio Buttazzo

    Ref: Real-Time Systems & Software   Alan Shaw 

    Processes - Tasks - Threads

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    Real-Time Systems

    Periodic and periodic

    Hard & Soft !eadlines

    Scheduling of Tasks " Static & !ynamic

    Sharing of Resources and Critical Sections

    !omino #ffect

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    Best #ffort Scheduling lgorithms

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    #$ample Cost %unctions What is a “good” system?

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    etrics for Performance #'aluation What is a “good” system?

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     ttempting to inimi(e )ateness

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    Scheduling nomalies

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    Precedence #$ample

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     dding a processor 

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    Reducing Computation Times *

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    +eaken Constraints

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    Resource Constraints

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    Successive events are separated by at least p

    The Sporadic process

    Shaw’s notation:

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    Cyclic Executives for Scheduling Periodic Tasks:

      Broken into ,locks:

      aor Cycles .CT/ " shortest period where all task periods align

      all tasks e$ecute in a maor cycle

      inor Cycles .mct/ " 0frames1 used to allocate tasks or portions of tasks

    #$ample: Say you ha'e two tasks:

      T2 3 .c24 p24 d2/ 3 .24 54 5/

      T5 3 .c54 p54 d5/ 3 .24 64 6/

      7$$$$$$8888887$$$$$$8888887$$$$$$888888799

      7888888$$$$$$8888887$$$$$$888888888888799

      ; 2 5 6 < = >

      CT 3 >

      mct 3 5 .or 2/

      llocations are: ?T24T5@ ?T24T5@ ?T2@4 repeat

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      Can this ,e scheduled*

    +hat is CT*

    +hat is mct*

    +hat is a schedule*

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     da-like implementation of this scheduling

    Clock returns the current time

    Delayuntil x  causes the task to ,lock until at least time x

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    Some principles a,out periodic scheduling:

      +e assume all processes are released at time t; +hy*

      The CT is the least common multiple .lcm/ of the periods +hy*

      The mct must di'ide the CT +hy*

      The mct must ,e A3 the longest computation ,lock +hy*

      mct 3 di for all  3 2494 n

      mct D .mct - gcd.mct4 pi// 3 di

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    !o"e work #$:  .)ecture