RealTime Applications Typical RealTime Applications Hard versus Soft RealTime systems

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Real-Time Applications Typical Real-Time
ApplicationsHard versus Soft Real-Time systems

• Internet Management Technology Lab.

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Contents

• Introduction
• Jobs and processors
• Release Times, Deadlines, and Timing Constraints
• Digital Control
• High-Level Controls
• Signal Processing
• Real-Time Databases
• Hard and Soft Timing Constraints
• Hard Real-Time Systems
• Soft Real-Time Systems
• Summary

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Introduction(1/2)

• What is a Real-Time System?
• RTS supports
• The execution of applications with timing
  constraints on it
• RTS consists of
• controlling subsystem systems that control
  resources
• controlled entities objects that are components
  of environments
• RTS is classified as
• Soft real-time systems and Hard real-time systems
• Periodic systems and Aperiodic systems
• Static systems vs. Dynamic systems

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Static vs. Dynamic Real-Time Systems
Controlled Entities
Controlled Entities
Controlling Subsystem
Controlling Subsystem
Resources
Resources
Program
Program
Real-Time OS
Static Real-Time System
Dynamic Real-Time System
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Jobs and Processors

• Job
• Each unit of work that is scheduled and executed
  by the system
• Every job executes on some resource
• Example
• Computation of a control law
• Computation of a FFT(Fast Fourier Transform) of
  sensor data
• Transmission of a data packet
• Retrieval of a file
• Task
• The set of related jobs which jointly provide
  some system function(application)
• Processors
• Active resources(on CPU, network, disk)

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Release Times, Deadlines, and Timing Constraints

• Release time
• The instant of time at which the job becomes
  available for execution
• Deadline
• The instant of time by which a job must be
  completed
• Response time
• Time from release time until completion of job
• Relative deadline
• Maximum allowable response time
• Absolute deadline
• Release time plus relative deadline
• Timing constraints
• Specified in terms of release time and deadlines

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Digital Control(1/2)

• Sampled Data system
• Implementation(Control loop)
• Set timer to interrupt periodically with period
  T
• At each time interrupt, do
• Do analog-to digital conversion to get y
• Compute control output u
• Output u and do digital-analog conversion
• End do

Controlled system (ex. Engine, brake, aircraft)
Multirate systems With multiple sensors
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Digital Control(2/2)

• Timing characteristics
• The workload consists of a few periodic
  control-law computations
• A control system consists of many digital
  controllers, each dealing with part of the system
• Together they demand perhaps hundreds of control
  laws be computed periodically

• More Complex Control-Law Computations
• The simplicity of digital controller depends on
  three assumptions
• 1. Sensors give accurate estimates of the
  state-variable values being monitored and
  controlled
• 2. Sensor data give the state of the system
• 3. All parameters representing the dynamics of
  the system are known

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High-Level Controls(1/2)
Air traffic/flight control hierarchy
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Real-Time Databases

• Real-Time Databases
• Refer to a diverse spectrum of information
  systems, ranging from stock price quotation
  systems, to track records databases, to real-time
  file systems
• Support
• Consistent of real-time data
• Concurrent access of shared data
• Transaction synchronization
• Detailed research topics
• Concurrency control protocols
• Memory resident transactions
• Fault Tolerance

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Hard and Soft Timing Constraints(1/2)

• Common definitions
• based on functional criticality, usefulness and
  determinism
• Hard deadline
• Leads to fatal fault
• Usefulness of result as functions of the
  tardiness of jobs falls off abruptly
• Tardiness difference between completion time
  and deadline
• Soft deadline
• Leads to undesirable(No serious harm)
• Usefulness of result as functions of the
  tardiness of jobs decreases gradually for soft

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Profit Graphs of a Real-Time System
Profit
Profit


time
time
0
0
-
-
deadline
deadline
release time
release time
Soft Real-Time System
Hard Real-Time System
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Hard and Soft Timing Constraints(2/2)

• Hard Timing Constraints and Temporal
  Quality-of-Service Guarantees
• Hard requires validation that the system always
  meets timing constraints
• Validation
• Demonstration by a provably correct procedure or
  by exhaustive simulation and testing
• Guaranteed services
• Soft requires no validation or only that the job
  meets some statistical constraint
• Best-effort services

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Hard Real-Time Systems

• The requirement places many restrictions on
  design and implementation of hard applications
• Some reason for Requiring Timing Guarantees
• Embedded systems require responsiveness of
  sensors and actuators monitored and controlled
• Control-law computation job of a flight
  controller
• High availability of critical systems
• The system must never be down
• More on Hard timing Constraints
• There may be no advantage in completing a job
  with a hard deadline early
• It is often advantageous, even essential to keep
  jitters in response times of a stream of jobs
  small
• Hard timing constraint to be specified
• Deterministic constraints
• Probabilistic constraints
• Constraints in terms of usefulness function

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Soft Real-Time Systems

• Example
• On-line transaction systems, telephone switches,
  electronic games
• It is more important to have small average
  response time and high throughput
• Occasional late or missed updates are tolerated
  as a trade-off for other factors
• Cost, availability, number of users
• Timing requirements often specified in
  probabilistic terms
• Telephone network
• QoS, validation, timing constraints of multimedia
  are soft

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Examples of RTS Applications

• Avionics control systems(H)
• Air traffic control systems(H)
• Medical monitoring(H/S)
• Process control systems(H/S)
• Nuclear power plant control(H)
• Stock market trading systems(S)
• Real-Time Multimedia(S)
• High speed communication systems(S)

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Research Areas in RTS

• Real-time Specification and Verification
• Real-time Operating Systems
• Real-Time Database Systems
• Real-Time Communications and Networks
• Real-time Programming Languages
• Real-time System Architecture

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Specification and Verification

• Characteristics
• Contract between system and environment
• Need to meet specification without fault
• Computation models
• Petri Net Model or State Model(Operational
  approach)
• Temporal Logic Model(Deductive approach)
• Hoare Logic Model(Axiomatic approach)
• Linear/Nonlinear Algebraic Model(Algebraic
  approach)
• Natural Deduction Model(Natual approach)
• Usually difficult and further exacerbated
  problems
• concurrency inherent and time presence
• logical correctness and physical property of
  timeliness

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Real-Time Operating Systems

• Goals are to provide
• Guaranteed timely execution
• Predictable system behavior
• Detailed research topics
• CPU or Processing elements scheduling
• Resource allocation and management
• Timing constraint validation
• Safe and predicatable kernel design
• Examples of Real-Time OS
• Maruti(University of Maryland),
  Springs(University of Massachusetts), Lynx
  (commercial system

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Real-Time Databases

• Support
• Consistent of real-time data
• Concurrent access of shared data
• Transaction synchronization
• Detailed research topics
• Concurrency control protocols
• Memory resident transactions
• Fault Tolerance
• Synchronization problems

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Real-Time Communications

• Supports
• Predictive communication scheduling
• Communication resource allocation and management
• Timely routing behavior
• Detailed Research Topics
• Multimedia on demand applications
• Communication protocols
• Channel and Buffer management
• Communication security

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RTS Programming Languages

• Need to be a high-level language
• Timing constraint representation
• Unknown timing factor handling
• Interrupts
• Unlimited loops
• Recursive functions
• Underlying languages
• Ada, RT-FLEX, Euclid, Reactive-C, CHaRTS

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Real-Time System Architecture

• Operation timing(Time measurement)
• Caching analysis
• I/O management
• Parallel processors
• Falt tolerance and monitoring
• Real-time processors
• Test and Measurement Processors
• HART Architecture(University of Michigan)

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Summary(1/2)

• Chapter 3
• Describes the general model of real-time systems
  and introduces the terms
• Chapter 4
• Gave a brief overview of the clock-driven,
  weighted round-robin and priority-driven
  approaches to scheduling
• Chapter 5
• Described crock-driven schedulers that schedule
  periodic tasks according to some cyclic schedule
• Chapter 6
• Focused on the performance of priority-driven
  algorithms for scheduling independent, preemptive
  periodic tasks
• Chapter 7
• Describes algorithms for scheduling aperiodic and
  sporadic jobs in a system of periodic tasks

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Summary(2/2)

• Chapter 8
• Described several protocols for controlling
  accesses to shared resources in systems
• Chapter 11
• Focused on algorithms and protocols for real-time
  communication
• Chapter 12
• Gave an overview of several commercial and widely
  used operating systems