real timeos updated

8/3/2019 Real TimeOS Updated

http://slidepdf.com/reader/full/real-timeos-updated 1/25

Presented by: Dalia Sobhy

Presented to : Dr. Fatma Zada

PRESENTATION

ON

R EAL TIME OPERATING SYSTEMS

(PRIORITY-DRIVEN SCHEDULING

OF PERIODIC TASKS)



WHAT ARE R EAL-TIME

SYSTEMS ?

Real-time systems

Those systems in which the correctness of the

system depends not only on the logical result of

computation, but also on the time at which the

results are produced.

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TYPES OF REAL TIME

SYSTEMS3

A hard real-time system is a system where a lateresult (not reacting to an event within a stricttimeframe) may lead to critical or catastrophic failures

such as physical damage or loss of life. Some examples of hard real-time systems: Heart

pacemaker, car engine control system, car airbag, carbraking system, nuclear power station, EW system.

A soft real-time system is a system that will toleratelateness with decreased service quality, but no criticalconsequences.

Some examples of soft real-time systems: DVD player,video game console, programmable thermostat,mobile telephone.



A SAMPLE REAL TIME

SYSTEM (1)4



REAL TIME OPERATING

SYSTEM(RTOS) KERNEL

RTOS Kernel provides an Abstraction layer that

hides from application software, the hardware

details of the processor / set of processors upon

which the application software shall run.

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Application Software

RTOS

Hardware



RTOS KERNEL FUNCTIONS7



TASK MANAGEMENT

Set of services used to allow application software

developers to design their software as a number of

separate chunks of software where

Each handles a distinct topic, a distinct goal, and

sometimes its own real-time deadline.

Main service offered is Task Scheduling

Controls the execution of application software tasks.

Can make them run in a very timely and responsive

fashion.

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TASK SWITCH

Each time the priority-based preemptive scheduler

is alerted by an External world trigger / Software

trigger it shall go through the following steps that

constitute a Task Switch:

Determine whether the currently running task shouldcontinue to run.

Determine which task should run next.

Save the environment of the task that was stopped (so itcan continue later).

Set up the running environment of the task that will runnext.

Allow the selected task to run.

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INTERTASK COMMUNICATION &

SYNCHRONIZATION

These services makes it possible to passinformation from one task to another

without information ever beingdamaged.

Makes it possible for tasks to coordinate& productively cooperate with eachother.

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MESSAGE PASSING IN RTOS

In RTOS, the OS copies a pointer to the messagedelivers the pointer to the message-receiver task,and then deletes the copy of the pointer with

message-sender task.

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Message Sender

Task

RAM

Message Receiver

Task RTOS

msg_ptr

msg_ptr

Message Message



TYPES OF SCHEDULING

ALGORITHMS Non-preemptive:

Picks a process to run and will allow it

to run forever until it chooses to end.

Preemptive:

Allows the process to run, but at

anytime it can force the process tomove into the ready state and allow a

higher prioritized process to run.

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TASK SCHEDULING

Non Real -time systems usually use Non-

preemptive Scheduling

Once a task starts executing, it completes its full

execution.

Most RTOS perform priority-based preemptive task

scheduling.

Basic rules for priority based preemptive taskscheduling

The Highest Priority Task that is Ready to Run, will

be the Task that Must be Running.

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PRIORITY BASED PREEMPTIVE

TASK SCHEDULING

Every Task in a software application is

assigned a priority.

Higher Priority = Higher Need for Quick

Response.

Follows nested preemption.

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NESTED PREEMPTION15

Timeline for Priority-based Preemptive Scheduling



TYPES OF PRIORITY

ALGORITHMS

Fixed-priority algorithms:

Task-Level: Jobs in a task assigned the same priority.

Job-Level: The priority of each job is fixed.

Dynamic-Priority Algorithms:

Task-Level: Jobs in a task assigned different priorities.

Job-Level: The priority of each job is dynamic.

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PRIORITY BASED PREEMPTIVE

TASK SCHEDULING (CONT·D)

Real-time priority scheduling assigns priorities based on

deadline or some other timing constraint:

Earliest deadline first.

Least slack time first.

FIFO Job queue is first-in-first-out by release time.

LIFO Job queue is last-in-first-out by release time.

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EARLIEST DEADLINE

FIRST(EDF)

Assign priority to jobs based on deadline.

Earlier the deadline, higher the priority.

Simple, just requires knowledge of deadlines.

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EXAMPLE

Consider 3 periodic processes scheduled using EDF, the

following acceptance test shows that all deadlines will

be met.

Compute the Utilization.

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Process Execution Time (C) Period = T

P1 1 8

P2 2 5

P3 4 10



EXAMPLE (CONT·D)

Rule :

The utilization will be:

(1/8) + (2/5) + (4/10) = 0.925 = 92.5%.

The theoretical limit for any number of processes is 100%and so the system is schedulable.

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LEAST SLACK TIME FIRST

(LST)

A job Ji has deadline d i , executi on time ei , and wasreleased at time r i.

At time t < d i :

Remaining executi on time trem = ei - ( t - r i )

Slack time t slack = d i - t trem

Assign pri ority t o j obs based on slack time, t slack

Slack time is the amount o f time left after a j ob if the j ob

was started now. T he smaller the slack time, the higher the pri ority.

Knowing the act ual executi on time is o ften difficult a pri ori,since it depends on the data, need t o use w orst caseestimates ( poor perf ormance)

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OPTIMALITY OF EDF AND

LST

These algorithms are optimal

i.e. they will always produce a feasible schedule if one

exists.

Constraints: on a single processor, as long as

preemption is allowed and jobs do not contend for

resources.

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RELATIVE MERITS

Fixed- and dynamic-priority scheduling algorithms havedifferent properties; neither appropriate for all scenarios.

Algorithms that do not take into account the urgencies of jobsin priority assignment usually perform poorly

Ex: FIFO, LIFO.

The EDF algorithm gives higher priority to jobs that have missedtheir deadlines than to jobs whose deadline is still in the future

Not necessarily suited to systems where occasional

overload unavoidable.

Dynamic algorithms like EDF can produce feasible schedules incases where RM and DM cannot

But fixed priority algorithms often more predictable, loweroverhead.

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ANY Q UESTIONS?



THANK YOU !

real timeos updated

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