Lecture Outline
TDDC47: Real-Time Systems and
Concurrent Programming
Block 1:
• Course information
• Examples of systems where real-time systems are found
• Definition of real-time system
• Terminology and characteristics
• The problems specific for real-time systems
• Operating systems
Block 1: Introduction
Mehdi Amirijoo
Real-Time Systems Laboratory
Department of Computer Science
Linköping University
[email protected]
TDCC47 – Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Personal
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Examinator/föreläsare:
– Mehdi Amirijoo ([email protected])
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Lektionshandledare och labbassistent:
– Mehdi Amirijoo
– Jonas Elmqvist ([email protected])
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Kursinnehåll
•
Kursen behandlar två delar:
1. Processprogrammering (block 2):
Processbegreppet, synkronisering och kommunikation
mellan processer, hantering av baklås
2. Realtidssystem (block 3 och 4):
Formulering av tidskrav, design map tidskrav,
realtidskommunikation
Kurssekreterare:
– Anne Moe ([email protected])
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
•
2006
Innehållet består av två delar som normalt ges i två kurser
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
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Kurslitteratur
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Finns ej ngn bok som täcker hela kursen
•
Två böcker rekommenderas:
1. C. Norström, K. Sandström, J. Mäki-Turja, H. Hansson, H.
Thane och J. Gustafsson, "Robusta realtidssystem“
– Tryckakademin, Kårallen
– (processer block 2), schemaläggning block 3,
realtidskommunikation block 4
Kursmål
Efter genomgången kurs ska studenten självständigt kunna:
– tydligt förklara principer bakom processer,
processoperationer, och kommunikation mellan processer
– karaktärisera delade resurser och kritiska sektioner, förklara
synkroniseringsmekanismer och tekniker för att hantera
låsning mellan processer
– använda ansatser för att synkronisera processer samt
tillämpa algoritmer för att undvika och hantera låsning
mellan processer
– beskriva designmetoder och ansatser för att tillgodose krav
på realtidssystem inklusive nätverksbaserade system
– uttrycka temporala krav på realtidssystem, välja lämplig
ansats för att tillgodose kraven på ett realtidssystem samt
jämföra prestanda av ansatser då flera existerar
– kunna redogöra för samband mellan temporala egenskaper
inom reglerteknik och realtidsteknik
– tillämpa metoder för att höja prestanda i reglersystem som
har implementerats med realtidsteknik
2. A. Burns and A. Wellings, ”Real-time Systems and their
programming languages”, Addison Wesley, 2001.
– Bokakademin, Kårallen
– processer block 2, schemaläggning block 3,
(realtidskommunikation block 4)
– Diskuterar även andra delar som inte tas upp i kursen
•
Fyra Artiklar
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Laborationer
•
•
•
•
•
•
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MATLAB, TrueTime, Simulink
– Kräver mindre erfarenhet med MATLAB
– Ingen programmering (C++, ADA, Java etc), fokus på
grundläggande koncept
Laborationsuppgifter:
1. Synkronisering av processer
2. Schemaläggning
3. Reglering av datorprestanda
Kräver att man har löst förberedelseuppgifter innan laboration
Schemalagda tillfällen ej dedikerade åt viss uppgift
Deadlines för varje laborationsuppgift
– Kan ses som riktlinje för hur man bör planera
Laborationsrapport
Registrering i WebReg
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
2006
Mer info - Websidor
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Kurshemsida: www.ida.liu.se/~TDDC47
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Kursinformation:
– Organisation, mål, kursutvärdering
– Kontorstid tisdagar 12:30-13:30 (ej 14:e nov)
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Kompletterande kurslitteratur
– MATLAB, Simulink
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Examination (gamla tentor)
•
Föreläsningar och lektioner
– Lektionsmaterial
– Föreläsningsslides finns tillgängligt senast 24h innan
föreläsning
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
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Background
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•
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Traditionally systems have been controlled by analog (electrical)
or mechanical systems
However, there is an increasing need in the use of computers for
controlling systems and to react to external stimuli
Why?
– To reduce costs, example:
• Cables in a car are very expensive!
• Replace cables with a computer network that uses one
”cable”
– Add more functionality that cannot be provided by analog or
mechanical systems:
• Anti-spinn, ABS breaks, navigation system etc.
– Easily upgrade system
• ”Plug in your car to the Internet and upgrade the software”
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Background
Ok, let’s use computers! But before we do that
let’s consider what type of systems we are
dealing with….
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Safety system
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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2006
Drive-by-wire system
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Networked system in a car
A simple fluid control system
Interface
Pipe
Input flow
reading
Flow meter
Processing
of
Control Algorithm
Valve
Output valve
angle
Time
Computer
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
A Process Control System
A Production Control System
Process
Control
Computer
Valve
Chemicals
and
Materials
Temperature
Transducer
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Production
Control
System
Stirrer
Finished
Products
Finished
Products
Parts
Machine Tools
Manipulators
Conveyor Belt
PLANT
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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What kind of computing systems?
“Every-day”
computing
systems
e.g. a PC
Average is not enough
Computing
systems in
previous slides
e.g., system in a car
Goal
Minimize average
response time of
user requests
React to
environment in a
predictive manner
Result of failing
goal
Annoyed user
Damage, loss of
human life
Conclusion
Computer controls
speed of user,
Computer-time
Computer has to
follow speed of the
environment,
Real-time
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
A person drowned in a river which an
average depth of 25 cm.
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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What is a real-time system?
Real-time systems
environment
•
A real-time system is any information processing system which
has to respond to externally generated input stimuli within a finite
and specified period called the deadline
action
event
– the correctness depends not only on the logical
result but also the time it was delivered
I/O
– failure to respond is as bad as the wrong response!
•
Clock
99% of all processors are for the embedded/realtime systems
market
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Real-time computing
system
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Terminology
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Event-triggered (ET) system
– The computations are driven by
occurrence of events
– Aperiodic processing
– Example: airbag
Example
Example: Actuate if the temperature falls below a
certain threshold.
environment
action
event
I/O
Clock
•
Time-triggered (TT) system
– Computation are driven by a clock
– Periodic processing
– Example: altitude control in an
airplane
Real-time computing
system
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
Terminology
Terminology
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.
video streaming.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
penalty
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
utility loss
•
benefit
Hard real-time systems
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Terminology
Soft Real-Time Systems
Soft real-time systems
Utility (cost function)
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Multiple requirements
– Deadlines can be missed occasionally, but with…
1. an upper limit of misses within a defined interval
2. a constraint on the maximum number of consecutive
deadline misses
– Service can occasionally be delivered late, with an upper
bound on lateness.
•
Hard real-time is hard, but soft real-time is harder!
– Hard real-time deals with meeting individual deadlines
– Soft real-time deals with maximizing the overall utility, which
is much harder from an algorithmic point of view
1
3
2
time
release
deadline
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Characteristics
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Large and complex — vary from a few
hundred lines of assembler or C to 20
million lines of Ada estimated for the
International Space Station Freedom
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Concurrent control of separate
system components — devices
operate in parallel in the real-world;
better to model this parallelism by
concurrent entities in the program
•
Facilities to interact with special
purpose hardware — need to be able
to program devices in a reliable and
abstract way
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Characteristics
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Extreme reliability and safe — systems typically control the
environment in which they operate; failure to control can result in
loss of life, damage to environment or economic loss
•
Guaranteed response times — we need to be able to predict with
confidence the worst case response times for systems; efficiency
is important but predictability is essential
•
Efficient implementation of the computations — the application
developer must be concerned with the execution time of using a
particular programming language feature.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Problem 1
Scheduling of computation (jobs,
processes, tasks)
Definition:
– Hard real-time systems:
Given a set of processes
sequence their execution
such that they all meet their
deadlines.
– Soft real-time systems: Given
a set of processes, sequence
their execution such that the
overall utility is maximized.
So – what are the problems?
Let’s take a peek look at some problems in
concurrent programming and real-time
systems…
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Problem 1
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Problem 2
Overload Management
Does the schedule really matter?
Definition (Overload):
The situation where there are too
many computations than the
computer can handle
This is shown in different ways:
• Deadline misses
• Overall utility is “too low”
A scheduling problem can be mapped to the problem of
assigning priorities to the processes.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Problem 2
Problem 2
Apollo 11, 1969, first lunar landing
•
The 1202 program alarm is being produced by unexpected flow
of data concerning radar.
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The computer has been programmed to recognize this data as
being of secondary importance and will ignore it while it does
more important computations.
•
Two possible solutions to overload:
– Use time-triggered approach instead of an event-triggered
approach
– Control the workload by rejecting (not executing) some of the
computations
Michael Collins: At five minutes into the burn.. "Program Alarm,"
barks Neil, "It's a 1202." What the hell is that?
I don't have the alarm numbers memorized for my own computer.
I jerk out my own checklist and start thumbing through it, but
before I can find 1202, Houston says, "Roger, we're GO on that
alarm." No problem, in other words.
My checklist says 1202 is an "executive overflow," meaning
simply that the computer has been called upon to do too many
things at once. A little farther along, at just three thousand feet
above the surface, the computer flashes 1201, another overflow
condition...
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
Problem 3
Problem 4
Priority Inversion
Definition:
The situation when a high priority process cannot be executed
since it needs a resource that is used by a low priority process.
Deadlock Avoidance and Management
Definition (Deadlocks):
A process is holding a resource and is waiting for another
resource that is held by other waiting processes
Deadline
High Priority
Process 1
Resource 1: File
Low Priority
Time
Process
Process 2
Resource 2: File
High Priority
Low Priority
Time
Common resource
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Problem 4
Mars Pathfinder:
• Autonomous robot designed to
collect data about mars and
transmit to earth
• Pathfinder used a priority-based
operating system. Each process
had a priority.
• The processes shared a common
resource – the network
•
•
Summary
•
Discussed
– event-triggered systems
– time-triggered systems
– hard real-time systems
– soft real-time systems
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The basic characteristics of a real-time or embedded computer
system are:
– largeness and complexity,
– extreme reliability and safety,
– concurrent control of separate system components,
– interaction with hardware interfaces,
– efficient implementation.
After a few days the robot started
restarting itself
It was found that the execution of
a high priority process was
delayed due to priority inversion,
leading to the computer to restart
itself.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
What is an Operating System?
Operating systems
TDDC47 - Real-Time Systems and Concurrent Programming
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A program that acts as an intermediary between a user of a
computer (user application) and the computer hardware.
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Operating system goals:
– Execute user programs
– Provide services/functionality to user programs
– Make the computer system convenient to use.
– Use the computer hardware in an efficient manner.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Computer System Organization
Computer-system operation
• One or more CPUs, device controllers connect through common
bus providing access to shared memory
• Concurrent execution of CPUs and devices
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Computer System Structure
Computer-System Operation
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Devices:
– Each device controller is in charge of a particular device type.
– Each device can operate in parallel with other devices
– Each device controller has a local buffer.
– CPU moves data from/to main memory to/from local device
buffers
– Device controller informs CPU that it has finished its
operation by causing an interrupt.
•
When an interrupt occurs…
– Interrupt transfers control to the interrupt service routine
– Incoming interrupts are disabled while another interrupt is
being processed.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
Four Components of a Computer System
Computer system can be divided into four components
• Hardware – provides basic computing resources
– CPU, memory, I/O devices
• Operating system
– Controls and coordinates use of hardware among
various applications and users
• Application programs – implement functionality and define
the ways in which the system resources are used
– Word processors, compilers, web browsers, database
systems, video games
• Users
– People, machines, other computers
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Operating System Definition
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•
OS is a resource allocator
– Manages all resources
– Decides between conflicting requests for efficient and
fair resource use
OS is a control program
– Controls execution of programs to prevent errors and
improper use of the computer
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
2006
Operating System Services
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User interface - Almost all operating systems have a user interface (UI)
– Varies between Command-Line (CLI), Graphics User Interface
(GUI), Batch
•
Program execution - The system must be able to load a program into
memory and to run that program, end execution, either normally or
abnormally (indicating error)
•
I/O operations - A running program may require I/O, which may involve
a file or an I/O device.
•
File-system manipulation - The file system is of particular interest.
Obviously, programs need to read and write files and directories, create
and delete them, search them, list file Information, permission
management.
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
Operating System Services (Cont.)
•
•
Communications – Processes may exchange information, on the
same computer or between computers over a network
– Communications may be via shared memory or through
message passing (packets moved by the OS)
Error detection – OS needs to be constantly aware of possible errors
– May occur in the CPU and memory hardware, in I/O devices, in
user program
– For each type of error, OS should take the appropriate action to
ensure correct and consistent computing
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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System Calls
•
System call provide an interface between a program and the
services provided by the OS (previous slides)
•
Types of systems call:
– Process control
– File management
– Device management
– Communications
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Example of System Calls
System Call – OS Relationship
System call sequence to copy the contents of one file to another file
TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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TDDC47 - Real-Time Systems and Concurrent Programming
M. Amirijoo, IDA, LiU
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Standard C Library Example
C program invoking printf() library call, which calls write() system call
TDDC47 - Real-Time Systems and Concurrent Programming
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