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

2. Target Manycore System 2

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

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

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

6. Photonic technology – photonic link 6

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

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

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

10. Silicon photonic link – Ring filter, photodetector 10 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

11. 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. 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 13

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

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

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

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

18. 18 Clos Network

19. 19 Photonic Clos for a 64-tile system

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

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 21

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 22

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

24. Concentrated Mesh with Photonic Express Channels 24 Express1Express2

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

26. Performance 26

27. Power - Electrical 27

28. Photonic vs Electric Power Comparison 28

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

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 30

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

32. Thanks for your time 32