L23_Reference Model for RT Systems

7/31/2019 L23_Reference Model for RT Systems

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CS C 424 Software for Embedded systems

Software for embedded systems

Cs 424First Semester

2012-13

Reference model for real time systems


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CS C 424 Software for Embedded systems 2

Definition

Coarse definition

A system which is required to complete itswork and deliver its services on a timelybasis

Ex:Digital control

Signaling

Radar systems

Guided missiles

Multi media applications

Alarm management


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Hard real time systems

Timing constraints:

Hard dead lines

Soft dead lines

Tardiness

Measured in deterministic or probabilistic terms

Hard real time job: User requires validation that the system always meet

the timing constraint.

Job must never miss its deadline

Usefulness of the results falls off abruptly

Need:

Required responsiveness

Safety


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Soft real time systems

Jobs have soft dead lines

Occasionally missed dead lines-tolerable

Aborted executions are tolerable

Quality specified in probabilistic terms

Ex:

Telephone exchange

Multimedia


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Examples

Hard real-time:Flight control

Railway signaling

Anti-lock brakes

Etc.

Soft real-time:Stock trading systemDVD player

Mobile phone

Etc


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Basic classification

Purely cyclic

Mostly cyclic

Asynchronous and some what predictable

Asynchronous and unpredictable


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A Reference Model of Real-Time Systems

A model to reason about real-time systems

Consistent terminology

Characteristics A workload model that describes the applications supported by

the system

A resource model that describes the system resourcesavailable to the applications

Algorithms that define how the application system uses theresources at all times


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CS C 424 Software for Embedded systems 8

Motivation

A model to analyze real-time systems

Focus on the important aspects of a system

Characteristics

Workload model

Resource model

Algorithms


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Jobs and tasks

Ajob is a unit of work that is scheduled and

executed by a systemcomputation of a control-law

computation of an FFT on sensor data,

transmission of a data packet,

retrieval of a file A task is a set of related jobs which jointly

provide some function

keeping an airplane flying at a constant altitude


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Processors and Resources

A job executes or is executed by theoperating system on a processor andmay depend on some resources

Processor(P):

active component on which jobs scheduledhave speed as attribute

Resource ( R ):

a passive entity upon which jobs may depend


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Use of Resources

If the system contains types of resources,

there are different types ofserially reusableresources

One job can use each unit at once (mutually

exclusive access) A job must obtain a unit of a needed resource,

use it, then release it

A resource is plentifulif no job is ever preventedfrom executingby the unavailability of units ofthe resource


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Execution Time

A job Ji will execute for time eiAmount of time required to complete the

execution of Ji when it executes alone and hasall the resources it needs

Execution times fall into an interval [ei-, ei

+];

Validate a system using ei+for each job


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Release and Response Time

Release time

The instant in time when a job becomesavailable for execution

Response time

The length of time from the release timeof the job to the time instant when itcompletes


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Deadlines and Timing Constraints Completion time the instant at which a job completes

execution Relative deadline the maximum allowable job response time

Absolute deadline the instant of time by which a job isrequired to be completed (often called simply the deadline) absolute deadline = release time + relative deadline

Feasible interval for a job Ji is the interval (ri, di]


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Example A system to monitor and control a heating furnace

The system takes 20ms to initialize For every 100 ms,

Samples , reads temparature

Computes the control-law,

Adjusts flow rates

The release time ofJk is 20 + (k 100) ms


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Example..


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Types of Tasks

Types:

Periodic

Aperiodic

Sporadic

Must be modeled differently Differing scheduling algorithms

Differing impact on system performance

Differing constraints on scheduling


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Modeling Periodic Tasks

A set of jobs that are executed repeatedly at

regular time intervals

Ti is a sequence of jobs Ji,1, Ji,2, , Ji,n

Phase of a task Ti =release time ri,1 of the first

job Ji,1

Period pi of a task Ti is the minimum length of all

time intervals between release times of

consecutive jobs

Execution time ei of a task Ti is the maximum

execution time of all jobs in the periodic task


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Modelling Periodic Tasks

Hyper-period of a set of periodic

tasks h= lcm(pi) for i= 1, 2, , n T1 : p1 = 3, e1 = 1

T2 : p2 = 5, e2 = 2


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Modeling Periodic Tasks..

Ui (utilization of taskTi)=ei/pi

Total utilization=U = ui

Usually assume the relative deadline for the

jobs in a task is equal to the period of the task


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Hard timing constraints

Deterministic constraints

Probabilistic constraints

Constraints in terms of some usefulnessfunctions.

Mostly specified as deterministic asvalidation is easy.


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Response to external events

Sporadic and aperiodic jobs

Inter arrival times between jobs varies widely.

Modeled as a random variable with some

probability distribution, A(x) .

Execution times are aperiodic

Aperiodic:

Have a soft deadline or no deadline.

The jobs have similar statistical behaviorSimilar timing requirements

Sporadic : Has hard deadlines


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Precedence Constraints and Dependencies

Jobs are constrained to execute in a particular

order

Ji is predecessor of Jk if Jk cannot begin

execution until the execution of Ji completes

Denoted by Ji < Jk

Immediate predecessorthere is no other job Jjsuch that Ji< Jj< Jk

A job with a precedence constraint becomesready for execution once when its release time

has passed and when all predecessors have

completed


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Task graph


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Types of dependencies

Data dependency

Temporal dependency

AND/OR precedence

Conditional branches

Pipeline relationship


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Functional Parameters

Preemptivity

A job is preemptable if its execution can be interrupted

A job is non-preemptable if it must run to completion

Preemptable jobs can have periods where they cannot be

preempted

Criticality Importance of job wrt to other jobs.

Measured by Priority/weight

During overload, scheduler optimizes by weighted priority.

Optional executions Scheduler can leave them executed or partially executed.

Laxity function Usefulness function gives the value of a job wrt to its tardiness.

Better never than late


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Resource graph

There is a vertex for each resource(processor or resource)

Attribute of a vertex is resource type

Edges represent relationship amongresources

Is a part ofConnectivity edges


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Scheduling

Jobs scheduled and allocated resources by a

scheduling algorithm Schedulerassigns jobs to processors

valid schedulesatisfies

Every processor is assigned to at most one job at any time

Every job is assigned at most one processor at any time

No job is scheduled before its release time

Amount of processor time assigned to every job is equal to its

maximum or actual execution time

All the precedence and resource usage constraints are satisfied


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Scheduling..

A valid schedule is also a feasible schedule if

every job meets its timing constraints.Miss rate is the percentage of jobs that are executed but

completed too late

Loss rate is the percentage of jobs that are not executed at all

A hard real time scheduling algorithm is optimalif the algorithm always produces a feasible

schedule if the given set of jobs has feasible

schedules

Many scheduling algorithms exist: main focus of

this module is understanding real-time

scheduling


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Summary

Outline of terminology and a reference

model:Jobs and tasks

Processors and resources

Time and timing constraints Hard real-time

Soft real-time

Periodic, aperiodic and sporadic tasks

Precedence constraints and dependencies Scheduling


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Summary

Concerned with release times and deadlines of jobs

Scheduler will ensure: Job is scheduled after its release time.

Job completes by its deadline.

Tasks can be hard or soft

Hard real time system may have several soft jobs and vice

versa

Validation:

1. Ensure the timing constraints are consistent withrequirements

2. Timing constraint of each component meets its timing if it is

executed alone and has all resources.

3. When scheduled together, the timing constraints are metwhen all applications contend for resources.

L23_Reference Model for RT Systems

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