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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 %unctionsWhat is a “good” system?

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etrics for Performance #'aluationWhat 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:

  aor Cycles .CT/ " shortest period where all task periods align

  all tasks e$ecute in a maor 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

Delayuntil 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