Introduction to Real Time Systems - Vanderbilt University

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Introduction

to

Real Time Systems

Akos Ledeczi

EECE 354, Fall 2012

Vanderbilt University

Disclaimer


Some of the material/slides are adapted from
various presentations found on the internet:


Johnnie W. Baker


Ian Sommerville


Alan Burns and Andy Wellings


Others


And Prof. Kopetz’s Real Time Systems book

Embedded vs. Real Time Systems


Embedded system: is a computer system that performs a
limited set of specific functions. It often interacts with its
environment.


RTS: Correctness of the system depends not only on the
logical results, but also on the time in which the results are
produced.

Embedded

Systems

Real Time

Systems

Examples?

Examples


Real Time Embedded:


Nuclear reactor control


Flight control


Basically any safety critical system


GPS


MP3 player


Mobile phone


Real Time, but not Embedded:


Stock trading system


Skype


Pandora


Embedded, but not Real Time:


Home temperature control


Sprinkler system


Washing machine, refrigerator, etc.


Blood pressure meter


Characteristics of RTS


Event
-
driven, reactive.


High cost of failure.


Concurrency/multiprogramming.


Stand
-
alone/continuous operation.


Reliability/fault
-
tolerance requirements.


Predictable behavior.


Time

digital clock

tick

granule

now

instant

instant

past

future

duration

event

event

time line

Definitions


Hard

real
-
time



systems

where

it

is

absolutely

imperative

that

responses

occur

within

the

required

deadline
.

E
.
g
.

Flight

control

systems
.


Soft

real
-
time



systems

where

deadlines

are

important

but

which

will

still

function

correctly

if

deadlines

are

occasionally

missed
.

E
.
g
.

Data

acquisition

system
.


Real

real
-
time



systems

which

are

hard

real
-
time

and

which

the

response

times

are

very

short
.

E
.
g
.

Missile

guidance

system
.


Firm

real
-
time



systems

which

are

soft

real
-
time

but

in

which

there

is

no

benefit

from

late

delivery

of

service
.


A single system may have all hard, soft and real real
-
time subsystems.

In reality many systems will have a cost function associated with missing each deadline

Control systems


Man
-
machine interface: input devices, e.g. keyboard and output devices,
e.g. display


Instrumentation interface: sensors and actuators that transform between
physical signals and digital data


Most control systems are hard real
-
time


Deadlines are determined by the controlled object, i.e. the temporal
behavior of the physical phenomenon

Operator

Controlled
Object

Real
-
Time
Computer
System

Man
-
Machine

Interface

Instrumentation

Interface

Control system example


Example:

A simple one
-
sensor, one
-
actuator control system.

control
-
law

computation

A/D

A/D

D/A

sensor

plant

actuator

r
k

y
k

y(t)

u(t)

u
k

reference

input r(t)

The system

being controlled

Outside effects

Control systems cont’d.

Pseudo
-
code for this system:

set timer to interrupt periodically with period
T
;

at each timer interrupt
do


do analog
-
to
-
digital conversion to get
y
;


compute control output
u
;


output
u

and do digital
-
to
-
analog conversion;

end do

T

is called the
sampling period
.
T

is a key design choice.
T
ypical

range for
T
: seconds to milliseconds.

Reliability and Safety


Reliability: probability that the system will provide the specified service for
a given time period. (Also see Failure Rate or Mean Time To Failure: MTTF)


Safety: reliability regarding critical failure modes


Fail
-
safe system: if the system has a guaranteed safe state that can be
reached in case of a critical failure. It is a property of the controlled object
and not the computer system.


Watchdog: external device that gets periodic life sign from the computer
system. If it does not get it, it forces the controlled object into a safe state.


Fail
-
operational system: no such safe state exists, so the computer system
must provided (limited) functionality in case of failures to avoid a
catastrophic failure.


Alarm monitoring.


Primary event


Secondary alarms. Temporal order is very important. Alarm shower


Rare events

12

Taxonomy of Real
-
Time Systems

13

Taxonomy of Real
-
Time Systems

14

Taxonomy of Real
-
Time Systems

15

Taxonomy: Static


Task arrival times can be predicted


Static (compile
-
time) analysis possible


Allows good resource usage (low idle time for
processors).


16

Taxonomy: Dynamic


Arrival times unpredictable


Static (compile
-
time) analysis possible only for
simple cases.


Processor utilization decreases dramatically.


In many real systems, this is very difficult to
handle.


Must avoid over
-
simplifying assumptions


e.g., assuming that all tasks are independent,
when this is unlikely.

17

Taxonomy: Soft Real
-
Time


Allows more slack in the implementation


Timings may be suboptimal without being
incorrect.


Problem formulation can be much more
complicated than hard real
-
time


Two common and an uncommon way of handling
non
-
trivial soft real
-
time system requirements


Set somewhat loose hard timing constraints


Informal design and testing


Formulate as an optimization problem

18

Taxonomy: Hard Real
-
Time


Creates difficult problems.


Some timing constraints are inflexible


Simplifies problem formulation.

19

Taxonomy: Periodic


Each task (or group of tasks) executes
repeatedly with a particular period.


Allows some static analysis techniques to be
used.


Matches characteristics of many real problems


It is possible to have tasks with deadlines
smaller, equal to, or greater than their period.


The later are difficult to handle (i.e., multiple
concurrent task instances occur).

20

Periodic


Single rate:


One period in the system


Simple but inflexible


Used in implementing a lot of wireless sensor
networks.


Multi rate:


Multiple periods


Should be harmonics to simplify system design



21

Taxonomy: Aperiodic


Are also called sporadic, asynchronous, or
reactive.


Creates a dynamic situation


Bounded arrival time interval are easier to
handle


Unbounded arrival time intervals are
impossible to handle with resource
-
constrained systems.

Example: Adaptive Cruise Control


Demo video



Control system


Hard Real Time


Multi
-
rate periodic



Camera


GPS


Low
-
speed mode for
rush hour traffic

United States Patent 7096109

Data Acquisition and Signal
-
Processing
Systems



Examples:


Video capture.


Digital filtering.


Video and voice compression/decompression.


Radar signal processing.



Response times range from a few milliseconds to a few
seconds.


Typically simpler than control systems

Other Real
-
Time Applications


Real
-
time databases.


Examples: stock market, airline reservations, etc.


Transactions must complete by deadlines.


Main dilemma
:

Transaction scheduling algorithms and real
-
time
scheduling algorithms often have conflicting goals.


Data is subject
temporal consistency

requirements.



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).

Are
All

Systems Real
-
Time Systems?


Question:

Is a payroll processing system a real
-
time 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.

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.

Example: Interactive/Multimedia
Applications

sufficient

but scarce

resources

abundant

resources

insufficient

resources

Requirements

(performance, scale)

1980

1990

2000

Hardware resources in year X

Remote

Login

Network

File Access

High
-
quality

Audio

Interactive

Video

The
interesting

real
-
time

applications

are here

OS or not?

Hardware

Operating

System

User Programs

Typical OS Configuration

Hardware

Including Operating

System Components

User Program

Typical Embedded Configuration

Foreground/Background Systems


Task
-
level, interrupt level


Critical operations must
be performed at the
interrupt level (not good)


Response time/timing
depends on the entire
loop


Code change affects
timing


Simple, low
-
cost systems

RTS Programming


Because of the need to respond to timing demands made by different stimuli/responses,
the system architecture must allow for fast switching between stimulus handlers.


Because of different priorities, unknown ordering and different timing requirements of
different stimuli, a simple sequential loop is not usually adequate.


Real
-
time systems are therefore usually designed as cooperating processes with a real
-
time
kernel controlling these processes.

Concurrent programming

Real Time Java?


Java supports lightweight concurrency (threads and
synchronized methods) and can be used for some soft
real
-
time systems.


Java is not suitable for hard RT programming but real
-
time
versions of Java are now available that address problems
such as


Not possible to specify thread execution time;


Uncontrollable garbage collection;


Not possible to access system hardware;


Etc.


Real
-
Time Specification for Java


Sun Java Real
-
Time System



Requires a Real Time OS underneath (e.g., no Windows support)



Classification of Scheduling Algorithms

All scheduling algorithms

static scheduling

(or offline, or clock driven)

dynamic scheduling

(or online, or priority driven)

static
-
priority

scheduling

dynamic
-
priority

scheduling

Scheduling strategies


Non pre
-
emptive scheduling


Once a process has been scheduled for execution, it runs to
completion or until it is blocked for some reason (e.g. waiting for I/O).


Pre
-
emptive scheduling


The execution of an executing processes may be stopped if a higher
priority process requires service.


Scheduling algorithms


Round
-
robin;


Rate monotonic;


Shortest deadline first;


Etc.

Real
-
time operating systems


Real
-
time operating systems are specialised operating systems
which manage the processes in the RTS.


Responsible for process management and

resource (processor and memory) allocation.


Do not normally include facilities such as file management.

14

Operating system components


Real
-
time clock


Provides information for process scheduling.


Interrupt handler


Manages aperiodic requests for service.


Scheduler


Chooses the next process to be run.


Resource manager


Allocates memory and processor resources.


Dispatcher


Starts process execution.

Interrupt servicing


Control is transferred automatically to a

pre
-
determined memory location.


This location contains an instruction to jump to

an interrupt service routine.


Further interrupts are disabled, the interrupt

serviced and control returned to the interrupted

process.


Interrupt service routines MUST be short,

simple and fast.

Metrics for real
-
time systems differ from that for time
-
sharing systems.












schedulability

is the ability of tasks to meet all hard deadlines


latency

is the worst
-
case system response time to events


stability

in overload means the system meets critical deadlines even if all
deadlines cannot be met


What’s Important in Real
-
Time


Time
-
Sharing
Systems

Real
-
Time
Systems

Capacity

High throughput

Schedulability


Responsiveness

Fast average response

Ensured worst
-
case
response

Overload

Fairness

Stability