drews@ohio.edu - PowerPoint PPT Presentation


PPT – drews@ohio.edu PowerPoint presentation | free to download - id: 64fb48-OWI4Z


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation



Real-Time Systems Introduction Frank Drews drews_at_ohio.edu Real-time Systems A real-time system is a system whose specification includes both logical and temporal ... – PowerPoint PPT presentation

Number of Views:4
Avg rating:3.0/5.0
Date added: 20 November 2019
Slides: 31
Provided by: Fra783
Learn more at: http://www.cs.kent.edu


Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: drews@ohio.edu

Real-Time Systems
  • Introduction
  • Frank Drews
  • drews_at_ohio.edu

Real-time Systems
  • A real-time system is a system whose
    specification includes both logical and temporal
    correctness requirements.
  • Logical Correctness Produces correct outputs.
  • Can by checked, for example, by Hoare logic.
  • Temporal Correctness Produces outputs at the
    right time.
  • In this course, we spend much time on techniques
    and technologies for achieving and checking
    temporal correctness.

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
  • Many embedded systems are real-time systems

Typical Characteristics of Real-Time Systems
  • Event-driven, reactive.
  • High cost of failure.
  • Concurrency/multiprogramming.
  • Stand-alone/continuous operation.
  • Reliability/fault-tolerance requirements.
  • Predictable behavior.

Misconceptions about Real-Time Systems
(Stankovic 88)
  • There is no science in real-time-system design.
  • We shall see
  • Advances in supercomputing hardware will take
    care of real-time requirements.
  • The old buy a faster processor argument
  • Real-time computing is equivalent to fast
  • Only to ad agencies. To us, it means PREDICTABLE

Misconceptions (Continued)
  • Real-time programming is assembly coding,
  • We would like to automate (as much as possible)
    real-time system design, instead of relying on
    clever hand-crafted code.
  • Real time is performance engineering.
  • In real-time computing, timeliness is almost
    always more important than raw performance
  • Real-time problems have all been solved in
    other areas of CS or operations research.
  • OR people typically use stochastic queuing models
    or one-shot scheduling models to reason about
  • In other CS areas, people are usually interested
    in optimizing average-case performance.

Misconceptions (Continued)
  • It is not meaningful to talk about guaranteeing
    real-time performance when things can fail.
  • Though things may fail, we certainly dont want
    the operating system to be the weakest link!
  • Real-time systems function only in a static
  • Note true. We consider systems in which the
    environment may change dynamically.

Are All Systems Real-Time Systems?
  • Question Is a payroll processing system a
    realtime system?
  • It has a time constraint Print the pay checks
    every two weeks.
  • Perhaps it is a real-time system in a
    definitional sense, but it doesnt pay us to view
    it as such.
  • We are interested in systems for which it is not
    a priori obvious how to meet timing constraints

The Window of Scarcity
  • Resources may be categorized as
  • Abundant Virtually any system design methodology
    can be used to realize the timing requirements of
    the application.
  • Insufficient The application is ahead of the
    technology curve no design methodology can be
    used to realize the timing requirements of the
  • Sufficient but scarce It is possible to realize
    the timing requirements of the application, but
    careful resource allocation is required.

Example Interactive/Multimedia Applications
Example Real-Time Applications
  • Many real-time systems are control systems
  • Example 1 A simple one-sensor, one-actuator
    control system

Simple Control System (Continued)
  • Pseudo-code for this system
  • T is called sampling period. T is a key design
    choice. Typical range for T seconds to

Multi-rate Control Systems
  • More complicated control systems have multiple
    sensors and actuators and must support control
    loops of different rates.
  • Example 2 Helicopter flight controller.
  • Note Having only harmonic rates simplifies the

Hierarchical Control Systems
Air Traffic Control
Reddaway et al. WMPP05
Signal-Processing System
  • Signal-processing systems transform data from one
    form to another.
  • Examples
  • Digital filtering.
  • Video and voice compression/decompression.
  • Radar signal processing.
  • Response times range from a few milliseconds to a
    few seconds.

Example Radar System
Internet/Multimedia Applications
  • Web farms hosting multiple web domains
  • Each web domain receives a certain share of the
    overall resources (CPU, network, file system)
  • Each web domain consists of an application pool
    (static content, dynamic content, streaming
    video/audio, etc.)
  • Challenges
  • Sharing the resource among domains (i.e.
    application pools) may be hard in general purpose
    Operating Systems
  • Guarantee of a uniform, steady, jitter-free
    execution of time critical multimedia
    applications while not starving other
  • Support of multiprocessor server systems

static content
dynamic content
streaming video/audio
Domain 1
Domain 3
Domain 2
Other Real-Time Applications
  • Real-time databases.
  • Transactions must complete by deadlines.
  • Main dilemma Transaction scheduling algorithms
    and real-time scheduling algorithms often have
    conflicting goals.
  • Data may be subject to absolute and relative
    temporal consistency requirements.
  • Overall goal reliable responses
  • Multimedia.
  • Want to process audio and video frames at
    steady rates.
  • TV video rate is 30 frames/sec. HDTV is 60
  • Telephone audio is 16 Kbits/sec. CD audio is
    128 Kbits/sec.
  • Other requirements Lip synchronization, low
    jitter, low end-to-end response times (if

Hard vs. Soft Real Time
  • Task A sequential piece of code.
  • Job Instance of a task
  • Jobs require resources to execute.
  • Example resources CPU, network, disk, critical
  • We will simply call all hardware resources
  • Release time of a job The time instant the job
    becomes ready to execute.
  • Deadline of a job The time instant by which the
    job must complete execution.
  • Relative deadline of a job Deadline - Release
  • Response time of a job Completion time -
    Release time.

Job is released at time 3. Its absolute deadline
is at time 10. Its relative deadline is 7. Its
response time is 6.
Hard Real-Time Systems
  • A hard deadline must be met.
  • If any hard deadline is ever missed, then the
    system is incorrect.
  • Requires a means for validating that deadlines
    are met.
  • Hard real-time system A real-time system in
  • which all deadlines are hard.
  • We consider hard and soft real-time systems in
    this course.
  • Examples Nuclear power plant control, flight

Soft Real-Time Systems
  • A soft deadline may occasionally be missed.
  • Question How to define occasionally?
  • Soft real-time system A real-time system in
    which some deadlines are soft.
  • Examples multimedia applications.

  • Two particular factors are important
  • How fast does a system respond
  • When it fails, what happens?

The Speed of Response
  • All required responses are time-critical
  • The designer should predict the delivered
    performance of the systems with the required
  • Unfortunately, it may not be possible to give
    100 guarantees

Periodic vs. Aperiodic Tasks
  • Periodic Tasks
  • Tasks run at regular, pre-defined intervals
  • Example closed loop digital controller having
    fixed, pre-set sampling rates

Execute control task
Execute control task
Idle time
Idle time
Sampling interval
Synchronous real-time clock signals
Periodic vs. Aperiodic Tasks
  • Aperiodic Tasks
  • Occur when the computer must respond to
    (generally) external events which occur at random
    (asynchronous or aperiodic) have either soft or
    no deadlines
  • Sporadic Tasks
  • Similar to aperiodic tasks however, the event
    must be serviced within a specific maximum time
    period hard deadline

Idle time
Execute event task
Execute event task
Execute event task
Idle time
Asynchronous events
Mixing Periodic and Aperiodic Tasks
  • We get into trouble in situations which involve a
    mix of periodic and aperiodic(sporadic) events,
    which are usual in real-time designs
  • Much thought and skill are needed to deal with
    the response requirements of periodic and
    aperiodic tasks

Real-Time Operating Systems (RTOSs)
  • RTOS specialized operating system for RTS
  • Main responsibilities
  • Process management
  • Resource allocation (processor, memory, network)
  • They may not include regular OS facilities such
    as file management, virtual memory, user/kernel
    level separation, etc.
  • Manage at least two priority levels
  • Interrupt level, for processes that need fast
  • Clock level, for periodic processes
  • Typical components real-time clock, interrupt
    handler, scheduler, resource manager, dispatcher
About PowerShow.com