real time computing

7/29/2019 Real Time Computing

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Real-Time Computing

Sunggu Lee

EE Dept., POSTECH


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Page 2Building Reliable Component-based SystemsEECE 426 - Embeddede Systems

Overview

Introduction

Characteristics and Challenges of Real-Time ComputingSystems

Definitions, Issues and Comparisons

Tasks and Scheduling

Worst-Case Execution Time Analysis

Real-Time Software

Real-Time Operating Systems

Middleware


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Introduction [Crnkovic 2002]

Embedded computers

[Examples:] Medical control equipment, mobile phones,and vehicle control systems.

Most [] such embedded systems can also be

characterized as real-time systems.

They must usually meet stringent specifications forsafety, reliability, limited hardware capacity etc


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Characteristics and Challenges of RTS

[Crnkovic 2002]

Real-time systems are computing systems in which themeeting of timing constraints is essential tocorrectness.

If the system delivers the correct answer, but after a

certain deadline, it could be regarded as having failed.


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Types of Real-Time Systems

Hard real-time system

A system where something very bad happensif the deadline is not metExamples: control systems for aircraft, nucluear reactors,

chemical power plants, jet engines, etc.

Soft real-time system

A system where the performance is degraded

below what is generally considered acceptableif the deadline is missedExample: multimedia system


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Utility Function (Task Value Function) [Kim

2005]

deadline Time0

1

Task with a hard deadlineTask with a soft deadline

Utility


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Issues in Real-Time Computing [Liu 2000]

Real-time computing deals with all problems incomputer architecture, fault-tolerant computing andoperating systems are also problems in real-timecomputing, with the added complexity of having to meetreal-time constraints

Real-time computer systems differ from general-purpose systems

They are more specific in their applications

The consequences of their failure are more drastic

Emphasis is placed on meeting task deadlines


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Example Problems in Real-Time Computing

Example 1: Task Scheduling

General-purpose system can use round-robinscheduling

This is NOT suitable for real-time systems becausehigh-priority tasks may miss their deadlines with round-

robin schedulingA priority mechanism is necessary

Example 2: Cache Usage Scheduling

A general-purpose system typically allows the processthat is currently executing the right to use the entirecache areaThis keeps the cache miss rate low

Side effect: task run times are less predictable

Thus, not so desirable for real-time systems


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Comparison of Typical Systems [Liu 2000]

Jet Engine Control System

Designer knows preciseworkload to be executed

System must be

designed to meet taskdeadlinesIf a deadline is not met, the jet

engine may explode

General-Purpose ComputerWorkstation

Workload is not known inadvance

System should bedesigned to be fast onthe averageTask execution time variance is

less important


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Tasks [Crnkovic 2002]

Real-time systems can be constructed [out] ofsequential programs, but typically they are built [out] ofconcurrent programs, called tasks.

Tasks are usually divided into:

Periodic tasks: consist of an infinite sequence ofidentical activities, called instances, which are invokedwithin regular time periods.

Non-periodic [or aperiodic] : are invoked by theoccurrence of an event.

[Sporadic : aperiodic tasks with a bounded interarrivaltime]


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Scheduling [Crnkovic 2002]

Offline scheduling:

The scheduler has complete knowledge of the task setand its constraints.

Online scheduling:

Make their scheduling decisions during run-time.

Deadline:

Is the maximum time within which the task mustcomplete its execution with respect to an event.

Real-time systems are divided into two classes, hardand softreal-time systems


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Schedulability Analysis [Crnkovic 2002]

At this point we must check that the temporalrequirements of the system can be satisfied, assumingtime budgets assigned in the detailed design stage.

In other words, we need to make a schedulabilityanalysis of the system based on the temporalrequirements of each component


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WCET Verification [Crnkovic 2002]

Performing a worst-case analysis can either be basedon measurements or on a static analysis of thesource code.

What is more interesting in the test cases is theexecution time behavior shown as a function of inputparameters as shown in the following slide.


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End-To-End Deadlines [Crnkovic 2002]

End-to-end deadlines

Are set such that the system requirements are fulfilled inthe same way as the time budgets are set

Should be specified for the input to and output from the

component since the WCET cannot be computed sinceits parts may be executing with different periods.


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Real-Time Operating System

Multi-tasking OS designed to permit tasks (processes)to complete within precisely stated deadlines

If deadline constraints cannot be met for a new task, itmay be rejected

If a new task would result in deadline violations for othertasks, it may be rejected

Example commercial operating systems

Vrtx Mentor Graphics Systems

VxWorks and pSOS Wind River Systems RTLinux FSMLabs, later acquired by Wind River

Systems


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Real-Time Middleware

ObjectWeb defines middleware as: "The software layerthat lies between the operating system and theapplications on each side of a distributed computingsystem in a network." [Wikipedia]

Collection of Tools for Real-Time Programming Real-Time Java

Real-Time Middleware Systems

Real-Time CORBA

CORBA: Component Object Request Broker Architecture

Time-triggered Message-triggered Object (TMO)

System developed at the University of California at Irvine
http://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Operating_system


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An example of a TMO

network

Base Intf TMO

SpM1

SvM1

ODSS1

Car Intf TMO

SpM1

SvM1

ODSS1

Base Intf TMO

SpM1

SvM1

ODSS1

Car Intf TMO

SpM1

SvM1

ODSS1 Car Intf TMO

SpM1

SvM1

ODSS1

Ex 2

Telematics in a Car Fleet [Kim 2005]


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References

Jane Liu, Real-Time Systems, Prentice-Hall, Upper SaddleRiver, 2000.

Ivica Crnkovic and Magnus Larsson, Building ReliableComponent-Based Software Systems, Artech House

Publishers, July 2002. PowerPoint slides fromhttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppt

Kane (K. H.) Kim, PowerPoint slides used for undergraduateclass: ECES 123: Introduction to Real-Time Distributed

Programming taught at the University of California at Irvine,http://dream.eng.uci.edu
http://www.idt.mdh.se/~icc/http://www.idt.mdh.se/~mlo/http://hhp.bvdep.com/artechhouse/default.asp?http://hhp.bvdep.com/artechhouse/default.asp?http://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://www.idt.mdh.se/cbse-book/presentations/13-chapterWC.ppthttp://hhp.bvdep.com/artechhouse/default.asp?http://hhp.bvdep.com/artechhouse/default.asp?http://www.idt.mdh.se/~mlo/http://www.idt.mdh.se/~icc/

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