RT Scheduling

8/9/2019 RT Scheduling

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Scheduling Goal To understand the role that scheduling and schedulability analysis

plays in predicting that real-time applications meet their deadlines

Topics Simple process model

The cyclic executive approach Process-based scheduling Utilization-based schedulability tests Response time analysis for PS and !" #orst-case execution time

Sporadic and aperiodic processes Process systems $ith " % T Process interactions& bloc'ing and priority ceiling protocols (n extendible process model "ynamic systems and on-line analysis Programming priority-based systems


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Scheduling

)n general& a scheduling scheme provides t$o features*

(n algorithm for ordering the use of system resources +inparticular the ,PUs

( means of predicting the $orst-case behaviour of the system$hen the scheduling algorithm is applied

The prediction can then be used to confirm the temporalre.uirements of the application


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Simple Process Model

The application is assumed to consist of a fixed set ofprocesses (ll processes are periodic& $ith 'no$n periodsThe processes are completely independent of each

other (ll system/s overheads& context-s$itching times and soon are ignored +i0e& assumed to have zero cost

(ll processes have a deadline e.ual to their period +that

is& each process must complete before it is nextreleased

(ll processes have a fixed $orst-case execution time


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Standard Notation

BCDIJNPRT

Ua-z

#orst-case bloc'ing time for the process +if applicable#orst-case computation time +#,!T of the process"eadline of the processThe interference time of the processRelease 1itter of the process2umber of processes in the systemPriority assigned to the process +if applicable#orst-case response time of the process3inimum time bet$een process releases +process period

The utilization of each process +e.ual to ,4TThe name of a process


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Cyclic Executives

5ne common $ay of implementing hard real-timesystems is to use a cyclic executive6ere the design is concurrent but the code is producedas a collection of procedures

Procedures are mapped onto a set of minor cycles thatconstitute the complete schedule +or ma1or cycle3inor cycle dictates the minimum cycle time3a1or cycle dictates the maximum cycle time

Has the advantage of being fully deterministic

Has the advantage of being fully deterministic


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Consider Process Set

Process Period ,T ,omputation Time ,C

a 25 10b 25 8

c 50 5 d 50 4 e 100 2


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Cyclic Executive

loop wait_fo _i!te "#t$ # oced" e_fo _a$ # oced" e_fo _b$ # oced" e_fo _c$ wait_fo _i!te "#t$ # oced" e_fo _a$ # oced" e_fo _b$ # oced" e_fo _d$ # oced" e_fo _e$ wait_fo _i!te "#t$ # oced" e_fo _a$ # oced" e_fo _b$ # oced" e_fo _c$ wait_fo _i!te "#t$ # oced" e_fo _a$ # oced" e_fo _b$ # oced" e_fo _d$end loop $


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Time-line for Process Set

a b c

Interrupt

a b d

Interrupt

e a b c

Interrupt Interrupt


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Properties

2o actual processes exist at run-time7 each minor cycleis 1ust a se.uence of procedure callsThe procedures share a common address space andcan thus pass data bet$een themselves0 This data does

not need to be protected +via a semaphore& for examplebecause concurrent access is not possible (ll 8 process periods must be a multiple of the minorcycle time


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Problems with Cycle Executives

The difficulty of incorporating processes $ith long periods7the ma1or cycle time is the maximum period that can beaccommodated $ithout secondary schedulesSporadic activities are difficult +impossible9 to incorporate

The cyclic executive is difficult to construct and difficult tomaintain : it is a 2P-hard problem

(ny 8process; $ith a sizable computation time $ill need tobe split into a fixed number of fixed sized procedures +this

may cut across the structure of the code from a soft$areengineering perspective& and hence may be error-prone3ore flexible scheduling methods are difficult to support"eterminism is not re.uired& but predictability is


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Process- ased Scheduling

Scheduling approaches

ixed-Priority Scheduling + PS !arliest "eadline irst +!"


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!ixed-Priority Scheduling "!PS#

This is the most $idely used approach and is the mainfocus of this course !ach process has a fixed& static & priority $hich iscomputer pre-run-time

The runnable processes are executed in the orderdetermined by their priority)n real-time systems& the 8priority; of a process isderived from its temporal re.uirements& not its

importance to the correct functioning of the system or itsintegrity


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Earliest $eadline !irst "E$!# Scheduling

The runnable processes are executed in the orderdetermined by the absolute deadlines of the processesThe next process to run being the one $ith the shortest+nearest deadline

(lthough it is usual to 'no$ the relative deadlines ofeach process +e0g0 >?ms after release & the absolutedeadlines are computed at run time and hence thescheme is described as dynamic


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%alue- ased Scheduling "% S#

)f a system can become overloaded then the use ofsimple static priorities or deadlines is not sufficient7 amore adaptive scheme is neededThis often ta'es the form of assigning a value to each

process and employing an on-line value-basedscheduling algorithm to decide $hich process to runnext


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Preemption and Non-preemption

#ith priority-based scheduling& a high-priority process maybe released during the execution of a lo$er priority one)n a preemptive scheme& there $ill be an immediate s$itchto the higher-priority process#ith non-preemption & the lo$er-priority process $ill beallo$ed to complete before the other executesPreemptive schemes enable higher-priority processes to bemore reactive& and hence they are preferred

(lternative strategies allo$ a lo$er priority process tocontinue to execute for a bounded timeThese schemes are 'no$n as deferred preemption orcooperative dispatchingSchemes such as !" and


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!PS and &ate Monotonic Priority 'ssignment

!ach process is assigned a +uni.ue priority based onits period7 the shorter the period& the higher the priority)0e& for t$o processes i and %&

This assignment is optimal in the sense that if anyprocess set can be scheduled +using pre-emptivepriority-based scheduling $ith a fixed-priorityassignment scheme& then the given process set canalso be scheduled $ith a rate monotonic assignmentscheme2ote& priority @ is the lo$est +least priority

P j P iT jT i >&P io it7. is

-- eac i!'ta!ce of t e ta') ca! a=e a -- diffe e!t # io it7

entry

e =ice1 &&&.$ entry e =ice2 &&&.$ pragma P io it7 P i.$end e =e '$

P i i C ili 1 2i


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Priority Ceiling 1oc2ing

Protected ob1ects need to maintain the consistency oftheir data3utual exclusion can be guaranteed by use of thepriority model

!ach protected ob1ect is assigned a ceiling priority $hichis greater than or e.ual to the highest priority of any ofits calling tas's#hen a tas' calls a protected operation& its priority isimmediately raised to that of the protected ob1ect)f a tas' $ishing to enter a protected operation isrunning then the protected ob1ect cannot be alreadyoccupied

C ili 1 2i


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Ceiling 1oc2ing

!ach protected ob1ect is assigned a priority using apragma)f the pragma is missing& P io it7 9a't is assumedP o a>_? o is raised if the calling tas'/s active

priority is greater than the ceiling)f an interrupt handler is attached to a protectedoperation and the $rong ceiling priority has been set&then the program becomes erroneous

#ith ceiling loc'ing& an effective implementation $ill usethe thread of the calling tas' to execute not only theprotected operation but also to execute the code of anyother tas's that are released as a result of the call

E l f C ili P i it


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Example of Ceiling Priority

protected @ate_Co!t o( is

pragma P io it7 28.$

entry to#_ !d_C(o'e$

procedure #e!$

private @ate 3 Boo(ea! 3; /a('e$

end @ate_Co!t o($

protected body @ate_Co!t o( is

entry to#_ !d_C(o'e

when @ate is

begin

@ate 3; /a('e$

end; procedure #e! is

begin

@ate 3; T "e$

end $end @ate_Co!t o($

E l


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Example

(ssume tas' T& priority >B& calls to#_ !d_C(o'e andis bloc'ed0 Iater tas' & priority >E& calls #e! 0 Thethread executing $ill underta'e the follo$ingoperations* the code of #e! for evaluate the barrier on the entry and note that T can no$

proceed the code to#_ !d_C(o'e for T evaluate the barrier again

continue $ith the execution of after its call on the protectedob1ect

There is no context s$itch

' ti P i iti


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'ctive Priorities

( tas' entering a protected operation has its priorityraised

( tas' s active priority might also change during*

tas' activation a tas' inherits the active priority of the parenttas' $hich created it +to avoid priority inversion during a rendezvous the tas' executing a rendezvous $ill

inherit the active priority of the caller if it is greater than itscurrent active priority

2ote* no inheritance $hen $aiting for tas' termination

$i t hi g


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$ispatching

The order of dispatching is determined by the tas's/active priorities"efault is preemptive priority based2ot defined exactly $hat this means on a multi-

processor system5ne policy defined by annex*/I/ _Ait i!_P io it7#hen a tas' becomes runnable it is placed at the bac'

on the run .ueue for its priority7 $hen it is preempted& itis placed at the front

E t 3 P li i


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Entry 3ueue Policies

( programmer may choose the .ueuing policy for atas'/s entry .ueue and the select statementT$o predefined policies* /I/ _ "e"i! +default andP io it7_ "e"i!

#ith P io it7_ "e"i! and the select statement& analternative that is open and has the highest priority tas'.ueued +of all open alternatives is chosen)f there are t$o open $ith e.ual priority tas's& the one$hich appears textually first in the program is chosenTas's are .ueued in active priority order& if active prioritychanges then no re.ueuing ta'es place7 if the basepriority changes& the tas' is removed and re.ueued

$ynamic Priorities


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$ynamic Priorities

Some applications re.uire the base priority of a tas' to change

dynamically* e0g0& mode changes& or to implement dynamicscheduling schemes such as earliest deadline scheduling

Pac2age Specification


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Pac2age Specification

with da&Ta')_Ide!tificatio!$

use da$ package da&D7!a>ic_P io itie' is

procedure et_P io it7 P io it7 3 7'te>& !7_P io it7$ T 3 Ta')_Ide!tificatio!&Ta')_Id 3;

Ta')_Ide!tificatio!&C" e!t_Ta').$

function @et_P io it7 T 3 T_Ide!tificatio!&Ta')_Id3; Ta')_Ide!tificatio!&C" e!t_Ta').

return 7'te>& !7_P io it7$ -- ai'e Ta')i! _? o if ta') a' te >i!ated -- Bot ai'e P o a>_? o if a N"((_Ta')_Id i' #a''ed

private -- !ot '#ecified b7 t e (a! "a eend da&D7!a>ic_P io itie'$

$ynamic Priorities


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$ynamic Priorities

The effect of a change of base priorities should be assoon as practical but not during an abort deferredoperation and no later than the next abort completionpoint

,hanging a tas'/s base priority can affect its activepriority and have an impact on dispatching and .ueuing

P S 4


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P.S 4

P5S)N supports priority-based scheduling& and has optionsto support priority inheritance and ceiling protocolsPriorities may be set dynamically#ithin the priority-based facilities& there are four policies*

) 5* a process4thread runs until it completes or it is bloc'ed Round-Robin* a process4thread runs until it completes or it is bloc'edor its time .uantum has expired

Sporadic Server* a process4thread runs as a sporadic server 5T6!R* an implementation-defined

or each policy& there is a minimum range of priorities thatmust be supported7 C> for ) 5 and round-robinThe scheduling policy can be set on a per process and a perthread basis

P S 4


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P.S 4

Threads may be created $ith a system contention option& in $hich case they compete $ith other systemthreads according to their policy and priority

(lternatively& threads can be created $ith a processcontention option $here they must compete $ith otherthreads +created $ith a process contention in the parentprocess )t is unspecified ho$ such threads are scheduled relative to

threads in other processes or to threads $ith global contention

( specific implementation must decide $hich to support

Sporadic Server


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Sporadic Server

( sporadic server assigns a limited amount of ,PUcapacity to handle events& has a replenishment period& abudget& and t$o prioritiesThe server runs at a high priority $hen it has somebudget left and a lo$ one $hen its budget is exhausted#hen a server runs at the high priority& the amount ofexecution time it consumes is subtracted from its budgetThe amount of budget consumed is replenished at thetime the server $as activated plus the replenishmentperiod#hen its budget reaches zero& the server/s priority is setto the lo$ value

ther !acilities


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.ther !acilities

P5S)N allo$s*

priority inheritance to be associated $ith mutexes+priority protected protocolF ),PP

message .ueues to be priority orderedfunctions for dynamically getting and setting a thread/sprioritythreads to indicate $hether their attributes should be

inherited by any child thread they create

&T /ava Threads and Scheduling


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&T /ava Threads and Scheduling

There are t$o entities in Real-Time Lava $hich can bescheduled*

Rea(ti>eT ead' +and No ea#Rea(ti>eT ead'7!?=e!t a!d(e +and Bo"!d 7!c?=e!t a!d(e

5b1ects $hich are to be scheduled must implement the c ed"(ab(e interface specify their

E c ed"(i! Pa a>ete '

E Re(ea'ePa a>ete 'E Fe>o 7Pa a>ete '

&eal Time /ava


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&eal-Time /ava

Real-Time Lava implementations are re.uired to support atleast >H real-time priority levels

(s $ith (da and P5S)N& the larger the integer value& thehigher the priority2on real-time threads are given priority levels belo$ theminimum real-time priority2ote& scheduling parameters are bound to threads atthread creation time7 if the parameter ob1ects are changed&they have an immediate impact on the associated thread

Ii'e (da and Real-Time P5S)N& Real-Time Lava supportsa pre-emptive priority-based dispatching policyUnli'e (da and RT P5S)N& RT Lava does not re.uire apreempted thread to be placed at the head of the run

.ueue associated $ith its priority level

The Schedulable nterface


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The Schedulable nterface

public interface c ed"(ab(e extends %a=a&(a! &R"!!ab(eG public void addTo/ea'ibi(it7 .$ public void e>o=e/ o>/ea'ibi(it7 .$

public Fe>o 7Pa a>ete ' etFe>o 7Pa a>ete ' .$ public void 'etFe>o 7Pa a>ete ' Fe>o 7Pa a>ete ' >e>o 7.$

public Re(ea'ePa a>ete ' etRe(ea'ePa a>ete ' .$ public void 'etRe(ea'ePa a>ete ' Re(ea'ePa a>ete ' e(ea'e.$

public c ed"(i! Pa a>ete ' et c ed"(i! Pa a>ete ' .$

public void 'et c ed"(i! Pa a>ete ' c ed"(i! Pa a>ete ' 'c ed"(i! .$

public c ed"(e et c ed"(e .$ public void 'et c ed"(e c ed"(e 'c ed"(e .$

H

Scheduling Parameters


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Scheduling Parameters

public abstract class c ed"(i! Pa a>ete 'G public c ed"(i! Pa a>ete ' .$ H

public class P io it7Pa a>ete ' extends c ed"(i! Pa a>ete 'G

public P io it7Pa a>ete ' int # io it7.$

public int etP io it7 .$ ete 'G

public I>#o ta!cePa a>ete ' int # io it7, int i>#o ta!ce.$ public int etI>#o ta!ce .$ public void 'etI>#o ta!ce int i>#o ta!ce.$ &&&

H

&T /ava0 Scheduler


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&T /ava0 Scheduler

Real-Time Lava supports a high-level scheduler $hosegoals are* to decide $hether to admit ne$ schedulable ob1ects according

to the resources available and a feasibility algorithm& and to set the priority of the schedulable ob1ects according to the

priority assignment algorithm associated $ith the feasibilityalgorithm

6ence& $hilst (da and Real-Time P5S)N focus on staticoff-line schedulability analysis& Real-Time Lavaaddresses more dynamic systems $ith the potential foron-line analysis

The Scheduler


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The Scheduler

public abstract class c ed"(eG public c ed"(e .$ protected abstract void addTo/ea'ibi(it7 c ed"(ab(e '.$ protected abstract void e>o=e/ o>/ea'ibi(it7 c ed"(ab(e '.$

public abstract boolean i'/ea'ib(e .$


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The Scheduler

Thec ed"(e

is an abstract classThe i'/ea'ib(e method considers only the set ofschedulable ob1ects that have been added to itsfeasibility list +via the addTo/ea'ibi(it7 and

e>o=e/ o>/ea'ibi(it7 methodsThe method c a! eIf/ea'ib(e chec's to see if itsset of ob1ects is still feasible if the given ob1ect has itsrelease and memory parameters changed)f it is& the parameters are changedStatic methods allo$ the default scheduler to be .ueriedor setRT Lava does not re.uire an implementation to providean on-line feasibility algorithm

The Priority Scheduler


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The Priority Scheduler

class P io it7 c ed"(e extends c ed"(eG public P io it7 c ed"(e .

protected void addTo/ea'ibi(it7 c ed"(ab(e '.$ ...

public void fi e c ed"(ab(e c ed"(ab(e 'c ed"(ab(e.$

public int etFa6P io it7 .$ public int etFi!P io it7 .$ public int etNo >P io it7 .$

public static P io it7 c ed"(e i!'ta!ce .$ &&&H

tandard preemptive priority based scheduling

ther !acilities


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.ther !acilities

Priority inheritance and ),,P +called priority ceilingemulationSupport for aperiodic threads in the form of processinggroups7 a group of aperiodic threads can be lin'edtogether and assigned characteristics $hich aid thefeasibility analysis

Summary


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Summary

( scheduling scheme defines an algorithm for resourcesharing and a means of predicting the $orst-casebehaviour of an application $hen that form of resourcesharing is used0#ith a cyclic executive& the application code must bepac'ed into a fixed number of minor cycles such that thecyclic execution of the se.uence of minor cycles +thema1or cycle $ill enable all system deadlines to be metThe cyclic executive approach has ma1or dra$bac's manyof $hich are solved by priority-based systemsSimple utilization-based schedulability tests are not exact

Summary


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Summary

Response time analysis is flexible and caters for* Periodic and sporadic processes =loc'ing caused by )P, ,ooperative scheduling (rbitrary deadlines

Release 1itter ault tolerance 5ffsets

(da& RT P5S)N and RT Lava support preemptive

priority-based scheduling (da and RT P5S)N focus on static off-line schedulabilityanalysis& RT Lava addresses more dynamic systems$ith the potential for on-line analysis

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RT Scheduling