1 Note content copyright © 2004 Ian Sommerville. NU-specific content copyright © 2004 M. E. Kabay....

1 Note content copyright © 2004 Ian Sommerville. NU-specific content copyright © 2004 M. E. Kabay. All rights reserved.

Real-timeSoftware Design

IS301 – Software EngineeringLecture # 19 – 2004-10-13

M. E. Kabay, PhD, CISSPAssoc. Prof. Information Assurance

Division of Business & Management, Norwich University

mailto:[email protected] V: 802.479.7937


Objectives

To explain the concept of a real-time system and why these systems are usually implemented as concurrent processes

To describe a design process for real-time systems

To explain the role of a real-time operating system

To introduce generic process architectures for monitoring and control and data acquisition systems


Topics

Introduction to R-T SystemsSystems DesignReal-Time ExecutivesMonitoring and Control SystemsData Acquisition Systems


Real-Time Systems

Systems which monitor and control their environment

Inevitably associated with hardware devicesSensors: Collect data from system

environmentActuators: Change (in some way) system's

environmentTime critical. Real-time systems MUST

respond within specified times


Definition

Real-time system:Correct functioning of system depends on

results produced by system and time at which these results produced

‘Soft’ real-time system:Operation degraded if results not produced

according to specified timing requirements ‘Hard’ real-time system

Operation incorrect if results not produced according to timing specification


Stimulus/Response Systems

Given stimulus, system must produce response within specified time

Periodic stimuli Occur at predictable time intervalsE.g., T° sensor polled 10 times/second

Aperiodic stimuli Occur unpredictablyE.g., system power failure


Architectural Considerations

Allow fast switching between stimulus handlers

Different stimuli have different timing demandsE.g., weather station – T° vs light intensityThus simple sequential loop not usually

adequateReal-time systems

Usually designed as cooperating processes

With real-time executive controlling these processes


Real-Time System Model

Real-timecontrol system

ActuatorActuator ActuatorActuator

SensorSensorSensor SensorSensorSensor


System Elements

Sensors control processesCollect information from sensors. May buffer data

Data processorCarries out processing of collected

informationComputes system response

Actuator controlGenerates control signals for actuator


Sensor/Actuator Processes

Dataprocessor

Actuatorcontrol

Actuator

Sensorcontrol

Sensor

Stimulus Response


System Elements

Sensor control processesCollect information from sensors. May

buffer information collected in response to a sensor stimulus.

Data processorCarries out processing of collected

information and computes the system response.

Actuator control processesGenerates control signals for the

actuators.


Topics



System Design

Design both hardware and software associated with systemPartition functions to either hardware or

softwareDesign decisions should be made on basis on

non-functional system requirementsHardware delivers better performance

But potentially longer developmentLess scope for change


Hardware and Software Design

Establish systemrequirements

Partitionrequirements

Hardwarerequirements

Hardwaredesign

Softwarerequirements

Softwaredesign


R-T Systems Design Process (1)

Identify stimuli to be processedRequired responses to these stimuli

For each stimulus and response, Identify timing constraints

Aggregate stimulus and response processing into concurrent processes Process may be associated with each

class of stimulus and response


R-T systems Design Process (2)

Design algorithms to process each class of stimulus and response. Meet given timing requirements

Design scheduling system Ensure processes started in time to meet

deadlines (during execution) Integrate using real-time executive or

operating system


Timing Constraints

Difficult to predictExtensive simulation and experiment to

ensure that these met by systemMay mean that certain design strategies such

as object-oriented design cannot be usedToo much overhead involved

May mean that low-level programming language features have to be used For performance reasons


State Machine Modeling

Effect of stimulus in real-time system may trigger transition from one state to another.

Finite state machines (FSM) can be used for modeling real-time systems

However, FSM models lack structure. Even simple systems can have complex

model. UML includes notations for defining state

machine modelsSee also Chapter 7


State Machine Modeling

Effect of stimulus in real-time system may trigger transition from one state to another.

Finite state machines can be used for modeling real-time systems.

However, FSM models lack structure. Even simple systems can have complex model.

UML includes notations for defining state machine models

See also Chapter 7.


Microwave Oven State Machine (As Shown Before)

Full power

Enabled

do: operateoven

Fullpower

Halfpower

Halfpower

Fullpower

Number

TimerDooropen

Doorclosed

Doorclosed

Systemfault

Start

do: set power = 600

Half powerdo: set power = 300

Set time

do: get numberexit: set time

Disabled

Operation

Timer

Cancel

Waiting

do: display time

Waiting

do: display time

do: display 'Ready'

do: display 'Waiting'


Petrol pump State Model

Cardinserted

into reader

Timeout

Resettingdo: display CC

error

Initialising

do: initialisedisplay

Paying

Stopped

Reading

do: get CCdetails

Waiting

do: display welcome

do:deliver fuel

do: debitCC account

Payment ack.

Ready Delivering

update displayNozzle

trigger on

Nozzle trigger off

Nozzle trigger on

Hose inholster

do: validatecredit card

Validating

Invalid card

Card removedCard OK

Hose out of holster

Timeout


Real-Time Programming

Hard real-time systemsOften in assembly language to ensure that

timing requirements metHigh-level languages such as C

Allow efficient programs to be written But do not have constructs to support

concurrency or shared resource management

AdaDesigned to support real-time systems

designIncludes general purpose concurrency

mechanism


Java as Real-Time LanguageJava

Supports lightweight concurrency threads and synchronized methods

Can be used for some soft real-time systems


Java (2)

Java 2.0 not suitable for hard RT programming or programming where precise control of timing requiredNot possible to specify thread execution

timeUncontrollable garbage collectionNot possible to discover queue sizes for

shared resourcesVariable virtual machine implementationNot possible to do space or timing analysis


Topics



Real-Time Executives

Real-time executives Specialized operating systemsManage processes in RTS

Standard RTE kernelUsed as-is; orCan be modified for particular application

LimitedNo file management facilities

14


Executive Components

Real-time clockInfo for process scheduling

Interrupt handlerManages aperiodic requests for service.

SchedulerChooses next process to be run

Resource managerAllocates memory and processor

resourcesDispatcher

Starts process execution


Non-Stop System Components

Configuration managerDynamic reconfiguration of system SW,

HW without stopping systemsReplace HW modulesUpgrade SW

Fault managerDetect SW & HW faultsTake appropriate actions

E.g. switch to backup disksEnsure that system continues operation


Real-Time Executive Components

Process resourcerequirements

Scheduler

Schedulinginformation

Resourcemanager

Despatcher

Real-timeclock

Processesawaitingresources

Readylist

Interrupthandler

Availableresource

list

Processorlist

Executingprocess

Readyprocesses

Releasedresources


Process Priority

Processing some types of stimuli must sometimes take priority

Interrupt level priorityHighest priority allocated to processes

requiring very fast responseClock level priority

Allocated to periodic processesFurther levels of priority may be assigned

within these levels


Interrupt Servicing

Control transferred automatically to pre-determined memory locationContains instruction to jump to interrupt

service routineFurther interrupts disabled, Interrupt servicedControl returned to interrupted process Interrupt service routines MUST be

short,simple and fast


Periodic Process Servicing

Several classes of periodic processDifferent periods (time between

executions)Different execution timesDifferent deadlines (time by which

processing must be completed)Real-time clock ticks periodically

Each tick causes interrupt which schedules process manager for periodic processes

Process manager selects process among those ready for execution


RTE Process Management

Resource manager

Allocate memoryand processor

Scheduler

Choose processfor execution

Despatcher

Start execution on anavailable processor


Process Switching

Scheduler Chooses next process to be executed by

processorDepends on scheduling strategy

May take process priority into accountResource manager

Allocates memory and processorDispatcher

Takes process from ready list, Loads it onto processorStarts execution


Scheduling Strategies

Non pre-emptive schedulingProcess runs to completionOr until blocked (e.g. pending I/O)

Pre-emptive schedulingExecution of executing processes may be

stoppedIf higher-priority process requires service

Scheduling algorithmsRound-robinRate monotonicShortest deadline first


Topics



Monitoring and Control Systems

Important class of real-time systems Continuously check sensors and take actions

depending on sensor valuesMonitoring systems examine sensors and

report their resultsControl systems take sensor values and

control hardware actuators


Generic architecture

S1

S2

S3

P (S1)

P (S2)

P (S1)

Monitoringprocesses

Controlprocesses

P (A1)

P (A2)

P (A1)

A1

A2

A3

P (A4) A4

Testingprocess

Control panelprocesses


Burglar Alarm System

Monitor sensors on doors and windowsDetect presence of intruders in building

When sensor indicates break-inSwitches on lights around areaCalls police automatically

Provision for operation even when external power fails / cut


Burglar Alarm SystemSensors

Movement detectors, window sensors, door sensors.

50 window sensors, 30 door sensors and 200 movement detectors

Voltage-drop sensorActions

When intruder detected, police called automatically

Lights switched on in rooms with active sensors

Audible alarm switched onSystem switches automatically to backup

power when voltage drop detected


R-T System Design Process

Identify stimuli and associated responsesDefine timing constraints associated with

each stimulus and responseAllocate system functions to concurrent

processesDesign algorithms for stimulus processing

and response generationDesign scheduling system which ensures that

processes will always be scheduled to meet their deadlines


Stimuli To Be Processed

Power failureGenerated aperiodically by circuit monitorSwitch to backup power within 50 ms

Intruder alarmStimulus generated by system sensorsResponse

Call policeSwitch on building lightsAudible alarm


Timing Requirements


Process Architecture

Lighting controlprocess

Audible alarmprocess

Voice synthesizerprocess

Alarm systemprocess

Power switchprocess

Building monitorprocess

Communicationprocess

Door sensorprocess

Movementdetector process

Window sensorprocess

560Hz

60Hz400Hz 100Hz

Power failureinterrupt

Alarmsystem

Building monitor

Alarmsystem

Alarm system

Alarm system

Detector status Sensor status Sensor status

Room number

Alert message

Room number

Room number

50 sensors @ 2/sec


Control Systems

Burglar alarm systemPrimarily monitoringCollects data from sensorsNo real-time actuator control

Control systemsSystem sends control signals to actuatorsE.g., temperature-control system

Monitors T°Switches heaters on and off


Temperature Control System

Thermostatprocess

Sensorprocess

Furnacecontrol process

Heater controlprocess

500Hz

500Hz

Thermostat process500Hz

Sensorvalues

Switch commandRoom number


Topics



Data Acquisition Systems

Collect data from sensors for subsequent processing and analysis.

Data collection processes and processing processes may have different periods and deadlines

Data collection may be faster than processingE.g. collecting information about explosion

Circular or ring buffers Mechanism for smoothing speed

differencesFile where new data added at bottom

displace old data at top


Data acquisition architecture

DisplayProcess

dataSensor data

bufferSensorprocess

Sensoridentifier and

value

Sensors (each data flow is a sensor value)


value

Processdata

Sensor databuffer

Sensorprocess


value


value

s1

s2

s3

s4

s5

s6


Reactor Data Collection

System collects data Sensors monitoring neutron fluxFrom nuclear reactor

Flux data placed in ring bufferFor later processing

Ring buffer Implemented as concurrent processCollection and processing processes may

be synchronizedSize of buffer must be set to account for

discrepancy of filling (sensors) and emptying (processing) speeds


Reactor Flux Monitoring

DisplayProcess

dataSensor data

bufferSensorprocess


value

Processedflux level

Sensors (each data flow is a sensor value)


Ring Buffer

Consumerprocess

Producerprocess


Mutual Exclusion (Locking)

Producer processes collect data and add it to buffer

Consumer processes take data from buffer and make elements available

Producer and consumer processes must be mutually excluded from accessing same element

Buffer must stop producer processes adding information to full buffer and consumer processes trying to take information from empty buffer


Key points

Real-time system correctness depends not just on what the system does but also on how fast it reacts.

A general RT system model involves associating processes with sensors and actuators.

Real-time systems architectures are usually designed as a number of concurrent processes.


Key points

Real-time operating systems are responsible for process and resource management.

Monitoring and control systems poll sensors and send control signal to actuators.

Data acquisition systems are usually organized according to a producer consumer model.


Homework

RequiredBy Wed 20 Oct 2004For 35 points,

15.1 (@5), 15.7 (@10), 15.8 (@20)Optional

By Wed 27 Oct 2004For a maximum of 15 points, answer either

or both of15.4 (@10)15.10 (@5)


DISCUSSION








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