Characteristics of real time system

ABSTRACT

Scheduling refers to the set of policies and mechanism

to control the order of work to be performed by a

computer system.process scheduling in real time

system has almost used many more algorithms like

FIFO, Round ROBIN ,Uniprocessor Multiprocessor

etc.The choices for these algorithms are simplicity

and speed in the operating system but the cost to the

system in the form of reliability and maintainability

have not been assessed.In this paper we describe the

distinguish characteristics of real time system which

can be expressed as a function of time.This paper is

also described in the form of a time driven model for

a real time operating system and also provide a tool

for measuring the effectiveness of a real time

|system .For this model ,we have generated a real time

system in which we measure a number of well known

scheduling algorithms.To meet the real time

constraints for scheduling the task different

algorithms were used.Most of the real time system

are designed using prioty based preemptive

scheduling and worst case execution time.

INTRODUCTION

In computer science ,scheduling is the procedure by

which threads, process or given the access to system

resources.The scheduling is mainly concerned with these

things Throughout:Total number of processes that

complete their execution per unit time.

Response time:when a request was submitted what

amount of time takes when first response produced.

Turnaround time:total time between completion and

submission of process.

Fairness:equal time to each process.

In real time operating systems like embedded system the

scheduler also perform that the process can meet

deadline that is necessary for stabling the

system.Schedular are sent to mobile devices and

managed by an administrative back end.A common

characteristic of many real-time systems isthat their

requirements specification

includes timing information in the form of deadlines. An

acute deadline is represented in Figure

1. The timetaken to complete an event is mapped against

the “value”thisevent has to the system. Here “value” is

loosely defined tomean thecontributionthisevent has to

the system‟s objectives. With thecomputationalevent

represented in Figure 1this value iszero beforethe starttimeand returns tozeroonce thedeadlineis passed. The

mapping of time tovalue between start-time and deadline

is application dependent.

SchedulingIn a general purpose computer system a

scheduler is considered good if it is fair and gives

execution time to all tasks equally. When scheduling a

real-time system what is most important is that all tasks

meet their deadlines and are executed so that any task

depending on them, meet their deadlines as well.

Types of scheduling

1Network scheduling

2 DISK Scheduling

3job scheduling

4Manual scheduling

5Multilevel queue scheduling6First in first out

7Process scheduling

Process scheduling is divided into four main types

1 long term scheduling which determines which

programs are admitted to the system for execution and

when, and which ones should be exited.

Concept of scheduling in real time system

2 medium schedulingWhich determines when

processes are to be a suspended and remain.

3 short term scheduling ordispatcher

which determine which process have cpu resources and

for how long.

Process scheduling in computer operating system is

instance of extensively studied problem from operation

research which in form of producing a sequence of jobs

which must a common resource.all decisions should be

made dynamically for example all jobs have to be

scheduled and processing time requirements are available

at the start of sequence time =0 the new job will not

come during processing.if it happens the priviouly

computed job is invalidated and scheduling must be

started overif is maintained.Now process

scheduling in real time system can categorized into

two categories

1 periodic

2 aperiodic

Periodic processes : which arrive at regular intervals

are called periodic process and aperiodic are those which

arrive at irregular.the main difference between real time

systems and other computer system have not

understood.the time to complete a process is important in

all computer systems but in real time response time play

a cruicial part in the correctness of the application

softwareReal-time systems are divided into two classes:

Hard real -time systems and soft real -time systems. Hard

real-time systems are those whose deadlines must

absolutely be met and system will be considered to have

failed whereas soft real-time systems allow

forsomemorendeadlines, at least occasionally, to be

missed with only adegradation performance but not a

complete failure of the system.In judge a number of

existing real time systems we study the DMA cycling

,interrupt processing blocking ang non-blocking.The

deadline scheduler gives no reasonable control over the

choice of which deadlines are delayed and which lead to

unperictable failures result to the impact on reliability

and maintainability of the real time system.real process

completion is handled by step function in which there is

no any value in completing the process after its deadline

the characteristic of a real-time system is that correctness

is determined not only by what is done, but when itis

done, we propose to use a representation of a process

completion value to measure the algorithms in real time

system

Computational model consists a set of processes every

process has a request time R ,Time interval c and avalue

function R.

Its value function become zero or negative.the value

function may be negative at R,not rise above zero the

request time R may be future time or past time .if request

time r is future time then process is not scheduable but

attributes in computations asre load in which current

scheduling decisions are made classical algorithms are.

1 Deadline:The earlist critical time in process at each

decision point

2FIFO:The longest request set is executed in process at

decision point

3Random:chosen from the request set and executed

4stack:the process with the smallest stack time is

executed in each decision point

5SPT:The shortest completion time is executed in each

decision point.

There are many approaches for utilizing a time driven

model in real time system.real time operating system

support or modify the value for the process or the set of

processes during run time.In this way application designr

can set and modify scheduling policy for various

systems.For describing these processes we assume

primtives to creat and kill processes already exist.There

are three real time interfaces

Time control primitive:

The arguments of these operating systems communicate

the information needed to implement the model but the

issue is the structure of informative that passed to the

operating system.In single primitive each parameter

would be flexible but in user might set inconsistent

parametres.

Scheduling policies:

In real time operating system it is compulsory to provide

a mechanism to express the scheduling model to

implentour model.the system should also able to modify

these policies to take advantage or flexibility of the

system.

Periodic policies:

There is one way to describe a periodic policies to using

optional arguments in a creative process.The creative

Process make new instance of process at a specific node

5 ways for scheduling the task in real time systems.

Real time system and scheduling techniques

Real time scheduling techniques are divided into

two main categories.

One is called static scheduling techniqueAnd other

is called dynamic scheduling technique.Dynamic

may be static perioty or dynamic perioty.

Static prioty is divided into two types

1 rate monolithic : rate-monotonic scheduling is

a scheduling algorithm used in real- time operating

systems with a static-priority scheduling class.

[2]

The

static priorities are assigned on the basis of the cycle

duration of the job: the shorter the cycle duration is, the

higher is the job’s priority.

These operating systems are generally preemptive and

have deterministic guarantees with regard to response

times. Rate monotonic analysis is used in conjunction

with those systems to provide scheduling guarantees for

a particular application.

2 deadline monolithic :

Dynamic prioty is also divided into two types

1)earlist time first

2)least stack time first

System and the task model

Each type and unit of work that is scheduled and execute

the system as a job.ALL the tasks are taken to be

periodic.the system knows all the things about arrival

time ,periodexecution time.the task are ready to execute

if it arrives the system.IN soft real time system each task

has a real positive value.The main goal of the task is to

obtain a value as much as possible

There are the two conditions if the task succeed the

system acquire that value if the task is not succeed

the ystem gain less value in a special case like soft

real systems the task has nothing a vale

3 Basic requirements of schedulars in real time

operating system

There are five basic requirements of scheduler in

real time operating system

1 Multitasking and preemptable

In real time operating applications real time

operating system should be multitask and

preemtable.the scheduler are able to preempt any

kind of task in the system and give the resources to

task that the system needs it

2 Dynamic deadline identification

With the earlist deadline RTOS should be able to

identify the task.deadline information may be

converted to prioty levels for resource allocation

3 predictable synchronization

To communicate multiple threads among

themselves in a timely fashionsynchronization

mechanism also reqired and also the abiliy to lock

or unlock is the resource to achieve dta integrity.

4 Sufficient perioty levels

The real time operating systems must also have a

sufficient number of priority levels for effective

implementation.Namelypurety,inheritance,ceiling

protocol need sufficient prioty levels

5 predefined latencies

the timing of system call define the following

specifications

Task awitchinglatency :time to save the context of a

current execution time and switch to another

Interrupt latency:the time elaped between first instruction

of the handler and execution time of the last instruction

of the interrupt task

Dynamic scheduling algorithm:

Dynamic algorithm at runtime assign perioties based on

the execution parametres of tasks the most important

dynamic scheduling with puriotysscheduling algorithms

are

1 EArlist deadline first algorithm

The perioty of each task based on the value of

itsdeadlinethe algorithm is simple and preemptive.

2 ACO Based scheduling algorithms.

The ACO algorithms are computational models for the

collective foraging behavior of ants .Ant is an agent that

generate a path.ANT do not need synchronization.ant

moves to the good looking neighbor for the crrent node

probabaisatically

Time Triggered

When scheduling a distributed system using offline

scheduling the whole system including the

communication is scheduled before the start of the

system giving a very rbust system but the cost of adding

a new node is high, complete rescheduling of the whole

system.To schedule a time-triggered distributed real-time

system we use the same techniques as we did with the

single processor system, but understandably with more

complex graphs.For example we might have a

precedence graph, where one task is preceded by several

tasks on different nodes. Making a schedule that have a

task precede by tasks on several nodes requires more of

the system then of the schedule, even the best schedule

will fail if the nodes time references is not synchronized

and tasks are not executed in time. This requires all

nodes to synchronize time with each other. This can be

done at an application level like in normal distributed

systems, with a common time reference with a

communication protocol that handles time

synchronization TTCAN, TTP and FlexRay to namea

few.

SUMMARY AND CONCLUSIONS

We have looked at the most commonly used scheduling

techniques and communication protocols used in

distributed realtimesystems.When we started this paper

we where looking at doing a survey of everything related

to distributed real-time systems,but found that to be a to

vast and growing field so we narrowed our field to

communication protocols and scheduling algorithms

used in distributed real-time systems. During our

research for this paper we have been looking

atschedulers that are created to work better with the

FlexRaycommunications bus or in a similar fashion.We

have taken a real interest in distributed real-time systems

and are looking forward to seeing the advances in

scheduling and communication for distributed real-time

systems and maybe one day join the research field our

selves.

With this paper we hoped to introduce the reader to the

problem of scheduling real-time tasks in distributed

systems. We presented the different interpretations of the

problem and the various options available to the solution

designers. Our analysis of some of the existing

scheduling algorithms tried to focus on the affect of the

specific problem on the choices made in the solution. We

hope that what we presented provides the reader with a

broad understanding of the problem and a range available

solutions. This paper was also aimed at providing the

reader with a solid foundation for further research on the

subject. Finally, we suggested possible future research

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