Hybrid Optoelectric On-chip Interconnect Networks

Yong-jin Kwon

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Target Manycore System

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On-chip network topology spectrum

Increasing radix

Increasing diameter

Mesh CMesh Clos Crossbar

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Related Works

[Shacham’07][Petracca’08]

[Vantrease’08][Psota’07][Kirman’06]

[NOCS’09][Pan’09]

Mesh CMesh Clos Crossbar

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Outline

• Technology Background• Previous Studies and Motivation• Performance Analysis• Power Analysis• Conclusion

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Photonic technology – photonic link

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Silicon photonic link – Coupler

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Coupler loss = 1 dB

Silicon photonic link – Ring modulator

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Modulator insertion loss = 0 – 1 dB

Energy spent in E-O conversion = 25 – 90 fJ/bt

(independent of link length)

Silicon photonic link – Waveguide

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Waveguide loss = 0 – 5 dB/cm

Silicon photonic link – Ring filter, photodetector

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Filter drop loss = 1.5 dB

Photodetector loss = 0.1 dB

Energy spent in O-E conversion = 25 - 60 fJ/bt

(independent of link length)

Receiver sensitivity = -20 dBm

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Silicon photonic link – WDM Through ring loss = 1e-

4 – 1e-2 dB/ring

• Dense WDM (128 λ/wg, 10 Gbps/λ) improves bandwidth density (30x!!)

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Silicon photonic link – Energy cost

• E-O-E conversion cost – 50-150 fJ/bt (independent of length)

• Thermal tuning energy (increases with ring count)• External laser power (dependent on losses in

photonic devices)

Silicon Photo

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Electrical technology

• Design constraints– 22 nm technology– 500 nm pitch– 5 GHz clock

• Design parameters– Wire width– Repeater size– Repeater spacing

FF FF FFRepeaters Repeaters

Repeater inserted pipelined wires

1.0 mm

2.5 mm

5.0 mm

7.5 mm

10.0 mm

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Electrical technology

• Design constraints– 22 nm technology– 500 nm pitch– 5 GHz clock

• Design parameters– Wire width– Repeater size– Repeater spacing

FF FF FFRepeaters Repeaters

Repeater inserted pipelined wires

1.0 mm

2.5 mm

5.0 mm

7.5 mm

10.0 mm

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Electrical vs Optical links – Energy cost

Thermal tuning energy

Transmitter-Receiver energy

Elec: ElectricalOpt-A: Optical-AggressiveOpt-C: Optical-Conservative

Optical laser power not shown

(dependent on the physical layout)

Outline

• Technology Background• Previous Studies and Motivation• Proposed Design• Performance and Power Analysis• Conclusion

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Clos Network

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Photonic Clos for a 64-tile system

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Power-Bandwidth tradeoff

CMeshX2Channel width = 128b

PClosChannel width = 64b

PClosChannel width = 128b

Off-chip laser power = 3.3 WComparable on-chip power for local traffic

Problems and Motivations

• A mesh-like topology is highly optimized for local communication and hard to beat– Solution: use a underlying mesh topology

• A fully photonic network has higher power numbers on low utilization – Solution: make the photonic channels to be

turned off at low utilization

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What Do We Need?

• An electrical network which connects all-to-all even when the laser is turned off

• A photonic network which (when turned on) provides benefits to the base electrical mesh

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Outline

• Technology Background• Previous Studies and Motivation• Proposed Design• Performance and Power Analysis• Conclusion

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Concentrated Mesh with Photonic Express Channels

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Express1 Express2

Outline

• Technology Background• Previous Studies and Motivation• Proposed Design• Performance and Power Analysis• Conclusion

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Performance

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Power - Electrical

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Photonic vs Electric Power Comparison

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Outline

• Technology Background• Previous Studies and Motivation• Proposed Design• Performance and Power Analysis• Conclusion

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Conclusion

• Maybe we do not need to shut down photonics on low utilization

• In order for photonics to be effective we need better devices– There is no power advantage in using photonics if

we can’t get to aggressive– We do win in bandwidth density but area is cheap

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Acknowledgement

• Ajay Joshi – Help in power calculations and images

• Chris Batten– Brainstorming help

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Thanks for your time

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