Hybrid Optoelectric On-chip Interconnect Networks

1. Hybrid Optoelectric On-chip Interconnect Networks Yong-jin Kwon

2. Target Manycore System

3. CMesh Mesh Clos Crossbar On-chip network topology spectrum Increasing diameter Increasing radix

4. CMesh Mesh Clos Crossbar Related Works [Vantrease’08] [Psota’07] [Kirman’06] [Shacham’07] [Petracca’08] [NOCS’09] [Pan’09]

5. Outline • Technology Background • Previous Studies and Motivation • Performance Analysis • Power Analysis • Conclusion

6. Photonic technology – photonic link

7. Silicon photonic link – Coupler Coupler loss = 1 dB

8. Silicon photonic link – Ring modulator Energy spent in E-O conversion = 25 – 90 fJ/bt (independent of link length) Modulator insertion loss = 0 – 1 dB

9. Silicon photonic link – Waveguide Waveguide loss = 0 – 5 dB/cm

10. Silicon photonic link – Ring filter, photodetector Energy spent in O-E conversion = 25 - 60 fJ/bt (independent of link length) Photodetector loss = 0.1 dB Filter drop loss = 1.5 dB Receiver sensitivity = -20 dBm

11. 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!!)

12. 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)

13. Silicon Photo

14. Electrical technology Repeaters Repeaters FF FF FF • Design constraints • 22 nm technology • 500 nm pitch • 5 GHz clock • Design parameters • Wire width • Repeater size • Repeater spacing Repeater inserted pipelined wires 10.0 mm 7.5 mm 5.0 mm 2.5 mm 1.0 mm

15. Electrical technology Repeaters Repeaters FF FF FF • Design constraints • 22 nm technology • 500 nm pitch • 5 GHz clock • Design parameters • Wire width • Repeater size • Repeater spacing Repeater inserted pipelined wires 10.0 mm 7.5 mm 5.0 mm 2.5 mm 1.0 mm

16. Electrical vs Optical links – Energy cost Elec: Electrical Opt-A: Optical-Aggressive Opt-C: Optical-Conservative Optical laser power not shown (dependent on the physical layout) Thermal tuning energy Transmitter-Receiver energy

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

18. Clos Network

19. Photonic Clos for a 64-tile system

20. Power-Bandwidth tradeoff Off-chip laser power = 3.3 W Comparable on-chip power for local traffic CMeshX2 Channel width = 128b PClos Channel width = 64b PClos Channel width = 128b

21. 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

22. 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

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

24. Concentrated Mesh with Photonic Express Channels Express1 Express2

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

26. Performance

27. Power - Electrical

28. Photonic vs Electric Power Comparison

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

30. 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

31. Acknowledgement • Ajay Joshi • Help in power calculations and images • Chris Batten • Brainstorming help

32. Thanks for your time