Process Variation Aware Crosstalk Mitigation for DWDM based Photonic NoC Architectures

MWSCAS 2015Fort 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

• Introduction

• Motivation and Contributions

• Related Work

• Impact of Localized Trimming on Crosstalk

• Double-bit Crosstalk Mitigation Technique

• Experimental Results

• Conclusion

Outline

1

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 NoCsSeveral new interconnect technologies are being explored

Benefits of Photonic Interconnects

3Source: 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 communicationNoCs that use photonic interconnects provide higher bandwidth with lower power consumption

Microring Resonator

4

Introduction to Photonic Elements

Modulator Detector

ElectricalBit-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

SiGeDopedWaveguide

Microring Resonator

Circular waveguide with diameter 5µm

Trans Impedance Amplifier (TIA)

E/O: Electrical to Optical and O/E: Optical to Electrical

101010010101010010100

Ideal Photonic Link Overview

5





MR Modulators

SiGe DopedMR Detectors Trans Impedance

Amplifier (TIA)

Waveguide

Four DWDM (Dense Wavelength Division

Multiplexing)

In real world, photonic link is not ideal

MR: Micro Ring

6

• Existence of process variation also incurs crosstalk in DMDM based photonic NoCs

MR Modulators

MR DetectorsSiGe 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

PV-Induced Crosstalk in Photonic Link

7



MR modulators

SiGe dopedMR detectors


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

PV-Induced Crosstalk in Photonic Link

8



MR modulators

SiGe dopedMR detectors


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

9

Voltage Tuning (Trimming):

=VONVR

Input Port Output Port

n+ p+ n+

Thermal Tuning:

Input Port Output Port

Micro Heater

Wavelength

Po

we

r Tr

ansm

issi

on

Voltage Tuning

Blue Shift

Wavelength

Po

we

r Tr

ansm

issi

on

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?

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

• Introduction

• Motivation and Contributions

• Related Work

• Impact of Localized Trimming on Crosstalk

• Double-bit Crosstalk Mitigation Technique

• Experimental Results

• Conclusion

Outline

11

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

Impact of Localized Trimming on Crosstalk

13

𝜱 𝒊, 𝒋 =𝜹𝟐

(𝒊 − 𝐣)𝑭𝑺𝑹𝒏

𝟐

+ 𝜹𝟐

𝑯𝒆𝒓𝒆 𝜹 =𝝀𝒋

𝟐𝑸′

𝝺𝑛𝝺𝑛+1 𝝺𝑛−1

TRA

NSM

ISSION

1

0

Ideal condition of MR passbands (without PV)

Increase in resonance wavelength• We model passband overlap

with coupling factor (𝚽)

• 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


TRA

NSM

ISSION

1

0

MR passbands with PV

Increase in resonance wavelength

Red Shift


TRA

NSM

ISSION

1

0

MR passbands with PV after trimming

Increase in resonance wavelength

Increase in passband overlap region

Coupling factor increases with trimming of MRs

• With localized trimming

Q-factor (Q’) of MR decreases

Coupling factor (𝚽) and crosstalk noise increases

Impact of Localized Trimming on Crosstalk

14

Our work decreases crosstalk noise and improves SNR in DWDM based PNoCs

0

2000

4000

6000

8000

10000

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

Q-f

acto

r

Incr

eas

e in

co

up

ling

fact

or

(φ)

Compensated PV-induced resonance shift (in nm)

increase in coupling factorQ-factor

Double-Bit Crosstalk Mitigation Technique

15

• Crosstalk noise in PNoCs increases with Coupling factor (𝚽) Signal strength of adjacent non-resonant

wavelengths

• Localized trimming increases 𝚽

• 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 (Δ𝛌max) in each MR Group

Yes

Enable DBCTM encoding in this

MR Group

Disable DBCTM encoding in this

MR GroupNo

Δ𝝺max> Δ𝝺th

DBCTM Technique

• 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 cores 256Technology node 22nmMemory controllers 32Main memory 32GB; DDR4@30nsPer Core:L1 I-Cache size/Associativity 16KB/Direct Mapped CacheL1 D-Cache size/Associativity 16KB/Direct Mapped CacheL2 Cache size/ Associativity 128KB/ Direct Mapped CacheL2 Coherence MOESIFrequency 5 GHzIssue Policy In-order

• 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 type EnergyEdynamic 0.42 pJ/bitElogic−dyn 0.18 pJ/bit

Photonic loss type Loss (in dB)Propagation loss -0.274 per cmBending loss -0.005 per 90o

Inactive modulator through loss -0.0005Active modulator power loss -0.6Passing detector through loss -0.0005Detecting detector power loss -1.6Active modulator crosstalk coefficient -16Detecting detector crosstalk coefficient -16

Performance and Energy Models

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

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

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 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

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

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


Results: Worst-case SNR comparison

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


Results: Corona Average Packet Latency

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


Results: Corona Average Packet Latency

24


• 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

• 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 knownprior work

Improves SNR by up to 31.30%

Reduces EDP by 16.4%

• DBCTM technique is effective in overcoming trimming-inducedcrosstalk in PNoCs to improve reliability

25

Conclusions

• Questions / Comments ?

Thank You

26

Process Variation Aware Crosstalk Mitigation for DWDM based Photonic NoC Architectures

Engineering

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