Real-time systems are systems whose specifications include both logical and temporal correctness requirements. They must produce outputs at the right time in addition to producing logically correct outputs. Many embedded systems are real-time systems. Real-time systems have characteristics like being event-driven, having high failure costs, requiring concurrency and reliability. They can be hard real-time systems where all deadlines must be met or soft real-time systems where some missed deadlines are allowed. Real-time operating systems help manage resources and priorities to meet the timing constraints of real-time applications.

1. 10/5/2023 Real-Time Systems
Real-Time Systems
Introduction

2. 10/5/2023 Real-Time Systems
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.

3. 10/5/2023 Real-Time Systems
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

4. 10/5/2023 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.

5. 10/5/2023 Real-Time Systems
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.

6. 10/5/2023 Real-Time Systems
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.

7. 10/5/2023 Real-Time Systems
Misconceptions (Continued)
• It is not meaningful to talk about
guaranteeing real-time performance when
things can fail.
– Though things may fail, we certainly don’t
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 doesn’t 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. WMPP’05]

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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
Server
Domain 1
Domain 2
Domain 3
static
content
dynamic
content
streaming
video/audio

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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.
It’s absolute deadline is at time 10.
It’s relative deadline is 7.
It’s 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

27. 10/5/2023 Real-Time Systems
Periodic vs. Aperiodic Tasks
• Periodic Tasks
– Tasks run at regular, pre-defined intervals
– Example: closed loop digital controller having
fixed, pre-set sampling rates
t0 t1 t2
time
Execute
control task
Execute
control task
Idle time Idle time
Sampling interval
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
e0
time
Execute
event task
Execute
event task
Idle
time
Idle time
Asynchronous events
e1 e2
Execute
event task

29. 10/5/2023 Real-Time Systems
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

30. 10/5/2023 Real-Time Systems
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