Post on 06-Apr-2017
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
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
ElectricalBit-stream
ElectricalBit-stream
ElectricalBit-stream
ElectricalBit-stream
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
ElectricalBit-stream
ElectricalBit-stream
MR modulators
SiGe dopedMR 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
PV-Induced Crosstalk in Photonic Link
8
ElectricalBit-stream
ElectricalBit-stream
MR modulators
SiGe dopedMR 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
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
𝝺𝑛𝝺𝑛+1 𝝺𝑛−1
TRA
NSM
ISSION
1
0
MR passbands with PV
Increase in resonance wavelength
Red Shift
𝝺𝑛𝝺𝑛+1 𝝺𝑛−1
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
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
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
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
• 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 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
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
Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Corona Average Packet Latency
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
• 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