Presented By: Pragya Arya

Btech(cs) 3rd year

Banasthali University

INTRODUCTION

Energy power efficient real time systems

are systems which are power efficient i.e.

uses less energy for computation and

processing in the specified time which

basically lead to light weighted devices

having longer battery life.

REAL TIME SYSTEMS

A real-time system is any information processing

system which has to respond to externally

generated input stimuli within a finite and

specified period.

Mainly Real time systems are divided in two

categories

Hard real time Systems

Soft real time systems

HARD REAL TIME SYSTEMS

An overrun in response time leads to potential

loss of life and/or big financial damage

Many of these systems are considered to be

safety critical.

Examples: Pacemakers , Airplane

control systems.

SOFT REAL TIME SYSTEMS

Deadline overruns are tolerable, but not desired.

There are no catastrophic consequences of

missing one or more deadlines.

Example: live video

streaming

EMBEDDED SYSTEMS

An embedded system is a computer system with

a dedicated function within a larger mechanical

or electrical system, often with a real time

constraint.

NEED OF ENERGY POWER EFFICIENCY

Critical design issue in real-time systems,

especially in battery- operated systems.

Smaller and lighter devices

Less heat generation

Better performance with unprecedented speed

Reduce cost

Reduce energy use

Longer battery life.

POWER ISSUE CAN BE ADDRESSED IN

Architecture Level

System Level

Application Level

SCHEDULING

Scheduling is method by which threads , process,

or data flow are given access to system resources.

This is usually done to load balance and share

system resources effectively.

There are various energy efficient scheduling

algorithms.

DYNAMIC VOLTAGE SCALING

Dynamic voltage scaling is a power management technique in computer architecture.

In which the voltage used in a component is increased or decreased, depending upon circumstances.

Dynamic voltage scaling to decrease voltage is known as undervolting. It is done in order to conserve power.

Dynamic voltage scaling to increase voltage is known as overvolting. It is done in order to increase computer performance.

SYSTEM MODELS

The real time system we are interested in consist of N independent periodic tasks, T = {τ1, τ2,⋅⋅⋅, τN} scheduled according to Earliest Deadline First (EDF).

τi = (Ci , Di ,Pi), is characterized by its worst case execution time Ci, deadline Di, and period Pi. We

assume Di<=Pi.

The Jth job of task τi is represented with Ji j = (ri j, ci j,di j),where ri j,ci j, and di j are the arrival time, actual execution cycles, and absolute deadline, respectively.

Each task τi is associated with a subset of

peripheral devices Φi = {M0,M1, ...,Mk}.

Φi can be further divided into shared part

Φi share and non-shared-part Φi nonshare.

Each peripheral device can be in one of the two

states Active State or Shut down state.

Energy overhead (Eo) andtime overhead (To) need

to be consumed to shut-down and later wake up

the processor or devices

Break even time is used to reflect

whether it is worthwhile to shut down the

processor /(peripheral devices) during idle

interval or not.

Ibe=max{Eo/P(idle),To}

DYNAMIC SCHEDULING ALGORITHM

Energy beneficial break even time Ieb is

computed dynamically for each job according to

the run-time conditions and shut down the

processor/devices whenever necessary.

THEOREM :

Given task set T = {τ1, τ2, ..., τN} with tasks

ordered by increasing value of Di.

All task deadlines can be guaranteed if any job

Ji of task τi is procrastinated by no more than Δi time units,

where Δi (called the procrastination time of task τi)

COROLLARY 1:

T – task set scheduled according to EDF

up(Ji)- upcoming job set in which each job Jk has

arrival time rk > ri .

If Ji is the only job in the ready queue, all jobs in

T can meet their deadlines if the starting

execution time of up(Ji) is delayed to tLS, where

tLS = min (rk +Δk) (1) Jk∈up(Ji )

COROLLARY 2:

T – task set scheduled according to EDF.

Ji current job to be executed in any time t. hp(Ji)

is the upcoming job set in which each job Jl has

arrival time rl > t and deadline dl < di.

Then all jobs in T can meet their deadlines if the

starting execution time of hp(Ji) is delayed to

˜tLS, where

˜tLS = min (rl +Δl) (2) l∈hp(Ji)

SCHEDULING ALGORITHM

1: Input: The current job Ji and the current

time t(cur);

2: if Ji is the only job in the ready queue then

3:Let edi = min{di, tLS};

//tLS is computed with 1

4: Compute Ii(eb) based on Ji’s current feasible

interval [t(cur),edi];

5: let fi be the expected completion time of Ji under

S(crit) ;

6: if (edi − fi) ≥ Ii(eb) then

CONT..

7: Execute Ji with S(crit) and shut down the

processor and devices in Φi at fi and set up the

wake up timer to be(tLS − fi);

8: // critical speed strategy with shut-down

9: else

10: Let t(na) be the earliest arrival time for the

upcoming jobs;

11: if fi ≥ t(na) and fi ≤ di then

12: Execute Ji with S(crit) ; // critical speed strategy

without shutting down shared devices in Φishare

13: else

14: Execute Ji with s′i = (ci×si)/(edi−(tcur)) non-

preemptively within [t(cur),edi]; // DVS strategy

stretching to edi;

µP

roc.

Sp

eed

Time

Start Deadline Start Deadline

Idle time

represent

s wasted

energy

Lower speed,

Lower voltage,

Lower energy

Energy ~ Work • Speed

Work

Work

CONT..

15: end if

16: end if

17: else

18: Compute ˜tLS based on equation (2);

19: Execute Ji with max{si, S(crit)} non-

preemptively within [t(cur),˜tLS] and shut down

its non-shared devices Φi nonshare upon its

completion;

20: end if

µP

roc.

Sp

eed

Time

S1 S2 S3 D2 D3

D1

W1 W2 W3

Task runs faster

to meet timing

constraints

APPLICATION AREAS Customer products:

Dish washers

Microwave ovens

Cars:

Airbag system

Engine control

Planes

Military

Weapons

Satellites

Robotics

Protection & security system

Intruder Alarm

Smoke/gas detection

LIMITATIONS

In DVS systems however, the performance level

is reduced during periods of low utilization such

that the processor finishes each task “just in time,” stretching each task to its deadline,

The primary caveat of overvolting is increased

heat: the power dissipated by a circuit increases

with the square of the voltage applied, so even

small voltage increases significantly affect power.

CONCLUSION

Dynamic Scheduling Algorithm is used to

minimize the system level energy consumption.

Approach of energy beneficial break even time is

is used to enhance the computation.

This approach can reduce energy consumption for

processor/devices significantly when compared

with other approaches.

REFERENCES

http://www.cse.unsw.edu.au/~cs9242/08/lectures/09-realtimex2.pdf - last seen 21/9/14

http://airccse.org/journal/ijcsea/papers/2212ijcsea16.pdf - last seen 24/9/14

http://www.dateconference.com/proceedings/PAPERS/2011/DATE11/PDFFILES/IP1_08.PDF - last

seen 21/9/14

http://www.ecs.umass.edu/ece/koren/architecture/RtDVS/intro.htm - last seen 28/9/14.