1. MWSCAS 2015 Fort Collins, Colorado, USA August 2-5, 2015 Process Variation Aware Crosstalk Mitigation for DWDM based Photonic NoC Architectures Sai Vineel Reddy Chittamuru, Ishan Thakkar and Sudeep Pasricha Department of Electrical and Computer Engineering Colorado State University, Fort Collins, CO, U.S.A. {sai.chittamuru, ishan.thakkar, sudeep}@colostate.edu DOI: 10.1109/ISQED.2016.7479176

2. Introduction Motivation and Contributions Related Work Impact of Localized Trimming on Crosstalk Double-bit Crosstalk Mitigation Technique Experimental Results Conclusion Outline 1

3. Introduction Execution of modern complex applications necessitates  Many-core processors To enable chip many-core processors (CMPs)  Efficient communication fabrics are essential  Eletrical buses are no longer scalable  Electrical networks-on-chip (NoCs) are more viable With increase in core count, electrical NoC has  Higher power dissipation  Reduced performance (increased latency) 2 Mellonox 72-core chip Intel Xeon Phi 60 core processor To address drawbacks of electrical NoCs  Several new interconnect technologies are being explored

4. Benefits of Photonic Interconnects 3 Source: L. Xu, et al. IEEE-PTL, 2012 and S. V. R. Chittamuru, et al. GLSVLSI 2015 Photonic interconnects are potential solution to address drawbacks of copper wire based electrical interconnects Advantages of photonic interconnects over copper wires:  High bandwidth (~40 Gbps) with DWDM (dense wavelength division multiplexing)  5× or higher compared to copper wires  Low latency (10.45 ps/mm)  10× faster than copper wires  Low power (7.9 fJ/bit)  Better scalability, no pin limits Photonic links for data communication NoCs that use photonic interconnects provide higher bandwidth with lower power consumption Microring Resonator

5. 4 Introduction to Photonic Elements ModulatorDetector Electrical Bit-stream Electrical Bit-stream 010101 Modulators and detectors perform E/O and O/E conversion of data Microring (MR) resonator operation with ON/OFF keying modulation  Modulator to write data  Detector to read data SiGe Doped Waveguide Microring Resonator Circular waveguide with diameter 5µm Trans Impedance Amplifier (TIA) E/O: Electrical to Optical and O/E: Optical to Electrical 101010 01010 1010 010 10 0

6. Ideal Photonic Link Overview 5 Electrical Bit-stream Electrical Bit-stream Electrical Bit-stream Electrical Bit-stream MR Modulators SiGe Doped MR Detectors Trans Impedance Amplifier (TIA) Waveguide Four DWDM (Dense Wavelength Division Multiplexing) In real world, photonic link is not ideal MR: Micro Ring

7. 6 Existence of process variation also incurs crosstalk in DMDM based photonic NoCs MR Modulators MR Detectors SiGe Doped TIA Waveguide Process variation causes resonance wavelength drift Unable to write on dedicated wavelengths Suppose modulation side successfully writes data Process variation causes wavelength drift in detector Read wrong data (data corruption) Process Variation Impact on Photonic Link MR: Micro Ring

8. PV-Induced Crosstalk in Photonic Link 7 Electrical Bit-stream Electrical Bit-stream MR modulators SiGe doped MR detectors Trans Impedance Amplifier (TIA) Waveguide Crosstalk noise in detector Crosstalk noise in waveguide Electrical bit-streams with noise PV-induced crosstalk noise in ring detectors  Decreases Signal to Noise Ratio (SNR)  Increases Bit Error Rate (BER)  Threatens reliable photonic communication Crosstalk noise in modulator MR: Micro Ring

9. PV-Induced Crosstalk in Photonic Link 8 Electrical Bit-stream Electrical Bit-stream MR modulators SiGe doped MR detectors Trans Impedance Amplifier (TIA) Waveguide Crosstalk noise in detector Crosstalk noise in waveguide Electrical bit-streams with noise PV-induced crosstalk noise in ring detectors  Decreases Signal to Noise Ratio (SNR)  Increases Bit Error Rate (BER)  Threatens reliable photonic communication PV-induced crosstalk noise in MR detector needs to be mitigated for reliable photonic communication Crosstalk noise in modulator MR: Micro Ring

10. 9 Voltage Tuning (Trimming): =V ON VRVR Input PortOutput Port n+n+ p+p+ n+n+ Thermal Tuning: Input PortOutput Port Micro Heater Wavelength Power Transmission Voltage Tuning Blue Shift Wavelength Power Transmission Thermal Tuning Red Shift These solutions increase intrinsic optical loss and crosstalk noise in MRs and motivate new crosstalk mitigation mechanisms How to Tolerate Process Variations?

11. Our Contributions 10 Analytical models for PV-aware crosstalk analysis  Impact of localized trimming on crosstalk  Crosstalk modeling for Corona PNoC Double bit crosstalk mitigation (DBCTM) technique  To reduce crosstalk noise in PV-affected PNoCs Explore impact of DBCTM on DWDM-based PNoCs  Analysis in terms of worst-case SNR  DBCTM performance implications PNoC: Photonic Networks-on-chip Corona PNoC

12. Introduction Motivation and Contributions Related Work Impact of Localized Trimming on Crosstalk Double-bit Crosstalk Mitigation Technique Experimental Results Conclusion Outline 11

13. Device Level Crosstalk: [C. H. Chen WOCC 2012] Crosstalk noise in single waveguide crossings is shown to be close to -47.58 dB [Q. Xu, et al. Opt. Exp. 2006] A cascaded MR-based modulator is proposed for low-density DWDM waveguides, with an extinction ratio of 13dB These works show that crosstalk noise is negligible at device level Network Level Crosstalk and Mitigation: [L.H.K. Duong, et al. IEEE D&T 2014] Crosstalk analysis for the Corona PNoC, where its data channels are studied and worst-case SNR is estimated to be 14dB [S. V. R. Chittamuru, et al. IEEE D&T 2015] two encoding techniques PCTM5B and PCTM6B are presented to mitigate the impacts of crosstalk noise in DWDM based PNoCs. These works do not consider process variations and their impact on crosstalk Related Work 12 None of these works consider PV-aware crosstalk mitigation

14. Impact of Localized Trimming on Crosstalk 13 TRANSMISSION 1 0 Ideal condition of MR passbands (without PV) Increase in resonance wavelength We model passband overlap with coupling factor () With PV, passband shifts due to change in refractive index Suppose PV induces red shift Trimming is used to compensate this resonance drift Passband overlap increases with trimming of MRs Passband overlap region TRANSMISSION 1 0 MR passbands with PV Increase in resonance wavelength Red Shift TRANSMISSION 1 0 MR passbands with PV after trimming Increase in resonance wavelength Increase in passband overlap region Coupling factor increases with trimming of MRs

15. With localized trimming  Q-factor (Q’) of MR decreases  Coupling factor () and crosstalk noise increases Impact of Localized Trimming on Crosstalk 14 Our work decreases crosstalk noise and improves SNR in DWDM based PNoCs

16. Double-Bit Crosstalk Mitigation Technique 15 Crosstalk noise in PNoCs increases with  Coupling factor ()  Signal strength of adjacent non-resonant wavelengths Localized trimming increases DBCTM reduces crosstalk noise  Modulates zero on alternate wavelengths  Modulated zeros are shield bits  Reduces signal strength of adjacent non-resonant wavelengths Resonance shift has linear dependency on length and width variation Divide MRs in each detecting node into groups of 8 MRs Determine the thickness and width variation in each MR using SE and CD-SEM Determine maximum PV-induced resonance red shifts (Δ max ) in each MR Group Yes Enable DBCTM encoding in this MR Group Disable DBCTM encoding in this MR Group No Δ max > Δ th DBCTM Technique

17. We analyzed our DBCTM technique by porting it to Corona PNoC  [D. Vantrease et al. MICRO 2009] Corona architecture with token slot arbitration and 64×64 multiple write single read (MWSR) crossbar CMP configuration for implementation for Corona PNoC Experimental Setup 16 Chip Many Core Configuration Number of cores256 Technology node 22nm Memory controllers32 Main memory32GB; DDR4@30ns Per Core: L1 I-Cache size/Associativity16KB/Direct Mapped Cache L1 D-Cache size/Associativity16KB/Direct Mapped Cache L2 Cache size/ Associativity128KB/ Direct Mapped Cache L2 CoherenceMOESI Frequency5 GHz Issue PolicyIn-order

18. Built a cycle accurate photonic network simulator in SystemC Trace driven simulations using GEM5 simulator (PARSEC benchmarks) 12 multithreaded application workloads from PARSEC benchmark Model and estimate PV in MRs using the VARIUS tool 100 process variation maps are considered for our evaluation Performance modeling using DSENT, CACTI 6.5, and circuit-level analysis Static and dynamic power/energy for photonic devices: Source: [P. Grani, et al. JETC 2014] and [L.H.K. Duong, et al. IEEE Design and Test, 2014] 17 Energy consumption typeEnergy E dynamic 0.42 pJ/bit E logic−dyn 0.18 pJ/bit Photonic loss typeLoss (in dB) Propagation loss-0.274 per cm Bending loss-0.005 per 90 o Inactive modulator through loss-0.0005 Active modulator power loss-0.6 Passing detector through loss-0.0005 Detecting detector power loss-1.6 Active modulator crosstalk coefficient-16 Detecting detector crosstalk coefficient-16 Performance and Energy Models

19. 18 Worst-Case SNR Sensitivity Analysis Corona DBCTM X%  Has X% ratio of shielding bits to data bits  Shielding bits are zeros between data bits  Shielding bits increase laser and static power In Corona DBCTM X%  Increase in shielding bits to data bits ratio  reduces crosstalk noise  Increases SNR  Increases power consumption Worst SNR of Corona with DBCTM compared to its baseline  25% shielding bits - 8.1% higher  50% shielding bits – 19.67% higher  75% shielding bits - 26% higher  100% shielding bits – 40.5% higher Increase in SNR with DBCTM Increase in Power with DBCTM Corona: D. Vantrease et al. MICRO 2009 Increase in shielding bits of DBCTM Power consumption of Corona with DBCTM compared to its baseline  25% shielding bits - 14% higher  50% shielding bits - 20.1% higher  75% shielding bits - 63.9% higher  100% shielding bits - 104.1% higher

20. 19 Worst-Case SNR Sensitivity Analysis Corona DBCTM X%  Has X% ratio of shielding bits to data bits  Shielding bits are zeros between data bits  Shielding bits increase laser and static power In Corona DBCTM X%  Increase in shielding bits to data bits ratio  reduces crosstalk noise  Increases SNR  Increases power consumption Worst SNR of Corona with DBCTM compared to its baseline  25% shielding bits - 8.1% higher  50% shielding bits – 19.67% higher  75% shielding bits - 26% higher  100% shielding bits – 40.5% higher Increase in SNR with DBCTM Increase in Power with DBCTM Corona: D. Vantrease et al. MICRO 2009 Increase in shielding bits of DBCTM Power consumption of Corona with DBCTM compared to its baseline  25% shielding bits - 14% higher  50% shielding bits - 20.1% higher  75% shielding bits - 63.9% higher  100% shielding bits - 104.1% higher To balance crosstalk reliability and power overheads  DBCTM uses 50% shielding bits to data bits

21. 20 Worst-case SNR improvements of Corona with DBCTM  19.28 to 44.13% compared to baseline  12.44 to 34.19% compared to PCTM5B  4.5 to 31.30% compared to PCTM6B Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015 Results: Worst-case SNR comparison

22. 21 Worst-case SNR improvements of Corona with DBCTM  19.28 to 44.13% compared to baseline  12.44 to 34.19% compared to PCTM5B  4.5 to 31.30% compared to PCTM6B Corona DBCTM (with 50% shielding bits)  Reduces crosstalk noise in the detectors by using shielding bits between data bits  Considers the PV profile of MRs to select MRs for shielding Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015 Results: Worst-case SNR comparison

23. 22 Average packet latency of Corona with DBCTM has  12.6% higher compared to baseline  3.4% higher compared to PCTM5B  2.1% higher compared to PCTM6B Corona: D. Vantrease et al. MICRO 2009 PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015 Results: Corona Average Packet Latency

24. 23 Average packet latency of Corona with DBCTM has  12.6% higher compared to baseline  3.4% higher compared to PCTM5B  2.1% higher compared to PCTM6B Delay due to encoding and decoding of data with DBCTM contributes to increase in average latency Corona: D. Vantrease et al. MICRO 2009 PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015 Results: Corona Average Packet Latency

25. 24 Corona: D. Vantrease et al. MICRO 2009 Corona with the DBCTM technique  Has 31.6% higher EDP compared to the baseline  Increase in average latency and bits (increase in photonic hardware)  Has 16.4% lower EDP compared to the best known crosstalk mitigation technique PCTM6B  Considerable laser, static power savings due to lower photonic hardware PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015 Results: Corona Energy Delay Product

26. Our proposed DBCTM technique with Corona PNoC  Reduces crosstalk noise in its detectors  Improves SNR by up to 44.13% compared to baseline Our proposed DBCTM technique compared to the best known prior work  Improves SNR by up to 31.30%  Reduces EDP by 16.4% DBCTM technique is effective in overcoming trimming-induced crosstalk in PNoCs to improve reliability 25 Conclusions

27. Questions / Comments ? Thank You 26