Devashish VedreCSU ID# 830147760

ECE 561Hardware/Software Design of Embedded

SystemsProject Presentation

Silicon Photonics and Photonic NoCs: A Survey

• Why photonic interconnects instead of metallic interconnects for intra-chip data transfer?1. Metallic interconnects have RC-delays which increase with increase in frequency.2. Slow data transfer rates which do not match with the current processing speed of the processing units on a chip.3. Increase the overall FLOPS of the system.

• How did the idea of silicon photonics began?1. First paper published in 1984 by Goodman et al.2. Photonics is being used for inter-chip communication and optical fibres are used widely for data transfer3. Now, it is possible to fabricate optical waveguides on silicon chips easily

• What things will be affected if photonic interconnects have to replace metallic ones?1. Entire or part of metallic NoCs will be replaced by photonic NoCs.2. Router structure, architectures, simulation tools will change

• In this survey, I have highlighted few works on routers for photonic interconnects, NoC architectures based on photonic interconnects and simulation tools which have been proposed to be used for simulation of these architectures and techniques for improvement in power, latency and bandwidth with minimizing overheads

Architectures• Assaf et al. proposed an architecture in 2006 for

chip-multiprocessor system

• Two layered structure : – Photonic interconnection network– Electronic control network

• Photonic router (fig)

• 4x4• PSE- photonic switching element

• Deflection routing algorithm for contention resolution.

• Failed because packets need they need alternate path and so they set constraint on network topology

• Brief power analysis was carried out but many questions unanswered

4x4 router by Assaf et al.

Architectures• Photonic switching elements• Modulators based on microring resonators fabricated in silicon• Switching time – 30ps• 5um ring radius• Power consumption <0.5mW when on, 1pJ to switch, negligible when

off• Crosstalk (>20dB)• Low insertion loss• Modulating several wavelenghts at tens of Gb/s• In this architecture, torus topology augmented with gateway access

points1. to facilitate injection and ejection without interference with the through

traffic on the torus, and2. to avoid blocking between injected and ejected traffic. • 3 types of switches in each GAP: (fig)• (1) a gateway switch is directly connected to the gateway in the

processor core; • (2) injection switches are located on the torus rows; and • (3) ejection switches are located on the torus columns.• Each injected message travels from the gateway switch to an injection

switch. It then travels on the network to the ejection• switch associated with its destination core from which it is sent to the

gateway switch and out of the network. The design of the GAPs and the traffic rules is used to avoid message blocking in them

Figure: 3 types of switches in each GAP

Figure: Layout of a 16node 2D folded torus with 2_ path overprovisioning. Solid (dashed) lines represent bidirectional (unidirectional) links. Network switches (squares) are shown in dark gray, and the GAPs, made of gateways (circles), gateway switches (squares), injection switches (triangles), and ejection switches (diamonds), appear in light gray. The GAP for one node is emphasized.

Architectures• Assaf et al. performed power analysis to compare their

photonic NoC with electronic NoC• Specsifications1. CMP 36 processing cores2. 800Gb/s peak bandwidth3. 512Gb/s average bandwidth4. Uniform traffic model5. Mesh topology6. XY dimension order routing

• Calculated average power 3.2W• Better than metallic NoC

Figure reference: Assaf Shacham, Keren Bergman, Luca P. Carloni, “the Case for Low-Power Photonic Networks on Chip”, DAC 2007, June 4-8, 2007, San Diego, California, USA.

Architecture• In 2009, S. Bahirat and S. Pasricha proposed a hybrid photonic architecture with a

clear comparison with the metallic NoC architecture• They compared the latency, throughput and power of electrical mesh, electrical

torus and their hybrid photonic for a defined number of nodes (4x4, 8x8 and 10x10 NoCs). As shown in fig. 5 for a 10x10 NoC, hybrid photonic NoC works the best in terms of latency, throughput and power

Figure: 3D implementation of the proposed hybrid photonic NoC -mesh of cores on the bottom layer, photonic waveguide on the top layer

Figure: Hybrid photonic vs. electrical mesh vs. electrical torus for a 10x10 NoC (a) latency, (b) throughput, (c) power

Figures Reference: Shirish Bahirat, Sudeep Pasricha, “Exploring Hybrid Photonic Networks-On-Chip for Emerging Chip Multiprocessors”CODES+ISSS’09, October 11-16, 2009, Grenoble, France

Architecture

• In 2009, Christopher et al. came up with a paper in which they showed the implementation of photonic interface for core to DRAM communication. They proposed a local mesh to global switches (LMGS) topology[10] They showed that ultimately, photonics enables almost an order of magnitude improvement in throughput at similar latency and power consumption

• In 2013, S. Bartolini et al. proposed an all-optical photonic network for low power applications (Fig.) called Olympic.

• Similarly, Y. Demir et al. proposed an architecture named Galaxy[18] in 2013. This is a multi-chip architecture which builds a many-core “virtual chip” by connecting multiple smaller chiplets through optical fibers. Galaxy is designed to push back the power constraints, in addition to overcoming the area and bandwidth limitations, while matching the high performance of tightly-coupled chips. They successfully demonstrate that Galaxy achieves 1.8-3.4x average speedup over competing single-chip designs, and achieves 2.6x lower energy-delay product (6.8x maximum). The careful design of optical paths in Galaxy minimize coupler crossings and allows it to scale beyond 4K cores, showing significant promise as the foundation of practical large-scale virtual chip designs. Finally, we show that a scaled-out Galaxy attains significant speedup and energy efficiency advantages over competing designs such as the Oracle Macrochip as it achieves at least 2.5x speedup with 6x more power-efficient optical links

Figure: Configuration Architecture of Olympic: low power all-photonic NoC

Figure Reference: Sandro Bartolin, Luca Lusnig and Enrico Martinelli, “Olympic: a Hierachical All-optic Photonic Network for Low-power Chip Multiprocessors”, 16th, Euromicro Conference on Digital System Design, 2013

Routing•N. Sherwood-Droz et al. proposed a work showing spatially non-blocking 4x4 optical router•Dynamically switched using thermo-optically tuned silicon microring resonators•wavelength shift to power ratio of 0.25nm/mW•can route four optical input to four outputs with individual bandwidths of up to 38.5 GHz•This work shows that the bridge between photonic NoCs and silicon is getting smaller and smaller and gives a detailed description of algorithm, design and fabrication as well as testing of these routers which are 4x4 and more of these can be used to extend to bigger chips•principle of operation is to use two microring resonators on the opposite sides of a waveguide intersection so that both signals can be routed to both ways simultaneously•wavelength of resonator is a function of temperature

Figure: Shown in color are four paths highlighting an arbitrary configuration using the maximum number of microrings

• In 2009, Andrew Poon et al. proposed a 5x5 microring resonator based optical switch in their paper. But the emphasis was on on improvement of 4x4 router.. They used various type of available microring resonators to combine them into a 5x5 router for photonic interconnects

Figure : (a) Schematic of a 5 _ 5 matrix switch comprising 20 identical microring resonator-based switch elements coupled to aMMI crossing grid array with 25 crossings. The dashed arrows illustrate some of the possible light paths connecting 5 different input-ports (Iwest, Ieast, injection, Inorth, and Isouth) to five different output-ports (Owest, Oeast, ejection, Onorth, and Osouth). (b) Physical layout of our proposed 5 _ 5 photonic matrix switch comprising 20 identical silicon microring resonator-based switch elements coupled to aMMI crossing grid array with 26 crossings. The pink arrow depicts one of the longest links (injection-to-east). The blue arrow depicts one of the shortest links (west-to-ejection). An additional MMI crossing is employed at the intersection between Isouth- and Oeast-ports. Black solid dots represent termination at the unused ports

Routing

Fig. 10. Connections between the add points and the drop points of N-port nonblocking optical router

Power loss estimation of different size routers compared to GWOR

•In 2012, Rui Min et al. proposed a N-port non blocking router uring microring resonators. They showed that the N-port router performs better than the previous routers cascaded to N-ports

•Xianfang Tan et al. proposed a low area-footprint microring resonator based photonic router in 2012. This router had less area footprint and power than previous types of routers as shown in fig. 10 above. But there was power loss that is weakening of the optical signal to some extent which was calculated and shown in this work.

Figure references:Rui Min, Ruiquiang Ji, Qiaoshan Chen, Lei Zhang and Lin Yang, “A Universal Method for Constructing N-Port Nonblocking Optical Router for Photonic Networks-On-Chip”, November 15, 2012, IEEE.

Xianfang Tan, Mei Yang, Lei Zhang, Yingtao Jiang and Jianyi Yang, “A Generic Optical Router Design for Photonic Network-on-Chips”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 3, FEBRUARY 1, 2012

Simulation Tools• Chen Sun et al. have presented a very prominent work in 2012 which gives information about the limitation of the extent till which we can

rely on software tools to calculate operation characteristics of optical NoCs as compared to electrical NoCs. Also DSENT is the first tool that is able to capture the interactions at electronic/photonic interface and their implications on a photonic NoC

Figure Reference: Chen Sun et al., “DSENT – A Tool Connecting Emerging Photonics with Electronics for Opto-Electronic Networks-on-Chip Modeling”, 2012, IEEE

Simulation tools• Photonic network technology, once relegated exclusively to large-scale telecommunication networks, has

in recent years gradually been penetrating into smaller scale networking domains, with the potential to eventually become a viable architectural solution for on-board and chip-scale systems.

• We have introduced PHOENIXSIM as a simulation environment for the design, analysis, and optimization of these high-performance interconnection networks in a manner that accurately captures the physical-layer aspects of the devices while enabling system performance evaluation.

• This combination of models and tools in a single integrated environment provides a unique resource for the design exploration of the new systems enabled by photonic networks PhoenixSim then was the result of recent work in photonics modeling, improving the architectural visibility concerning the trade-offs of photonic networks.

• PhoenixSim provided parameterized models for photonic devices. • However, PhoenixSim lacked electrical models, relying instead on Orion for all electrical routers and links. • As a result, PhoenixSim used fixed numbers for energy estimations for electrical interface circuitry, such as

modulator drivers, receivers, and thermal tuning, losing many of the interesting dynamics when transistor technology, data-rate, and tuning scenarios vary.

• PhoenixSim in particular could not capture trade-offs among photonic device and driver/receiver specifications that results in an area or power optimal configuration.

Conclusion• A survey of the important researches on the use of photonics for System-On-Chips has been

done. • While most of the works done until now suggest that the use of photonics NoCs for on-chip

data transfer is beneficial, some works suggest that it is not so. • We have also seen many novel architectures which are entirely photonic NoCs or hybrid

photonic NoCs with some metallic components. • The increase in number of GFLOPS of processors due to photonic NoCs is evident based on

software simulation results. • We have also seen the best of the routing techniques suggested for routing data in form of

light signals through photonic NoCs. • Use of photonics for multi-core system-on-chips cannot be yet considered as a practically

viable option as the tools for design synthesis and fabrication of opto-electronic components have not yet been developed completely as we saw earlier that the simulators had some drawbacks of their own and.

• Hence, it can be concluded that the potential power, bandwidth and latency difference between photonics-based NoCs and their electronic counterparts is immense and with the perfection in technology for the simulation and fabrication of photonic components on silicon fabric, the on-chip realm is going to see a revolution.