Real Time System.doc

8/10/2019 Real Time System.doc

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Priority-driven Scheduling of

Periodic Tasks (1)

Sistem Waktu Nyata

Kuliah 5


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Lecture Outline

Assumptions

Fixed-priority algorithmso Rate monotonic

o Deadline monotonic

Dynamic-priority algorithmso Earliest deadline rst

o !east slack time

Relati"e merits o# xed- and dynamic-priorityscheduling

Schedula$le utili%ation and proo# o# schedula$ility

&aterial in lectures 5 ' ( corresponds to chapter ( o#!iu)s $ook


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Assumptions *riority-dri"en scheduling o# periodic tasks on a single

processor Assume a restricted periodic task model+

o A xed num$er o# independent periodic tasks exist ,o$s comprising those tasks+

Are ready #or execution as soon as they are released

an $e pre-empted at any time

Ne"er suspend themsel"es

Ne. tasks only admitted a#ter an acceptance test/ may $e

re0ected 1he period o# a task dened as minimum inter-release time o#

0o$s in task

o1here are no aperiodic or sporadic tasks

o Scheduling decisions made immediately upon 0o$ releaseand completion

Algorithms are e"ent dri"en2 not clock dri"en

Ne"er intentionally lea"e a resource idle


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o ontext s.itch o"erhead negligi$ly small/ unlimited priority

le"els


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ynamic versus Static Systems Recall #rom lecture 3+

o 4# 0o$s are scheduled on multiple processors2 and a 0o$ can$e dispatched toany o# the processors2 the system isdynamic

o 4# 0o$s are partitioned into su$systems2 each su$system

$ound statically toa processor2 .e ha"e a static systemo Dicult to determine the $est- and .orst-case per#ormance

o# dynamicsystems2 so most hard real-time systems $uiltare static

4n static systems2 the scheduler #or each processorschedules the 0o$s in its su$system independent o#the schedulers #or the other processors

Results demonstrated #or priority-dri"en

uniprocessor systems are applica$le to eachsu$system o# a static multiprocessor system


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o1hey are not applica$le to dynamic multiprocessor systems


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!i"ed- and ynamic-Priority Algorithms A priority-dri"en scheduler is an on-line scheduler

o 4t does not pre-compute a schedule o# tasks60o$s+ insteadassigns prioritiesto 0o$s .hen released2 places them on arun 7ueue in priority order

o When pre-emption is allo.ed2 a scheduling decision is

made .hene"er a0o$ is released or completedo At each scheduling decision time2 the scheduler updates

the run 7ueues andexecutes the 0o$ at the head o# the7ueue

,o$s in a task may $e assigned the same priority 8tasklevel fxedpriority9 or di:erent priorities 8task leveldynamic-priority9

1he priority o# each 0o$ is usually xed 8job levelfxed-priority9/ $ut some systems can "ary the priorityo# a 0o$ a#ter it has started 8job level dynamic-priority9

o,o$ le"el dynamic-priority usually "ery inecient


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#ate $onotonic Scheduling

;est kno.n xed-priority algorithm is rate monotonicscheduling

Assigns priorities to tasks $ased on their periodso1he shorter the period2 the higher the priority

o1he rate 8o# 0o$ releases9 is the in"erse o# the period2 so

0o$s .ith higherrate ha"e higher priority 8?2 =9 rate > @

o1 > 852 9 rate > =65

o13 > 8B2 59 rate > =6B

o Relati"e priorities+ 1= C 1 C 13


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%"ample& #ate $onotonic Scheduling


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Time Ready To Run Running

0 T1,T2,T3 T1

1 T2,T3 T2

2 T2,T3 T2

3 T3 T3

4 T1,T3 T1

5 T2,T3 T2

6 T2,T3 T2

7 T3 T3

8 T1,T3 T19 T3 T3


10 T2,T3 T2

11 T2,T3 T2

12 T1,T3 T1

13 T3 T3

14 T3 T3

15 T2 T2

16 T1,T2 T1

17 T2 T218 - -

19 - -


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eadline $onotonic Scheduling

1he deadline monotonic algorithm assigns taskpriority according to relati"e deadlines the shorterthe relati"e deadline2 the higher the priority

When relati"e deadline o# e"ery task matches itsperiod2 then rate monotonic and deadline monotonicgi"e identical results

When the relati"e deadlines are ar$itrary+


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o Deadline monotonic can sometimes produce a #easi$le

schedule in cases.here rate monotonic cannoto

;ut2 rate monotonic al.ays #ails .hen deadline monotonic#ails

Deadline monotonic pre#erred to rate monotonico 4# deadline period


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ynamic-Priority Algorithms Discussed se"eral dynamic-priority algorithms in

lecture 3+o Earliest deadline rst 8EDF9

1he 0o$ 7ueue is ordered $y earliest deadlineo !east slack time rst 8!S19

1he 0o$ 7ueue is ordered $y least slack time

1.o "ariations+ Strict !S1 scheduling decisions are made also .hene"er a7ueued 0o$)s slacktime $ecomes smaller than the executing0o$)s slack time huge o"erheads2 notused

Non-strict !S1 scheduling decisions made only .hen 0o$srelease or complete

o First in2 rst out 8F4F9

,o$ 7ueue is rst-in-rst-out $y release timeo !ast in2 rst out 8!4F9

,o$ 7ueue is last-in-rst-out $y release time

Focus on EDF as commonly used example


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%"ample& %arliest eadline !irst


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0 T1,T2 T1

0,5 T1,T2 T1

1 T2 T21,5 T2 T2

2 T1,T2 T1

2,5 T1,T2 T1

3 T2 T2

3,5 T2 T2

4 T1,T2 T2

4,5 T1,T2 T2


5 T1,T2 T1

5,5 T2 T26 T1,T2 T1

6,5 T1,T2 T1

7 T2 T2

7,5 T2 T2

8 T1,T2 T1

8,5 T1,T2 T1

9 T2 T2

9,5 T2 T2


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#elative $erits

Fixed- and dynamic-priority scheduling algorithmsha"e di:erent properties/ neither appropriate #or allscenarios

Algorithms that do not take into account the

urgencies o# 0o$s in priority assignment usuallyper#orm poorly

o Eg F4F2 !4F


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1he EDF algorithm gi"es higher priority to 0o$s thatha"e missed their deadlines than to 0o$s .hose

deadline is still in the #utureo Not necessarily suited to systems .here occasional

o"erload una"oida$le

Dynamic algorithms like EDF can produce #easi$leschedules in cases .here R& and D& cannot

o ;ut xed priority algorithms o#ten more predicta$le2 lo.ero"erhead


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%"ample& 'omparing ierent Algorithms

ompare per#ormance o# R&2 EDF2 !S1 and F4Fscheduling

Assume a single processor system .ith tasks+o1= > 82 =9

o1 > 852 59 H > =B

1he total utili%ation is =B

no slack timeo Expect some o# these algorithms to lead to missed

deadlinesGo1his is one o# the cases .here EDF .orks $etter than

R&6D&


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%"ample& #$ %! LST and !*!O

Demonstrate $y exhausti"e simulation that !S1 andEDF meet deadlines2 $ut F4F and R& don)t


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Schedula+ility Tests

Simulating schedules is $oth tedious and error-proneH can .e demonstrate correctness .ithout.orking through the scheduleI

Jes2 in some cases 1his is a schedulability testo A test to demonstrate that all deadlines are met2 .hen

scheduled using aparticular algorithmo An ecient schedula$ility test can $e used as an on-line

acceptance test/clearly exhausti"e simulation is tooexpensi"e


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Schedula+le ,tiliation

Recall+ a periodic task Ti is dened $y the ?-tuple 8 i2pi2 ei2 Di9 .ith utili%ation ui > ei 6pi

1otal utili%ation o# the system ==n

i iuU

1 .here B U =

A scheduling algorithm can #easi$ly schedule anysystem o# periodic tasks on a processor i# U is e7ual toor less than the maximum schedula$le utili%ation o#the algorithm2 UAL

o 4# UAL > =2 the algorithm is optimal

Why is kno.ing o# UAL importantI 4t gi"es aschedula$ility test2 .here a system can $e "alidated$y sho.ing that U UAL


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Schedula+le ,tiliation& %!

1heorem+ a system o# independent preempta$leperiodic tasks .ith Di >pi can $e #easi$ly scheduledon one processor using EDF i# and only i# U =

o U!D" > = #or independent2 preempta$le periodic tasks .ithDi >pi

LExpected since EDF pro"ed optimalin lecture 3 see the $ook #or proo#M

o orollary+ result also holds i# deadline longer than period+U!D" > = #orindependent preempta$le periodic tasks .ith Dipi

Notes+

o Result is independent o# io Result can also $e sho.n to apply to strict !S1


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might still $e #easi$le i# C =8.ould ha"e to run theexhausti"e simulation to pro"e9


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Schedula+le ,tiliation& %!

Po. can you use this in practiceIo Assume using EDF to schedule multiple periodic tasks2

kno.n executiontime #or all 0o$shoose the periods #or the tasks such that the

schedula$ility test is met

Example+ a simple digital controller+o ontrol-la. computation task2 T=2 takes e= > ms2 sampling

rate is =BB P%8iep= > =B ms9

u= is B

the system is guaranteed to $e schedula$leo Want to add a $uilt-in sel# test task2 T2 taking 5Bms - .ill

the system still.orkI


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Schedula+le ,tiliation of #$

1heorem+ a system o# n independent preempta$leperiodic tasks .ith Di >pi can $e #easi$ly scheduled

on one processor using R& i# and only i# U n

8=6n

=9

o U#$8n9 > n8=6n =9

o For large n ln

8ie n B(3=?T=B5(H9

o L*roo# in $ook -

complicatedGM


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o U U#$8n9 is a sucient2 $ut not necessary2 condition ie

a #easi$le ratemonotonic schedule is guaranteed to exist i#U U#$8n92 $ut might still $epossi$le i# U C U#$8n9


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Schedula+le ,tiliation of #$ What happens i# the relati"e deadlines #or tasks are

not e7ual to their respecti"e periodsI Assume the deadline is some multiple o# the period+

Dk > pk

4t can $e sho.n that+


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Schedula+le ,tiliation of #$


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Real Time System.doc