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CS 425/625 Software Engineering

Real-Time Software Design

Based on Chapter 15 of the textbook [SE-7] Ian Sommerville,Software Engineering, 7th Ed., Addison-Wesley, 2004 and on theCh15 PowerPoint presentation available at the book’s web-site

October 17, 2005

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Outline

Introduction Real-Time Systems (RTS): A Characterization RTS Design RT Operating Systems Generic RTS architectures:

Monitoring and Control Systems Data Acquisition Systems

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Introduction.…

Real-Time Systems: systems whose correct operation depends not only on the correctness of the results produced but also on the time at which these results are produced

Embedded Systems [www.webopedia.com]: An embedded system is “a specialized computer system that is part of a larger system or machine. Typically, an embedded system is housed on a single microprocessor board with the programs stored in ROM. Virtually all appliances that have digital interfaces (e.g., watches, microwaves, VCRs, cars) utilize embedded systems […]”

Usually, embedded systems are RTS

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.Introduction...

RTS receive stimuli (both external and internal) and provide responses to these stimuli

Stimuli: Periodic: occur at preset intervals of time (e.g.,

every 20 ms) Aperiodic: have irregular occurrences

The sensor-system-actuator model of RTS: sensors provide inputs (stimuli), computational units elaborate responses, and actuators convey outputs (responses)

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..Introduction..

Three types of processes: Sensor management Computational Actuator management

Since many stimuli need immediate treatment software handlers are needed. Handlers can run concurrently, hence RTS are usually designed as a set of concurrent processes.

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...Introduction.

General model of an RTS [Fig. 15.1, SE-7]

Real-timecontrol systemActuatorActuator ActuatorActuatorSensorSensorSensor SensorSensorSensor

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.…Introduction

Sensor/actuator processes [Fig. 15.2, SE-7]

Dataprocessor ActuatorcontrolActuatorSensorcontrol

SensorStimulus Response

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RTS: A Characterization……

This section of the presentation is based on [Dascalu01] “A real-time system must respond to externally generated

stimuli within a finite, specifiable time delay” [Everett95] An RTS differs from a “regular” (non-RTS) system in at

least the following aspects [Stankovic88]: Have deadlines attached to some or all tasks Faults in the system may lead to catastrophic

consequences Must have the ability to deal with exceptions Must be fast, predictable reliable, adaptive

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.RTS: A Characterization.….

“Development of most software focuses on how to handle a normal situation, but real-time, critical-application development also focuses on how to handle the abnormal situation” [Everett95]

RTS “must operate under more severe constraints than ‘normal’ software yet perform reliably for long periods of time” [Douglass99]

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..RTS: A Characterization….

A classification of RTS:

Utility

Timeth (hard deadline)

(a) Hard RTS

Utility

th (hard)Time

ts (soft)

(b) Firm RTS

Utility (c) Soft RTS

Timets (soft deadline)

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…RTS: A Characterization…

Requirements for RTS: Timeliness

Reaction to stimuli “on time” (deadlines must be met) Relative and absolute timing constraints

Reliability Many errors have roots in incorrect specification Formal techniques needed for safety-critical systems

Intensive dynamics Models to describe behavior are necessary (based on

finite state machines)

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….RTS: A Characterization..

Requirements for RTS (cont’d): Exception handling

Priorities should be assigned to stimuli/events Mechanisms for handling interrupts need be developed

Concurrency Parallel tasks are inherent in RTS The environment is also “concurrent” in nature

Distribution & resource allocation Distribution is not necessarily a characteristic of RTS,

but should be taken into consideration in larger applications

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…..RTS: A Characterization.

Requirements for RTS (cont’d): Communication and synchronization

Synchronous and asynchronous communication mechanisms should be designed

Size In larger applications, there are numerous processes

and threads Size is associated with continuous change Decomposition in smaller units is needed, as are

mechanisms for modeling hierarchical structures

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.…..RTS: A Characterization

Requirements for RTS (cont’d): Non time-constrained activities

Worst case scenarios cannot be easily evaluated Computations & data modeling

In process control systems computations can be complex

In RT databases data must have temporal validity Reuse

RTS are poor candidates for reuse (are too specialized) However, OO design may provide solutions

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RTS Design…

Both the hardware and the software of the system must be designed and system functions allocated to either hardware or software

RTS design process should result in a system model that can be implemented in either software or hardware

Special-purpose hardware: Better performance, but Longer development time, and Less suitable to change

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.RTS Design..

An RTS design process focuses on events (stimuli) rather than on objects or functions

Suggested RTS design process: Identify stimuli and associated responses Identify timing constraints on stimuli and responses Choose an execution platform for the system:

hardware & RTOS Aggregate stimulus and response processing

activities in several concurrent processes Design computational algorithms for each

stimulus/response association Design the scheduling software

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..RTS Design.

RTS modeling relies on the use of state machines [e.g., Fig. 8.5. and 8.7, Somm00]

Timing constraints: May require extensive simulation and experimentation May preclude the use of an object-oriented

development approach (because of the overhead involved at run-time)

May require, for performance reasons, programming in assembly languages or system-level languages such as C

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…RTS Design

RT programming: System-level languages (e.g., C) allow elaboration of

efficient code but the burden to express concurrency and to manage shared resources is on the programmer

Specially designed languages with good synchronization mechanisms such as Ada still have a number of limitations (e.g., lack of exceptions when deadlines are not met, strict FIFO policy for task queues)

Java has several facilities for lightweight RT programming (threads, synchronized methods) but also a number of limitations (e.g., garbage collector not controllable, JVM has various implementations)

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RT Operating Systems...

RTOS: specialized operating system for RTS Main responsibilities:

Process management Resource allocation (processor, memory)

They may not include regular OS facilities such as file management

Manage at least two priority levels: Interrupt level, for processes that need fast response Clock level, for periodic processes

Typical components: real-time clock, interrupt handler, scheduler, resource manager, dispatcher

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.RT Executives.. Typical structure of an RTOS [Fig. 15.4, SE-7]

Process resourcerequirementsSchedulerSchedulinginformationResourcemanagerDespatcher

Real-timeclockProcessesawaitingresourcesReadylist

InterrupthandlerAvailableresourcelistProcessorlistExecutingprocessReadyprocessesReleasedresources

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..RTOS.

Process management: Coordination of the system’s set of concurrent

processes Periodic processes run at pre-set intervals of time Process period: time between executions Process deadline: the time by which the process

must be complete The executive uses the real-time clock to determine

when a process must execute; a real-time tick period is usually several milliseconds long

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...RTOS

RTOS actions to start a process [Fig. 15.5, SE-7]

Scheduling strategies: Non-preemptive: a process scheduled for execution runs until

completion or until blocked (e.g., waiting for an input) Pre-emptive: a higher-priority process can take over a lower-

priority process Scheduling algorithms, examples: round-robin, shortest

deadline first, rate monotonic

Resource managerAllocate memoryand processorSchedulerChoose processfor execution DespatcherStart execution on anavailable processor

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Generic RTS Architectures..….Generic RTS Architectures..….

Typical classes of RTS (each with a characteristic architecture): Monitoring and control systems [MCS]:

Monitoring systems examine sensors and report their results; may take action in exceptional cases

Control systems read sensors and continuously command actuators

Data acquisition systems [DAS] collect data from sensors for later processing and analysis

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Generic RTS Architectures..….Generic RTS Architectures..….

S1S2S3P (S1)P (S2)P (S1)MonitoringprocessesControlprocessesP (A1)P (A2)P (A1)A1A2A3P (A4)A4

TestingprocessControl panelprocesses

Generic MCS architecture [Fig. 15.6, SE-7]

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.Generic RTS Architectures..…

An intruder alarm system (monitoring system): Monitors sensors on doors and windows to detect

the presence of intruders in a building; also monitors movement sensors in rooms

When a sensor indicates a break-in, switches on lights around the area and calls police automatically

Powered by a main power supply but also has provisions for battery backup; includes a power circuit monitor

Timing requirements for the system are shown on the next page [Fig.15.7, SE-7]

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..Generic RTS Architectures....

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...Generic RTS Architectures… The architecture of the intruder alarm system [Fig. 15.8, SE-7]

Lighting controlprocessAudible alarmprocess Voice synthesizerprocessAlarm systemprocessPower switchprocessBuilding monitorprocess CommunicationprocessDoor sensorprocessMovementdetector process Window sensorprocess560Hz

60Hz400Hz 100HzPower failureinterruptAlarmsystem

Building monitorAlarmsystem

Alarm systemAlarm system

Detector statusSensor statusSensor statusRoom numberAlert messageRoom numberRoom number

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….Generic RTS Architectures.. Architecture of a temperature control system [Fig. 15.10, SE-7]

ThermostatprocessSensorprocess

Furnacecontrol processHeater controlprocess

500Hz500Hz

Thermostat process500HzSensorvalues

Switch commandRoom number

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…..Generic RTS Architectures. Generic DAS architecture [Fig. 15.11, SE-7]

DisplayProcessdataSensor databufferSensorprocessSensoridentifier andvalueSensors (each data flow is a sensor value)Sensoridentifier andvalueProcessdataSensor databufferSensorprocessSensoridentifier andvalueSensoridentifier andvalue

s1s2s3s4s5s6

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…..Generic RTS Architectures. A neutron flux data acquisition system [Fig. 15.12, SE-7]

OperatordisplayFluxprocessingFlux databufferA-DconvertorSensoridentifier andflux valueProcessedflux levelNeutron flux sensors

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……Generic RTS Architectures A ring buffer for data acquisition [Fig. 15.13, SE-7]A ring buffer for data acquisition [Fig. 15.13, SE-7]

ConsumerprocessProducerprocess

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Additional References

[Dascalu01] Dascalu, S., Combining Semi-forma and Formal Notations in Software

Specification: An Approach to Modelling Time-Constrained Systems, PhD thesis, Dalhousie University,

Halifax, NS, Canada, 2001. [Douglass99] Douglass, B.P., Doing Hard Time: Developing

Real-Time Systems with UML, Objects, Frameworks and Patterns, Addison-Wesley, 1999.

[Everett95]Everett, W., and Honiden, S., “Reliability and Safety of Real-Time Systems,” IEEE

Software, 12(3), May 1995, p. 12-16[Gibbs94] Gibbs, W.W., “Software’s Chronic Crisis,”

Scientific American, Sep. 1994, p. 86-95. [Stankovic88] Stankovic, J.A., and Ramamritham, K.,

Tutorial: Hard Real-Time Systems, IEEE Computer Society Press, 1988.