Real Time System KUKUM Last lecture- Summary Definition of Real Time System Types of RTS Slow/Fast...

Real Time System KUKUM

Last lecture- Summarybull Definition of Real Time Systembull Types of RTSbull SlowFast RTSbull What is OS amp its functionality Kernel RTOS

how is it different from OSndash Multitasking ndash pre-emptable (priority)ndash Process synchndash Interrupt latency Predictablendash Mask interruptndash Sys call


Concurrency and Concurrent Programming


Objectives

bull Able to explain thendash Concept of concurrencyndash Interaction between concurrent processesndash Identify pathological cases ( to be avoided)

bull Able to design and implement concurrent software


Lecture contents

bull Concept of concurrencybull Abstraction of concurrent programmingbull Correctness of concurrent programbull Writing concurrent program


Concept of Concurrency

In programming

There are two fundamental types of programming

Sequential parallel


Sequential Programmingbull In SP processes are executed in order one

followed by anotherbull Analogy shopping in a supermarket

bull Buy meat then buy vegetables then buy drinks then pay at the counter

bull For a given input the computer always executes the same sequence instructions

bull Program is easier to write (logical)bull Most probably not suitable to solve real-time

problems


Example of SP

function add_two_list( int1 int2 integer)beginsum= 0sum = int1sum = sum + int2return sumend add_two_list

A single sequence of

control


Concurrent programming (CP)

bull In CP processes are executed in parallelbull Analogy shopping in supermarket

bull Mother buys meat father buys veg son buys drinks ( at once) then all meet by the pay counter

bull Harder to model and to programbull Each time the sequence of execution of

instructions is non deterministicbull Appropriate for time critical real time problems


Conceptuallyhellipbull We study it by looking at the

ndash Examples abstraction and model of CPndash Classical problems which illustrate the concepts

bull Programming-wisendash Some languages have no support for

concurrency so we need other means to achive it eg OS support (UNIX system calls)

ndash Using lsquotaskrsquo in ADA or lsquothreadrsquo in C++


Definition ndash concurrent programmingldquoConcurrent programmingrdquo is the term given to

programming notations and techniques for expressing potential parallellism and for solving the related sycnhronization and communication problemsrdquo

Eg x yz= 6015120

Where xy and z are all assigned their respective values concurrently( undeterministic which assignment is done first)


Definition ndash Program amp Process(Ben-Arirsquos definition)

bull A concurrent program is a set of ldquo sequential programsrdquo which are executed in abstract parallelism

bull Each sequential program is called a process and the whole set of process is called the concurrent program


Examples of concurrencyoverlapped IO and computation

Main

IO

Start IO

Wait for IO

End of IO


hellip examples of concurrencyMulti programming

bull A concurrent execution of several independent programs

bull Eg running excel word and email concurrently on a PC where the excel is calculating something a file is being edited and email is being constantly checked for mail arival


hellip examples of concurrencyMulti tasking

bull Solving a problem by decomposing it into several sub-tasks Examples

bull Sort algorithmsbull Summation of huge set of numberbull Computing binomial coefficient


Why do we need concurrency

ndash Improve speed of execution o processesndash Different peripherals have different speedndash True to reality- real time behavior of problems

in everyday life


Types of concurrency

bull Semi-concurrencyndash Two or more processes will run on the same

processor one at a time using and releasing the processor managed by the OS

bull True concurrencyndash Many processes run on several processors at the

same time (transputer environment)


Try this outThis exercise is intended to get a feeling of the problem inherent in CPndash Assume you can write an SP program which produces an endless series

of the following sentence

ldquoA person with one watchknows what time it isa person with two watchesis never surerdquo

ndash Now write a pseudo-CP program to do the above using two processes where P1 prints out the first two lines and P2 the rest What is the output look like


Abstraction of concurrent programming

TimingInterleaving

Atomicity of operation


Abstraction

bull An abstraction is an idealizedsimplified model of a phenomenon The model can then be used to study a more complex system which uses the abstraction

bull Examplendash Human body organs and the various systemsndash Computer OS-gt instructions -gt electronicsndash Complex software system components and their

interconnections


hellipAbstractionbull To understand CP we need a number of

abstractions about processesndash Interleavingndash Timingndash Atomicity of operations in a process

bull We want to use these to identify the problems and also prove the correctness of a CP


Interleavingbull For analysis of CP we say that the execution sequences of

concurrent processes are interleaved and each process has its own processor time

bull Between interleaving processes there can be contention and communicationndash Contention processes compete for resourcesndash Communication processes may need to exchange

informationbull In fact there are various scheduling mechanism used to

manage the interleaving order of processes To show that a CP is incorrect we must be able to show that there exist at least one bad interleaving sequence which does not meet the system specification


hellip Interleaving

bull A concurrent program must be correct under all kinds of interleaving

bull Furthermore fairness must be guaranteed ie The scheduler manages is such a way that eventually every instruction in each process must be executed

bull If it can be shown that a process is deferred forever in its execution under a sequence of interleaving then the process is said to be ldquostarvedrdquo


Timingbull The interleaving abstraction also makes no assumption about absolute timing

between the execution of processesndash If process-P must finish before process-Q is started it is irrelevant if it finishes 1 second or 1

hour before

bull This is a weak specification and can be misleading But it is a necessary abstraction becausehellipndash Execution sequence are more appropriate in proving correctness rather than absolute timingndash Systems are always upgraded with newer and faster hardware and if correctness depends on

absolute timing we always need to recheck its correctness every time a new device with a new timing is installed

bull Of course in a real system the specification must include the absolute real time requirement otherwise the system may be theoretically correct but still does not meet its requirement


Atomic Instructionsbull Interleaving of processes are done on

sequences of atomic (undivisible) instructions

bull The correctness of a CP depends on the level of atomicity used by the computer when processes interleaved

bull For example consider the following two processes which execute in parallel but at different atomic levels

bull Instruction levelbull Register level


hellip Atomic instructionsN Integer = 0 -- a global var

process P is --specification of process PBegin

N= N+1End P

process Q is --specification of process QBegin

N= N+1End Q


Instruction levelProcess Instruction Value of N(Initially) 0P INC N 1Q INC N 2

Process Instruction Value of N(Initially) 0Q INC N 1p INC N 2

Result is correct since there are 2 processes each incrementingN by 1 so final value of N=2 whichever process executes first


Reqister levelAt the register level instruction the statement N=N+1 is equivalent to the series of instructions as follow

LOAD Reg NADD Reg 1STORE N Reg

Where LOAD ADD and store are atomic instructions and N is the global


Register level

Process Instruction N Reg(1) Reg(2)(Initially) 0 - -P LOAD Reg N 0 0 -Q LOAD Reg N 0 0 0P ADD Reg 1 0 1 0Q ADD Reg 1 0 1 1P STORE Reg N 1 1 1Q STORE Reg N 1 1 1

Result is not correct based on the specification in the requirement


By comparisonbull It shows that a different level of atomicity leads to

a wrong result when processed are interleavedbull Obviously there are tradeoffs between the level of

atomicity vs correctness and between the level of atomicity vs difficulty in implementation

bull Generally prefer a higher level of atomicity although need to consider speed


Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming

tech to parallelismSolve synch amp comm

True Conc

Semi ConcAbstraction

Atomicity

Interleaving

Timing

Last lecture- Summary


Objectives

Lecture contents


Sequential Programming

Example of SP


Conceptuallyhellip

Definition ndash concurrent programming

Definition ndash Program amp Process (Ben-Arirsquos definition)

Examples of concurrency overlapped IO and computation

hellip examples of concurrency Multi programming

hellip examples of concurrency Multi tasking



Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions

hellip Atomic instructions

Instruction level

Reqister level

Register level

By comparison

Slide 30




Objectives




Lecture contents




In programming


Sequential parallel







problems


Example of SP



control
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30


Lecture contents




In programming


Sequential parallel







problems


Example of SP



control
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30



In programming


Sequential parallel







problems


Example of SP



control
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30







problems


Example of SP



control
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30


Example of SP



control
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30
















Eg x yz= 6015120








Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30







Main

IO

Start IO

Wait for IO

End of IO












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30












in everyday life














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30














TimingInterleaving



Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30


Abstraction



interconnections












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30












hellip Interleaving







hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30




hour before













N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30










N= N+1End P


N= N+1End Q










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30










Register level









Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30







Concurrency

Implementation

C++ thread

OS support

Example

Defn

Types

Multi tasking

Multi programming


True Conc


Atomicity

Interleaving

Timing



Objectives

Lecture contents



Example of SP


Conceptuallyhellip








Try this out


Abstraction

hellipAbstraction

Interleaving

hellip Interleaving

Timing

Atomic Instructions


Instruction level

Reqister level

Register level

By comparison

Slide 30

Real Time System KUKUM Last lecture- Summary Definition of Real Time System Types of RTS Slow/Fast...

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Transcript of Real Time System KUKUM Last lecture- Summary Definition of Real Time System Types of RTS Slow/Fast...