Transcript 1- intro to rts

Real Time System
KUKUM
Real Time System
Lecture Outline
• Administrivia
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A few things about me
Aims and Objective
Intended Learning Outcomes
Prerequisites
Module Outline and Timetable
Reading List
Assessment
• Introduction to Real Time System (RTS)
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Real Time System
Lecturer Contact Details
En. Zahereel Ishwar Abdul Khalib
Email: [email protected]
Off. Phone: 04 979 8641
Phone: 019 477 9177
Education: MSc. Real Time S.E. (UTM) 2005
BSc. Computer Engineering (Cal. USA) 2000
If u need to discuss/question on the materials
covered, please do tell me and we can arrange
as per when and where can we meet and
discuss
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Real Time System
Aims of the module
• To introduce and explore the concept
relying behind RTS.
• To understand the challenges and
grab the techniques necessary in
developing RTS.
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Real Time System
Intended Learning Outcomes
• By digesting this course, the participants are
expected to be able to:
• Differentiate the different issues relating to designing real
time system.
• Explain the various concepts of time relating to RTS.
• Comprehend the various static and dynamic scheduling
mechanism in achieving a real time system.
• Utilize the Unix system calls mechanism to write synchronize
concurrent programs, which exhibits Real Time System
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Prerequisite
• You need to know C language, as we will
dive deeper in it to perform Unix system
level programming which exhibits Real
Time System.
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Module Outline
Introduction to Real Time System
• Defining RTS
• OS and RTOS… what’s up
• Issue in developing an RTS compared to non-RTS
Real Time Programming and Concurrency
• Concept of Concurrency
• Abstraction of Concurrent programming
• Writing Concurrent Programs
Synchronization in RTS
• The mutual exclusion problem & Solution
• Semaphore
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Module Outline …cont
Scheduling in RTS
• A Real Time Model
• Types of Jobs & Dynamic vs Static
• Clock driven scheduling algorithms
• Priority driven scheduling algorithms
• RMA,DMA,EDF,Round Robin
POSIX programming
• Multi process programming
• Interprocess communication
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Signal
Pipe
Shared memory
Semaphore
Msg Queue
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Timetable
• Lecture: 2+1  3 hours per week
Monday: 3.00 -5.00
DKW1 (Dewan Keikhlasan)
KWSP
• Lab:
2 hours per week
TUESDAY: 4.00-6.00
MKY 5 (Kg Wai)
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Jumaat: 11.00 -12.00
DKP 4 (Automart)
Assessment
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Final Exam:
Course Work:
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50%
50%
Assignment
Test
Mini Project
10%
20% - 2 test (10% each)
20% - 2 mini project
( 10% each)
Bonus: Pop Quiz Max of 3%
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Real Time System
Readings
Introduction to Real Time System
1. Lecture Notes
Real Time programming and Concurrency
1. Lecture Notes
2. M. Ben-Ari, Principles of Concurrent and
Distributed
Prog., Prentice Hall, 1990
(chp: 1-5)
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…/Readings
Synchronization in RTS
1. Lecture Notes
Scheduling in RTS
1. Lecture Notes
2. Jane W. S. Liu, “Real-Time Systems”,
Prentice Hall, 2000, ISBN 0130996513
POSIX programming
1. Lecture Notes
2. Graham Glass, Unix for Programmers and
Users, Prentice Hall
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Introducing
Real Time System
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Lecture Outline
• What is real time system (RTS)
• OS and RTOS
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Real Time System
A System
• Consists of a set of components that collaborate
to achieve a common purpose.
• A system comprises of hardware and software
components which are put together through a
well-defined interface.
• Components include:
• Hardware: sensors, keypads, display, communication chips
• Software: concurrent processes, objects and etc.
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Describing RTS
• A system can be define as a collection of
components that respond to inputs from the user
or the environment or from itself.
• Typically, there is an amount of time spent
between receiving the input and responding to
the input.
• In any system it is quite natural to expect a
response within a specific time interval.
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../ Describing RTS
• But, out there exists systems which has
very strict deadlines ( not necessarily
short) to be met. These are the Real Time
System.
“ A late answer is wrong answer”
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Overall Structure of RT Systems
• Hardware (CPU, I/O device etc)
– a clock!
• A real time OS (function as standard OS,
with predictable behavior and well-defined
functionality)
• A collection of RT tasks/processes (share
resourses, communicate/synchronize with
each other and the environment)
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Real Time System
Types of RTS
• Hard RTS:
– A system where, a missing deadline could cause lost
of life/ massive property destruction.
• eg: Aircrafts, biomedical inst, nuclear reactor
• Soft RTS:
– A system where a few missed deadlines may not
cause any significant inconvenience to the user.
• eg:
Internet streaming ( lost of some packets can
be afforded)
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What’s up with Fast & Slow RTS
• Typically, any system that operates within
subsecond response time can be
classified as fast.
• While systems that can take a second or
more to respond is classified slow.
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Real Time System
…/ Slow Fast RTS
• Note: soft real time system can be “fast”.
ie: a few packets can be lost across a
high-speed router that works with
nanosecond deadlines.
• Similarly hard real time system can be
“slow”. i.e: the cadmium rods inside a
nuclear reactor need not be pulled out at a
lighting speed.
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Determinism in RTS
• The concept of determinism differentiates
real time programming from normal
application programming.
• The time interval between input
occurrence and output occurrence must
be “deterministic” or predictable. ie: the
system always respond within a specified
(known) period of time.
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Which is RTS
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Mobile phones
Cars, Trains, Aircrafts
Washing machines
Games
...
• All computers that do not look like a computer
– Consumming 99% of processors produced in the
world!
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Digital control
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Components of RTS
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Characteristics of a RTS
• Large and complex
– vary from a few hundred lines of assembler or C to 20
million lines of Ada estimated for the Space Station
Freedom
• 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
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../ Charac of RTS
• Extreme reliability and safety
– Embedded 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
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So what have I comprehend ?
• A real-time system is any information processing system
which has to respond to externally generated input
stimuli within a finite and specified period
– the correctness depends not only on the logical result
– but also the time it was delivered
– failure to respond is as bad as the wrong response!
• A system is classified as an RTS if it is required to
respond to the stimulus of real world events and
complete its work & deliver its service on a timely basis
– Not necessarily fast, but must meet some timing deadline
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Remember
• In RT systems, the correctness of
computation depends not only on the
results but also on the time when outputs
are produced.
– Real Time =/= Fast
– Real Time =/= Time Sharing
– Real Time = just in Time (predictable)
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Example: A car controler
• Activities of a car control system. Let
• 1. C = worst case execution time
• 2. T = (sampling) period
• 3. D = deadline
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Speed measurement: C=4ms, T=20ms, D=5ms
ABS control:
C=10ms,T=40ms, D=40ms
Fuel injection:
C=40ms, T=80ms,D=80ms
Other software with soft deadlines e.g audio, air
condition etc
• Construct a controller meeting all the deadlines!
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Programming the car controller
(1)
Process Speed:
Loop
read
sensor,compute,display...
sleep (0.02) /*period*/
End loop
Process Fuel
Loop
read data, compute, inject ...
sleep(0.08)
End loop
Process ABS:
Loop
read
sensor,compute,display...
sleep (0.04) /*period*/
End loop
Soft RT Processes
Loop
read temperature
el hiss, stereo
....
End loop
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Any problem?
• We forgot the execution times !
• e.g. Process speed:
• 20ms = execution time + sleep(X)
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Programming the car controller
(2)
Process Speed:
Loop
next := get-time + 0.02
read sensor,compute,display...
sleep until next
End loop
Process Fuel
Loop
next:=get-time + 0.08
read data, compute, inject ...
sleep until next
End loop
Process ABS
Loop
next:=get-time + 0.04
Read sensor, compute, react
sleep until next
End loop
Soft RT Processes
Loop
read temperature,el hiss, stereo
....
End loop
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What is the problem now?
• We don’t know if the deadlines are met!
– We need to know the execution times
– We need to do schedulability analysis
– We need to construct a schedule
– We need to implement/buy an RT operating
system
– Run-time system (in programming language
design)
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Programming the car controller (3)
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Main desirable properties of RT
Systems(1)
• Timeliness: not only outputs but also times they are
produced
• Robustness: must not collapse when subject to peak
load, exception, manage all possible scenarios
• Fault tolerance: hardware and software failures should
not cause the system to crash
• Predictability: able to predict the future consequences
( in relation to time) of current actions
• Maintainability: modular structure to ease system
modification
• Testability: easy to test if the system can meet all the
deadlines
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How to achieve RTS ( the time factor)
• How can u guarantee that your system
fulfill the timing requirement?
– A lot of things need to be considered, but here
some of the most useful terms related to the
answer
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Scheduling
Concurrency
Synchronization
Re-entrant code
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Those are expensive stuff !
• Answer: RTOS
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Operating System
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OS
• A computer system composes of resources like
CPU, memory, I/O devices and others
• All this resources needed to be used by the
programs which runs on the system
• Thus, OS acts as manager which manages the
system resources to all of the command issued
by the programs running on the computer
system.
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OS
• So, OS is a set of sw which manages the operation of
computer system.
• The central module of OS is called kernel
• It is the part of the operating system that loads first, and
it remains in main memory (RAM).
• Because it stays in memory, it is important for the kernel
to be as small as possible while still providing all the
essential services required by other parts of the
operating system and applications.
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OS : basic task
– Process management
» Creation,Deletion,Synchronization, Comm,
– Main memory management
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Which part of memory being used by whom
Load new process when memory available
Allocate and de-allocate memory space as needed
Memory protection
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../ OS : basic task
– 2ndry storage management
» File creation and deletion.
» Directory creation and deletion.
» Support of primitives for manipulating files and
directories.
» Mapping files onto secondary storage.
» File backup on stable (nonvolatile) storage media.
– I/O management
– File management
– Protection System
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Process
• A process is a program in some “state” of execution. A
process needs certain resources, including CPU time,
memory, files, and I/O devices, to accomplish its task.
• process execution must progress in sequential fashion.
• A process includes:
– program counter
– stack
– data section
• Two different processes from the same program have
everything different but the text section, and may or may
not share a copy of the text section
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Process State
• As a process executes, it changes state
– new: The process is being created.
– running: Instructions are being executed.
– waiting: The process is waiting for some
event to occur.
– ready: The process is waiting to be assigned
to a processor – not waiting for anything or
resource.
– terminated: The process has finished
execution
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Diagram of process state
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Process State Transition Descp.
• The job scheduler or long-term scheduler decides to
admit a process from "new" into "ready". It uses the
loader to do this.
• The CPU scheduler or short-term scheduler decides to
dispatch a process from "ready" into "running". This
requires a context switch (more on that later.)
• The CPU scheduler also decides to remove a process
from "running" to "ready". It does this on receiving a
timer interrupt. An interrupt service routine handles the
timer interrupt.
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Real Time System
../ Process State Transition Descp.
• The process itself decides to go into a "wait" state. It
does this by executing a system call to wait for I/O (or for
an event). An interrupt service routine handles the
system call.
• Usually, a process in a "wait" state is waiting for I/O; thus
an interrupt from the I/O device causes the process to
move to the "ready" state, and the interrupt is processed
by an interrupt service routine.
• The process itself can decide to exit, by making a
system call, or the process can be abnormally
terminated in a number of ways.
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Kernel
Kernel contains a collection of primitives
which are used to build the OS
OS implements policy
Kernel implements mechanisms
Hardware
kernel
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Real Time System
Operating system
The Unix Kernel
• program that runs directly on the hardware
• loaded at boot time and initializes system,
– creates some initial system processes.
– remains in memory and manages the system
• Resource manager/mediator
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Time share (time-slice) the CPU,
coordinate access to peripherals,
manage virtual memory.
Synchronization primitives.
• Well defined entry points:
– syscalls, exceptions or interrupts.
• Performs privileged operations.
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What makes an OS an RTOS?
• A RTOS (Real-Time Operating System) has to be multitasking and pre-emptable.
• The notion of process/thread priority has to exist.
• The OS has to support predictable process
synchronization mechanisms
• A system of priority inheritance has to exist
• OS behaviour should be known
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The following figures should be good as well
• The interrupt latency (i.e. time from interrupt to task run)
: this has to be compatible with application requirements
and has to be predictable. This value depends on the
number of simultaneous pending interrupts.
• For every system call, the maximum time it takes. It
should be predictable and independent from the number
of objects in the system;
• The maximum time the OS and drivers mask the
interrupts.
• The following points should also be known by the
developer:
– System Interrupt Levels.
– Device driver IRQ Levels, maximum time they take, etc.
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