dhruva rtos iete 2.00pm

7/29/2019 Dhruva Rtos Iete 2.00pm

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RR Dhruva, CVR College of Engg.

Real TimeOperating Systems

R R Dhruva

Sr. Asst. Professor E. C. E. Dept.


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Contents

Introduction to RTOS

Tasks

Preemption

Scheduling

Scheduling algorithms


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The Challenge

The Rising Software Intensity:


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Mobile Terminals

> 10X

Software

Intensity

2 G 3 G

4 M bit Memory 64 M bit30MIPS Radio Channel 200MIPS

3-30MlPS Speech Codec 30MIPS-- Voice Control 50MIPS-- Video Codec 100MIPS

8-16 bits CTRL Processor 16-32bits10 M Hz Speed 50 M Hz


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The Rising SoftwareIntensity:

Automobiles


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How to meet it...

A more complex software architecture is

needed to handle multiple tasks,

coordination, communication, and

interrupt handling

an RTOS architecture


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What Does Real-Time Mean?

Main difference to other computation: time time means that correctness of system depends

not only on logical results

but also on the time the results are produced real indicates: reaction to external events must

occur during their evolution.

system time ( internal time ) has to be measuredwith same time scale

as controlled environment ( external time )


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Definition

A real-time Computer system is a computer

system in which the correctness of the

system behaviour depends not only on the

logical results of the computation, but also

on the physical instant at which these results

are produced.-Kopetz


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What is RTOS?

A multitasking operating system intended for

real-time applications. Embedded systems, industrial robots, spacecraft,

industrial control and scientific researchequipment.

Facilitates the creation of a real-time system

Does not guarantee the final result will bereal-time; this requires correct development

of the software.


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RTS: Why use a RTOS?

For many years small embedded RTS havenot used a RTOS

Profound (but not immediately apparent)

effects of using an RTOS - major impact onsoftware: Dependability productivity maintainability

Significantly simplifies development ofmedium/large RTS


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Real-Time meanspredictable ! Main difference between real time and non-

real-time : deadline

Critical applications: result after deadline not

only late but wrong deadline to be met under all (even worst)

circumstances

real time means predictable


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Hard RT vs. Firm vs. Soft RT

RT task is called hard if missing its deadline may cause catastrophic consequences on

the environment under control

RT task is called firm if missing its deadline makes the result useless, but missing does

not cause serious damage

RT task is called soft if meeting its deadline is desirable (e.g. for performance reasons)

but missing does not cause serious damage RTOS that is able to handle hard RT tasks is called hard

real-time operating system


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Hard RT vs. Firm vs. Soft RT


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Typical Application Areas

Hard RT systems: automotive : power-train control, air-bag control, steer by wire,

brake by wire

aircraft : engine control, aerodynamic control

Firm RT systems: Weather forecast

Decisions on stock exchange orders

Soft RT systems: communication systems (voice over IP!)

user interaction

comfort electronics (body electronics in cars)


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Real-Time 'is NOT' Fast


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Desirable Features of Real-Time Operating Systems Timeliness

OS has to provide kernel mechanisms for time management

handling tasks with explicit time constraints

Design for peak load

Predictability

Fault tolerance

Maintainability


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Tasks

Basic SW entity handled by the OS:Process

Process = task in our context

One processor, concurrent tasks... CPU has to be assigned to tasks

with respect to predefined criterion:scheduling

policy

implementation of scheduling policy: schedulingalgorithm

allocation of selected task to CPU: dispatching


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Introduction to Scheduling

We observe that processes require alternate

use of processor and I/O in a repetitive

fashion.

Each cycle consist of a CPU burst followed byan I/O burst.


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In multiprogramming systems, when there is more

than one runnable tasks (i.e., ready), the operating

system must decide which one to activate.

The decision is made by the part of the operatingsystem called the scheduler, using a scheduling

algorithm.

The scheduler is concerned with deciding policy, not

providing a mechanism.


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In general, a scheduling scheme provides two

features: An algorithm for ordering the use of system

resources (CPU) A means of predicting the worst-case behavior of

the system when the scheduling algorithm is

applied.


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Types of Constraints

The following are general types of constraints:

Timing constraints meet your deadline

Precedence constraints respect pre-requisites

Resource constraints access only available resources


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Task Scheduling

Three main states of tasks: active : can potentially execute

ready : is waiting for CPU

running : is executing on CPU

All ready tasks are kept in ready queue

Execution

scheduling

activation dispatching termination


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Example for Schedule

Assume three tasks J1, J2, J3

At times t1, t2, t3, t4 the processor performs a

context switch

3

2

1

t1 t2 t3 t4t

J1 J2 J3idle

Scheduling obtained by executing threetasks J1, J2, and J3

(t)


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Preemption

Running task may be interrupted at any point

to allow a more important task to gain the

processor immediately.

Execution

scheduling

activation dispatching termination

preemption


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Importance of Preemption

Tasks performing exception handling may

need to preempt running tasks to ensure

timely reaction

Tasks may have different levels of criticalness.This can be mapped to a preemption scheme

More efficient schedules can be produced with

preemption


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Scheduling Goals

All systems Fairness - giving each process a fair share of the CPU

Policy enforcement - seeing that stated policy is carried out

Balance - keeping all parts of the system busy

Interactive systems Response time - respond to requests quickly

Proportionality meet users expectations

Real-time systems Meeting deadlines - avoid losing data

Predictability avoid quality degradation in multimedia systems


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Real-Time Scheduling Taxonomy


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RR Dhruva, CVR College of Engg. 28

Basic Notations (1)

set of periodic tasks

i a generic periodic task

i j instance j of task i

i,j release time of i,j

i phase of i (= i,1, i.e. release time of first instance)

i periodof i (= interval between two consecutive activations)


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Basic Notations (2)

Di relative deadline of i (relative to release time)

di,j absolute deadline of i,j

(di,j = i + (j - 1) Ti + Di)

si,j start time of

i,j(s

i,j r

i,j)

fi,j finishing timeof i,j (fi,j di,j )


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Feasible and Schedulable Sets ofTasks Each interval [ti, ti+1) with (t) constant for t[ti, ti+1)

is called time slice

A preemptive schedule allows a running task to be

suspended at any time. tasks may be executed in disjoint intervals of time

A schedule is called feasible if all tasks can be

completed according to a set of specified constraints

A set of tasks is called schedulable if there exists atleast one algorithm that can produce a feasible

schedule


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Example of PreemptiveSchedule


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Ji

ai Si fi di

t

Ci

Typical parameters of a real-time task

Parameters to Characterize RT-task Ji Arrival time ai :

the time Ji becomes ready for execution

also called request time or release time, denoted by ri

Computation time Ci: time necessary for execution without interruption


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Ji

ai Si fi di

t

Ci

Typical parameters of a real-time task

Parameters to Characterize RT-task Ji

Deadline di: time before which task has to be completed its execution

Start time Si: time at which Ji start its execution

Finishing time fi: time at which Ji finishes its execution


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Real Time Scheduling

Aperiodic Scheduling Earliest Deadline First (EDF)

Modified EDF

Periodic Scheduling Rate Monotonic Priority Assignment (RM)

Earliest Deadline First (EDF)

Servers Fixed Priority

Dynamic Priority


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Periodic and Aperiodic Tasks

Periodic task i consists of infinite sequence of identical activities, calledinstances or jobs regularly activated at a constant rate

Activation of first instance ofi is called phase i

activation time for the kth instance ofi is ai,k= i + (k-1) Ti

Ti is called period of the task

Note: activation time


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Periodic and AperiodicTasks: Example


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Processor Utilization Factor

Given a set of a periodic tasks the processorutilization factor U

is the fraction of processor time spent in the execution of the taks set.

Ci/Ti is the fraction of processor time spent in executing task i

U=i= 1

nC

i

Ti


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Upper Bound of ProcessorUtilization Factor

U can be improved by

increasing tasks computation times

decreasing tasks periods

up to a maximum value below which is schedulableand above with is not schedulable

Limit depends on

task set (particular relations among tasks periods)

algorithm used to schedule the tasks

Uub(, A) = upper bound of processor utilization factor for a task set

under a given algorithm A

Least Upper Bound of


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Least Upper Bound ofProcessor Utilization

FactorU = Uub (, A) => fully utilize the processor.

is schedulable using A but any increase of computation time in one of thetasks makes set infeasible.

For given A least upper bound Ulub (A) is the minimum of the utilization

factors over all task sets, that fully utilize the processor:

Ulub (A) = min Uub (, A)


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Schedulability Test

Ulub allows to easily verify the schedulability of a task set:

Uub (i) Ulub (A) => i schedulable.

Uub (i) > Ulub (A) => i may be schedulable, if the periods of the

tasks are suitable related.

Uub (i) > 1 => i is not schedulable


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Example for Least Upper Boundfor U


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Rate Monotonic Scheduling

More precisely : Rate Monotonic Priority Assignment

Priorities are assigned to tasks according to their

request rates.

Tasks with higher request rates (i.e. shorter periods)get assigned higher priority.

Periods constant => RM is fixed-priority assignment

RM is intrinsically preemtive: currently executing

task is preempted by a newly released task withshorter period.


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Rate Monotonic Scheduling:Example


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Ulub for n Tasks

For an arbitrary set of periodic

tasks, the least upper bound of the processor utilization factor under RM :

Ulub = n (21/n - 1)

This value decreases with increasing n and converges to

Ulub = ln 2 0.69


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Ulub for n Tasks

n Ulub

1.000

0.828

0.780

0.757

O.743

1

2

3

4

5

n Ulub

0.735

0.729

0.724

0.721

O.718

6

7

8

9

10


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Concluding remarks on RM

RM is optimal among all fixed priority assignment

RM guarantees that an arbitrary set of periodic tasks is schedulable if

the total processor utilization U does not exceed the value of 0.69

Ulub = 0.69 is sufficient but not necessary to guarantee the feasibility of

a given task set.


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Earliest Deadline First (EDF)

EDF algorithm: Dynamic scheduling rule Selects tasks according to their absolute deadlines

Tasks with earlier deadlines will be executed at

higher priorities Absolute deadline of periodic task depends on

current j-th instance:

di,j= i + (j - 1) Ti + Di

=> EDF is dynamic priority assignment Intrinsically preemptive Applicable also for aperiodic tasks


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EDF SchedulabilityAnalysis

Schedulability of periodic task set handled by EDF can be verified

through the processor utilization factor

Ulub here is 1 , i.e. 100 % utilization achievable

Theorem

A set of periodic tasks is schedulable with EDF if and only if

i=1

n

ci

Ti

1


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Example for EDF


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Example: EDF vs RM


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Survey of RTOS

LynxOS www.lynx.com Microkernel and plug in

modules. UNIX compatible.

pSOSystem www.windriver.com Iridium satellites.

VxWorks www.windriver.com Mars Pathfinder QNX www.qnx.com Suitable for multiprocessors on

big (shared memory) machines.

Symbian www.symbian.com Mobile phones


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Suggested for FurtherReading Response Time

Priority Inversion

Priority Inheritance

Priority Ceiling


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Questions Please...

dhruva rtos iete 2.00pm

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