parascale : exploiting parametric timing analysis for real-time schedulers and dynamic voltage...

ParaScale : Exploiting Parametric Timing Analysis for Real-Time

Schedulers and Dynamic Voltage Scaling

Sibin Mohan1

Frank Mueller1,William Hawkins2, Michael Root3, Christopher

Healy3, David Whalley4

1 : North Carolina State University, Centre for Embedded Systems Research

2 : University of Illinois, Urbana-Champaign, 3 : Furman University,

4 : Florida State University

Dec 5-8, 2005 Sibin Mohan : Real-Time Systems Symposium 2005 2

Outline

Motivation Static Timing Analysis Parametric Timing Analysis Using Parametric Timing Analysis Framework Experiments and Results Conclusion


Motivation

Worst-case execution time (wcet) obtained by Static timing analysis Dynamic timing methods (proven unsafe)

Static Timing Analysis Upper limits on loop bounds known at compile

time Results in large overestimations Provides single numeric value

Places limits on real-time designersType of code that may be used in tasks


Static Timing Analysis

for( i = 0 ; i < n ; ++i )Loop Body ;

Consider the following piece of code…

Value must be known at compile time

Result : WCET = 1000 cycles (say)


Parametric Timing Analysis

Calculate : formula /closed form for WCET

Formula depends on number of loop iterations

Evaluated at run-time

Results used for… scheduling decisions power savings etc.


Parametric Timing Analysis (contd.)

For the previous example…

for( i = 0 ; i < n ; ++i )Loop Body ;

Result : WCET = 102 * n (say)

Number of cyclesto execute Loop Bodyfor above example

Value must be known prior to loop entry


Parametric Timing Analysis (contd.)

In practice…

for( i = 0 ; i < n ; ++i ) Loop Body ;

Call IntraTaskScheduler(evaluate_loop_k(n)) ;

int evaluate_loop_k( int loop_bound){

return ( 102 * loop_bound ) ;

}


Flow of Parametric Timing Analysis

C source file

Parametric Timing Analyzer

C source file annotated with

Parametric Evaluation functions

Has Parametric formula changed?

YES

Send Annotated C source file to Parametric Timing Analyzer

NO

Use Annotated C source file for execution on Simulator


Insert Parametric Formulae into Task Code

Timing analysis of function with parametric loop Loop analyzed and parametric evaluation function generated

Numerical Analysis of generated function complete Timing analysis if function with parametric loop complete


Use of Parametric Timing Analysis

Evaluate parametric expressions at run-time Based on actual loop bounds at run-time

Transfer control to dynamic scheduler

Newly-calculated WCET sent to scheduler Calculate savings in execution time

Savings exploited by scheduler to… admit additional tasks reduce operating frequency/voltage save

power


Unique advantages of our work

Exploit early knowledge of execution of tasks Especially knowledge about future execution of loops

Tightly bound execution for remainder of task

Intra-task DVS algorithm slows down processorAs execution proceeds processor slowed down furtherSaves power!

Other DVS algorithms:Task execution sped up as deadlines approach


Framework

C Source FilesC Source

FilesC Source Files

gcc PISA Compiler

Numeric WCET Bound

Numeric Timing

Analyzer

Instruction/data info

p-compiler

assembly

Parametric Timing

Analyzer

C Source Files & Parametric

Functions


Functions


Functions

Scheduler

Energy/power values

Wattch Power Model

SimpleScalar Simulator


Framework (contd.)

p-compiler Works on assembly files as input Extracts information about instructions and data

Timing Analyzers Numeric : provides constant, numeric WCET bound Parametric : provides parametric formulae as WCET

Schedulers Frequency/Voltage lowered during execution Set to levels for last executing task instance

SimpleScalar simulation framework Capable of handling multiple threads of execution Can be configured for simple processor, SMT, CMP, etc.


Experiments

Energy measurement techniques…Perfect Clock Gating (PCG)Perfect Clock Gating with Leakage (PCGL)

ParaScaleOur combined intra-task and static inter-task DVS

Parameter Range of Values

Utilization 20%, 50%, 80%

Ratio WCET/PET* 1x, 2x, 5x, 10x, 15x, 20x

Base

DVS Algorithms Static DVS

Parametric

ParaScale

* Parametric Execution Time

Dec 5-8, 2005 15

Energy Consumption : 2x, PCG

0

50

100

150

200

250

300

350

400

450

500

base static DVS parametric ParaScaleUtilization

En

erg

y C

on

su

mp

tio

n(m

J) 20%

50%

80%

Energy consumptions, assuming no leakage, for various utilizationsDVS Schemes

Least consumption: ParaScale (for all cases)

Dec 5-8, 2005 16

Energy Consumption : 2x, PCGL

0

500

1000

1500

2000

2500

3000


En

erg

y C

on

su

mp

tio

n(m

J) 20%

50%

80%

Energy consumptions, leakage considered, for various utilizationsDVS Schemes

Least consumption: ParaScale (for all cases)

Dec 5-8, 2005 17

Energy Consumption: 10x, PCG

0

20

40

60

80

100

120

140

base static DVS parametric ParaScale

Energ

y C

onsu

mpti

on(m

J)

20%

50%

80%

Energy consumptions, assuming no leakage, for various utilizations and DVS Schemes for 10xLeast Consumption : ParaScale

Overall savings are lower due to greater slack


Energy Consumption: 10x, PCGL

0

500

1000

1500

2000

2500

3000


Energ

y C

onsu

mpti

on(m

J)

20%

50%

80%

Energy consumptions, leakage considered, for various utilizations and DVS Schemes for 10xLeast Consumption : ParaScale

Overall savings are lower due to greater slack

Dec 5-8, 2005 19

Energy Consumption trends (PCG)

0

50

100

150

200

250

1x 2x 5x 10x 15x 20xWCET Factor

Ene

rgy

Con

sum

ptio

n(m

J)20%

50%

80%

Energy consumption (hence savings) for ParaScale:Drops as ratio of WCET/PET is increasedDue to greater slack available in the system

Dec 5-8, 2005 20

Energy Consumption trends (PCGL)

0

50

100

150

200

250

300

350

400

1x 2x 5x 10x 15x 20xWCET Factor

En

erg

y C

on

sum

pti

on

(mJ)

20%

50%

80%

Energy consumption affected byLeakage power prevalent in systemRelative savings lower

Dec 5-8, 2005 21

Scheduler Overhead: Utilization 50%

0

500

1000

1500

2000

2500

3000

1x 2x 5x 10x 15x 20x

Ex

ecu

tio

n T

ime u

sed

by

Sch

ed

ule

r (

ns)

base static DVS

parametric ParaScale

Scheduler overheads greatest for ParaScale Power savings still significant

Due to lowering of voltage/frequency for schedulers

22

Conclusion

Fixed-point approach To embed parametric formulae Bounds WCET of application + parametric code

Provide lower WCET bounds during execution Prior to entry into loops

Quantify savings in terms of Power savings ParaScale: combined inter and intra-task DVS

Savings of 66-80% ! Power savings over DVS-oblivious techniques.

Mainly due to knowledge about: Past execution Future execution due to parametric formulae


Thank You !

Questions ?

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