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Dr Shamala Subramaniam 1

Real-Time SystemsDr Shamala SubramaniamDept. Communication Technology & Networks

Faculty of Computer Science & IT, UPM

e-mail : [email protected] : 03-89466512

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Synopsis

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Real-Time SystemsDr Shamala SubramaniamDept. Communication Technology & Networks

Faculty of Computer Science & IT, UPM

e-mail : [email protected] : 03-89466512

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Definition

Any system where a timely response by the

computer to external stimuli is vital is a real-

time system.

The statement is True

because it is almost

Content Free

Real-Time Systemruns

task that have deadlines

Not necessary for the task to done within the

dateline

aircraft vs. video game

What do you do with a real-

time tasks that misses its

deadline?

Done Not necessarily to be done ,

calculating the value ofT vs. accounts

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Definition A real-time system is one which a substantial

fraction of the design effort goes into makingsure that task deadlines are met.

Oxford Dictionary of Computing Any system in which the time at which the output

is produced is significant. This is usually becausethe input corresponds to some movement in thephysical world, and the output has to relate to thesame movement. The lag from the input to theoutput must be sufficiently small for acceptabletimeliness.

Young (1982) Any information processing activity or system

which has to respond to externally generated inputstimuli within a finite and specified period.

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Hard versus Soft

HARD: miss a deadline and youre in trouble! (planes, trains,factory control, nuclear facilities, ...)

SOFT: try to meet deadlines, but if not, system still works,although with degraded performance (multimedia, thermostat,...)

FIRM: late results are worthless (deadlines can be missedoccasionally), but you are not in trouble (no benefit from latedelivery)

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Driving a Car

Relating to a Real-Time System

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Example: Car-and-DriverDrivers Mission: Reaching the destination safely whilesatisfying the constraints.

Controlled System/Process: Car.

Operating environment: Road conditions and other cars.

Controlling SystemHuman driver:Sensors - Eyes and Ears of the driver.(Real-time Controller)Computer:

Sensors - Cameras, Infrared receiver, and Laser telemeter.

Controls: Accelerator, Steering wheel, Break-pedal.

Actuators: Wheels, Engines, and Brakes.

Constraintsof the driver

Reachdestinationwithout

colliding andwithin thespeed limit

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Quantifying the drivers performance

Measure the outcome of the drivers actions, taken in

context of the operating environment.

1. Getting to the destination

2. The Time taken to reach to

the destination

15mph (well?) during

snowstorm and

on a dry weather

Not just the

outcome but whatcaused the driver

to get there

(landing into a

ditch)

Obvious

Outcomes

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Performance is not an absolute commodity.

Performance must be measured instead in

terms of what the condition allows.

In other words, performance measures the

goodness of the outcome relative to the bestoutcome possible under the circumstances.

Conclusion Performance

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Critical Task Tuning the Radio vs. Steering

and Braking

Critical Task varying Deadlines Not Constant; they vary

with the operating environment (driving 6pm/am).

About the drivers condition:

Drivers Physical Condition 1s sleep vs. micro-sleeping

Discrimination - on the basis of secondary factors

additional fuel consumption

Performance (cont.):

The task the driver must perform

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Try Out !!

Write a detail specification on what the driver must do ineach conceivable eventuality & then ensure that the

specifications were complete.

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Wouldnt you agree that:

Writing out formal specifications and validating them areperhaps the most difficult task in real-time systems. They

are also the tasks about which researchers know the least.

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Issues in Real-Time Computing

A real-time computer must be more reliable than its

individual hardware and software components.

It must be capable of working in : Harsh environments

Rich in electromagnetic noise

Elementary-particle radiation

Face of rapidly changing computation loads.

The field of real-time computing is especially rich in

research problems.

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Issues in Real-Time Computing (cont.)

Task execution time Cache Example

A executes B executes A executes

t1T2.2

A preempted by B A resumes A completes

Real-Time Systems differs from general purpose counterparts in 2 ways:

Much more specific in their applications

Consequences of their failure are more drastic

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The Oracle in Delphi, GreeceThe Oracle in Delphi, Greece

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Since the consequences

are severe Such systems need to be specified

more carefully

Their performance validated

specification languages and

performance measures capable of

expressing timing requirements

Example : jet engine x general purpose

workstation

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Structure of A RealStructure of A Real--Time SystemTime System

ControlledControlled

ProcessProcess SensorsSensors Job ListJob List

TriggerTrigger

GeneratorGenerator

ExecutionExecution

DisplayDisplay

ClockClock

OperatorOperator

ActuatorsActuators

State of the controlledState of the controlledprocess and of theprocess and of theOperating EnvironmentOperating Environmentis acquired by sensorsis acquired by sensors

EnvironmentEnvironmentProvides input to theProvides input to thecontroller, RT computercontroller, RT computer

The s/w for the jobs areThe s/w for the jobs arepreloadedpreloaded schedulingscheduling

used to triggerused to triggerthe execution ofthe execution ofindividual jobsindividual jobs

can becan bepreloaded orpreloaded or

pre=determinedpre=determined

output ofoutput oftrigger is fed intrigger is fed inthe actuatorsthe actuatorsFaultFault--TolerantTolerant

techniquestechniques

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Structure of A Real-Time System (cont.)

The control computer exhibits dichotomy in terms of the

data rates.

Central clusterCentral clusterControlalgorithmsareexecutedControlalgorithmsareexecuted

ProcessingofthedataProcessingofthedataintothenextstageintothenextstage

MIDDLEMIDDLE

Sensors,actuators,displaysandinputpanelsSensors,actuators,displaysandinputpanels

LOW RATESLOW RATES

Schematic decompositionofaSchematic decompositionofarealreal--time control computertime control computer

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TASK CLASSES

Task can be classified in two ways:

Predictability of their arrival

Consequences of not being executed on time

Periodic speed of the aircraft

aperiodic aircraft turn

Critical

Non Critical

- every ni iterations of critical periodtask i

- the goal is to maximize no. of jobs

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More examples RT SystemMore examples RT System cars: engine control, ABS, drivecars: engine control, ABS, drive--byby--wirewire

planes: stability, jet engine, flyplanes: stability, jet engine, fly--byby--wirewire

computers: peripherals, applicationscomputers: peripherals, applications

military: weapons, satellitesmilitary: weapons, satellites

domestic: microwave, thermostat, dishwasherdomestic: microwave, thermostat, dishwasher

medical: pacemaker, medical monitoringmedical: pacemaker, medical monitoring

protection: intruder alarm, smoke/gasprotection: intruder alarm, smoke/gas

detectiondetection

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Common MisconceptionsCommon Misconceptions real fast is real-time: a computer system may

satisfy an applications requirement, but no

predictability (no real-time resource management). hardware over-capacity is enough: again, without

real-time resource management no appropriate

balance of resource distribution.

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Issues in RealIssues in Real--Time ProgrammingTime Programming

Performance MeasuresPerformance Measures

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Characterizing Real-Time

Systems and Task

Dr Shamala SubramaniamDept. Communication Technology & NetworksFaculty of Computer Science & IT

Universiti Putra Malaysia43400 Serdang, Selangor

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Contents Performance measures for real-time systems

Properties of Performance Measures

Traditional Performance Measures

Performability Cost Functions & Hard Deadlines

Estimating Program Run Times Analysis of Source Code

Accounting for Pipeline

Caches

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Introduction

2 Questions ?

Which performance measures are

the most appropriate for real-time

systems?

How can engineers estimate theworst-case run time of a program,

given the source code and target

architecture?

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Performance Measures for Real-Time

SystemsReal-time systems are used for critical

applications

Thus, the system must be carefully :

designed

validated before operation

Checking design correctnessusing formal and informal

methods

Characterizing performance &

reliability

Preparatory

Step:

The choice of

performance

measures

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Performance Measures

The choice of performance measures iscrucial

Are yardsticks by whichcharacterization is expressed

Must be concise encapsulate in veryfew numbers the performance of asystem

Analogy: Languages through which we convey

system performance Natural languages determines how our thoughts

gets expressed but also which ideas arise the

richness of the vocabulary

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Performance Measures (cont.) Eg: System A and B have system response times with thepdf.

Prob

abilitydensity

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Performance Measures (cont.) Eg: System A and B have system : weighted sum of themean M and variance V of the response time

a1M+

a2(V)

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The mean is a measure of the center or most likely value ina distribution. For N samples

x0, x1, x2,....xN-1from some population, the mean (also sometimes called the

average orexpected value) is defined as

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The variance A measure of the average distance betweeneach of a set of data points and their mean value; equal to thesum of the squares of the deviation from the mean value.

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Example 2.2 Performance perceived by a userdepends in a complex way on the

system response time

P

erformance

- Viewpoint of a Typist

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Example 2.3 Pocket Pager simple functions

Performance metrics : Not response time

Battery Power

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Example 2.4System C

special array-processing

unit 2 matricesmultiplication

Clock frequency 5 Mhz

System D

- no such unit

- Clock frequency 10Mhz

+

x0y0

x1y0x2y0

xny0

0

x0y1+ x1y1

0

+ x0y2+ x1y2+ 0+

P(2n-1) P(2n-2) P0

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Example 2.5

System E

Simple instruction set

Machine code twice as

long as F

1.5 times faster than F

System F

Complex instruction

set

Number of instructions

executed per second

MIPS

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Quite often even if good performance measureQuite often even if good performance measure

that accurately represents performance as thethat accurately represents performance as the

user perceives it, theuser perceives it, the application determinesapplication determines howhow

systems are ranked.systems are ranked.

PointsPoints

Frequent possibility for reasonableFrequent possibility for reasonable soundingsounding

performance measures to be misleading.performance measures to be misleading.

Performance perceived by the user depends onPerformance perceived by the user depends onso many factors that it is difficult in most casesso many factors that it is difficult in most cases

to find a measure that is perfect in all respects.to find a measure that is perfect in all respects.

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Properties of Performance

Measures A good performance measure must:

Represent an efficient encoding of relevant

information

Provide an objective basis for the ranking of

candidate controllers for a given application

Provide objective optimization criteria

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