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Real-Time Systems

• Introduction
• Frank Drews
• drews_at_ohio.edu

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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.

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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

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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.

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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
  computing.
• Only to ad agencies. To us, it means PREDICTABLE
  computing.

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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
  systems.
• In other CS areas, people are usually interested
  in optimizing average-case performance.

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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
  environment.
• Note true. We consider systems in which the
  environment may change dynamically.

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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

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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
  application.
• Sufficient but scarce It is possible to realize
  the timing requirements of the application, but
  careful resource allocation is required.

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Example Interactive/Multimedia Applications
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Example Real-Time Applications

• Many real-time systems are control systems
• Example 1 A simple one-sensor, one-actuator
  control system

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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
  milliseconds.

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Time
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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
  system

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Hierarchical Control Systems
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Air Traffic Control
Reddaway et al. WMPP05
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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.

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Example Radar System
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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
  applications
• Support of multiprocessor server systems

static content
dynamic content
streaming video/audio
Server
Domain 1
Domain 3
Domain 2
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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
  frames/sec.
• 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
  interactive).

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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
  section.
• We will simply call all hardware resources
  processors.
• 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
  time.
• Response time of a job Completion time -
  Release time.

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Example
Job is released at time 3. Its absolute deadline
is at time 10. Its relative deadline is 7. Its
response time is 6.
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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
  control.

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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.

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Performance

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

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The Speed of Response

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

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Periodic vs. Aperiodic Tasks

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

Executecontrol task
Executecontrol task
Idle time
Idle time
time
Sampling interval
t0
t1
t2
Synchronous real-time clock signals
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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
Executeevent task
Executeevent task
Executeevent task
Idle time
time
e0
e1
e2
Asynchronous events
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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

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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
  response
• Clock level, for periodic processes
• Typical components real-time clock, interrupt
  handler, scheduler, resource manager, dispatcher