Computational Sprinting on a Hardware/Software Testbed

Arun Raghavan*, Laurel Emurian*, Lei Shao#, Marios Papaefthymiou+, Kevin P. Pipe+#, Thomas F. Wenisch+, Milo M. K. Martin*

University of Pennsylvania, Computer and Information Science*

University of Michigan, Electrical Eng. and Computer Science+

University of Michigan, Mechanical Engineering#

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3

Overview

• Computational sprinting [HPCA’12]• Targets responsiveness in thermally constrained environments• Far exceed sustainable power for short bursts of computation• Simulation based feasibility study

• This work: what can we learn with today’s hardware?• Engineer hardware/software testbed for sprinting

• Reduce heat venting capacity• Sustain only lowest power mode

• Can sprint on today’s system• Longer with phase-change material• Sprinting improves energy-efficiency

• Even for sustained computations

Tmaxp

ow

erte

mp

erat

ure

Computational Sprinting Using Dark Silicon [HPCA’12]

5

Tmaxp

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erte

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Effect of thermal capacitance

Computational Sprinting Using Dark Silicon [HPCA’12]

6

Tmaxp

ow

erte

mp

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ure

Effect of thermal capacitance

Computational Sprinting Using Dark Silicon [HPCA’12]

7

Tmaxp

ow

erte

mp

erat

ure

Effect of thermal capacitance

Computational Sprinting Using Dark Silicon [HPCA’12]

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Tmaxp

ow

erte

mp

erat

ure

State of the art:Turbo Boost 2.0

exceeds sustainable power

with DVFS (~25%)

Our goal: 10x

Effect of thermal capacitance

Computational Sprinting Using Dark Silicon [HPCA’12]

9

Evaluating Sprinting

• Simulation-based feasibility study [HPCA’12]

• Thermal models: buffer heat using thermal capacitance• Electrical models: stabilize voltage with gradual core activation• Architectural models:

• Large responsiveness improvements • Little dynamic energy overheads

• Next steps: understanding sprinting on a real system• Build a real chip? • Sprint on today’s mobile chips?

Our approach: study sprinting on hardware available today

10

This Work: Testbed for Computational Sprinting

• How long can the testbed sprint?• How to select sprint intensity?• How can we extend sprint duration?• How does sprinting impact energy?

11

Designing a testbed for sprinting

sprinting

sustainable

Quad-core Intel i7-2600With heatsink and fan: 95W

12

Remove heatsink, slow fan;10W thermal design (TDP)

Cores Freq. PowerNormalized

Power

PeakSpeedu

p

1 core 1.6 GHz 10 W 1x 1x

4 cores 1.6 GHz 20 W ~2x 4x

4 cores 3.2 GHz 50 W ~5x 8x

3 operating modes:

Sprinting Performance

0

1

2

3

4

5

6

7

8

norm

aliz

ed s

peed

up

sobel disparity segment kmeans feature texture

Cores + Frequency (3.2GHz): 6.3x speedup Cores only (1.6 GHz): 3.5x speedup

Max 4 core, 3.2 GHz

Max 4 core, 1.6 GHz

Baseline(no sprint)

3.2GHz

3.2GHz

3.2GHz

3.2GHz

3.2GHz

3.2GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

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How long can the testbed sprint?

Testbed Thermal Response40

5060

700

2040

60

Po

wer

(W

)

time (s)

Tem

p (

°C)

time (s)

Tmax

15

-5 0 5 10 15 20 25 300

1 0

2 0

3 0

4 0

5 0

6 0

sustainedsprint-3.2GHz

-5 0 5 10 15 20 25 30

sustainedsprint-3.2GHz

020

4060

4050

6070

sustained

sustained

-5 0 5 10 15 20 25 300

1 0

2 0

3 0

4 0

5 0

6 0

sustainedsprint-3.2GHz

-5 0 5 10 15 20 25 30

sustainedsprint-3.2GHz

Testbed Thermal Response40

5060

700

2040

60

Po

wer

(W

)

time (s)

Tem

p (

°C)

time (s)16

5x

3s

-5 0 5 10 15 20 25 300

1 0

2 0

3 0

4 0

5 0

6 0

sustainedsprint-3.2GHz

-5 0 5 10 15 20 25 30

sustainedsprint-3.2GHz

020

4060

4050

6070

sustainedsprint (3.2 GHz)

sustainedsprint (3.2 GHz)

20g copper

Δ 25oC, ~188J 3.5s @ 50W

Heat spreader

-5 0 5 10 15 20 25 30

sustainedsprint-3.2GHzsprint-1.6GHz

-5 0 5 10 15 20 25 300

1 0

2 0

3 0

4 0

5 0

6 0

sustainedsprint-3.2GHzsprint-1.6GHz

Testbed Thermal Response40

5060

700

2040

60

Po

wer

(W

)

time (s)

Tem

p (

°C)

time (s)17

5x

3s

-5 0 5 10 15 20 25 30

sustainedsprint-3.2GHz

020

4060

4050

6070

-5 0 5 10 15 20 25 300

1 0

2 0

3 0

4 0

5 0

6 0

sustainedsprint-3.2GHz

21ssustainedsprint (3.2 GHz)sprint (1.6 GHz)

sustainedsprint (3.2 GHz)sprint (1.6 GHz)

2x

20g copper

Δ 25oC, ~188J 3.5s @ 50W

Heat spreader

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What if computation doesn’t complete during the sprint?

Truncated Sprint Performance

19

012345678

4 cores, 1.6 GHz4 cores, 3.2 GHz

computation length

nor

ma

lize

d s

pe

edu

p

Little benefit

Near-peak performance for short tasks

Truncated Sprint Performance

012345678

4 cores, 1.6 GHz4 cores, 3.2 GHz

computation length

nor

ma

lize

d s

pe

edu

p

20

Little benefit

Near-peak performance for short tasks

Lower peak performance; benefits longer tasks

• Best sprint intensity depends on task size• How to sprint when task size is unknown?• Sprint pacing

• Max intensity sprint for half thermal capacitance• Cores-only sprinting for other half

Truncated Sprint Performance

012345678

4 cores, 1.6 GHz4 cores, 3.2 GHz

computation length

nor

ma

lize

d s

pe

edu

p

21

• Best sprint intensity depends on task size• How to sprint when task size is unknown?• Sprint pacing

• Max intensity sprint for half thermal capacitance• Cores-only sprinting for other half

Truncated Sprint Performance

012345678

adaptive4 cores, 1.6 GHz4 cores, 3.2 GHz

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• Best sprint intensity depends on task size• How to sprint when task size is unknown?• Sprint pacing

• Max intensity sprint for half thermal capacitance• Cores-only sprinting for other half

computation length

nor

ma

lize

d s

pe

edu

p

Increasing sprint duration

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Two Ways of Adding Thermal Capacitance

• Specific heat capacity: introduce thermal mass

• Latent heat: absorb heat to change phase (e.g. melting)

tem

per

atu

rete

mp

era

ture

time (s)

time (s)

Phase-change absorbs heat while holding temperature constant

Baselinesprinting

More specific heat takes longer to heat

20g copper, Δ 25oC ~188J

1g of wax ~200J

25 Computational Sprinting on a Hardware-Software Testbed

Extending Sprints with Phase Change Material

4g of wax, 1g of aluminum foam

Copper shim

26

0 50 100 150 200 250 300 350 40040

50

60

70

80

Impact of Phase Changete

mp

erat

ure

(°C

)air

time (s)

27

0 50 100 150 200 250 300 350 40040

50

60

70

80

tem

per

atu

re (

°C)

Small extension from heat capacity of encasement…

foamair

time (s)

Impact of Phase Change

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0 50 100 150 200 250 300 350 40040

50

60

70

80

tem

per

atu

re (

°C)

Small extension from heat capacity of encasement……6x increase in sprint duration with phase change

foam waxair

time (s)

Impact of Phase Change

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0 50 100 150 200 250 300 350 40040

50

60

70

80

tem

per

atu

re (

°C)

Small extension from heat capacity of encasement……6x increase in sprint duration with phase change

due to phase change

foamairwater

time (s)

Impact of Phase Changewax

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Phase Change Material in Action

Time lapse 15x

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How does sprinting impact energy?

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Energy Impact of Sprinting

0

0.5

1

1.5

norm

aliz

ed e

nerg

y

sobel disparity segment kmeans feature texture3.2GHz

3.2GHz

3.2GHz

3.2GHz

3.2GHz

3.2GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

1.6GHz

Race-to-idle: 7% energy savings!

Sprint 3.2GHz

Sprint 1.6GHz

Idle

If sprinting is more energy efficient, why not sprint all the time…

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0 50 100 150 200 250 300 350 400 450 50005

10152025

sustainedsprint-rest

Sprint-and-Rest

seconds

pow

er (

W)

0 50 100 150 200 250 300 350 400 450 500304050607080

sustainedsprint-rest

seconds

tem

pera

ture

(o C

)

0 50 100 150 200 250 300 350 400 450 5000

50100150200250300

sustainedsprint-rest

seconds

cum

ulat

ive

wor

k

sprint-and-rest

sprint-and-rest

sprint-and-rest

34

35% faster

5s @ 20W sprint, 12s @ 5W rest < 10W average

Conclusions

• Testbed confirms sprinting improves responsiveness

• Sprint pacing can extend benefits of sprinting

• Exploiting phase change allows longer sprints

• Sprinting can save energy in thermally limited systems

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www.cis.upenn.edu/acg/sprinting/