firefly: illuminating future network-on-chip with nanophotonics

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Firefly: Illuminating Future Network-on-Chip with Nanophotonics. Yan Pan, Prabhat Kumar, John Kim † , Gokhan Memik , Yu Zhang, Alok Choudhary. EECS Department Northwestern University Evanston, IL, USA {panyan,prabhat-kumar,g-memik, yu-zhang,a-choudhary} @northwestern.edu. - PowerPoint PPT Presentation

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  • Firefly: Illuminating Future Network-on-Chip with NanophotonicsYan Pan, Prabhat Kumar, John Kim, Gokhan Memik, Yu Zhang, Alok ChoudharyEECS Department Northwestern UniversityEvanston, IL, USA{panyan,prabhat-kumar,g-memik, yu-zhang,a-choudhary} @northwestern.edu CS Department KAISTDaejeon, Koreajjk12@cs.kaist.ac.kr

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    On-Chip Network TopologiesMesh [MIT RAW] [TILE64] [Teraflops] C-Mesh [Balfour06] [Cianchetti09]Crossbar [Vantrease08] [Kirman06]Others: Torus[Shacham07], Flattened Butterfly[Kim07], Dragonfly[Kim08], Hierarchical(Bus&Mesh)[Das08], Clos[Joshi09], Ring[Larrabee], Network-on-chip is critical for performance.

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Signaling technologiesElectrical signalingRepeater insertion neededBandwidth density (up to 8 Gbps/um) [Chang HPCA08]NanophotonicsBandwidth density ~100 Gbps/ m !!! [Batten HOTI08]Generally distance independent power consumptionSpeed of light low latencyPropagationSwitching [Cianchetti ISCA09]

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Nanophotonic componentsBasic componentsoff-chiplaser sourcecouplerresonant modulators resonant detectorsGe-dopedwaveguide

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Resonant RingsSelectiveCouple optical energy of a specific wavelength

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Putting it togetherModulation & detection ~100 Gbps/m bandwidth density [Batten HOTI08]

    1101010111010101100010111000101164 wavelengths DWDM3 ~ 5m waveguide pitch10Gbps per link

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Whats the catch?Power CostRing heatingLaser PowerE/O & O/E conversionsDistance insensitiveFor short links (2.5mm)NanophotonicsElectricalRC lines with repeater insertionFor long linksNanophotonicsCost stays the sameElectricalCost increases

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Here is the idea Design an architecture that differentiates traffic.Use electrical signaling for short links.Use nanophotonics only for long range traffic.What do we gain?Low latencyHigh bandwidth densityHigh power efficiencyLocalized arbitrationScalability

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    OutlineMotivationArchitecture of FireflyEvaluationConclusion

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Layout View of 64-core FireflyConcentration4 cores share a router16 routers

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Layout View of 64-core FireflyConcentrationClustersElectrically connectedMesh topology4 routers per cluster4 clustersCluster 0 (C0)

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Layout View of 64-core FireflyConcentrationClustersAssembliesRouters from different clustersOptically connectedLogical crossbars

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Layout View of 64-core FireflyClustersElectrical CMESHAssembliesNanophotonic crossbarsEfficient nanophotonic crossbars needed!

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Nanophotonic crossbarsSingle-Write-Multiple-Read (SWMR) [Kirman06] (CMXbar)Dedicated sending channelMulticast in natureReceiver compare & discard High fan-out laser power

    SWMR Crossbar [Joshi NOCS09]

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Nanophotonic crossbarsMultiple-Write-Single-Read (MWSR)[Vantrease08] (DMXbar)Dedicated receiving channelDemux to channelGlobal arbitration needed!

    MWSR Crossbar [Joshi NOCS09]

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Reservation-assisted SWMRGoalAvoid global arbitrationReduce powerProposed designReservation channelsNarrowMulticast to reserveDestination IDPacket lengthUni-cast data packetR-SWMR Crossbar

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Router MicroarchitectureVirtual-channel routerAdded optical link ports and extra buffer.

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    RoutingIntra-cluster routingTraversing optical link

    Routing(FIREFLY_dest)

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Firefly another lookClustersShort electrical linksConcentrated meshAssembliesLong nanophotonic linksPartitioned crossbarsBenefitsTraffic localityReduced hardwareLocalized arbitrationDistributed inter-cluster bandwidth

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    OutlineMotivationArchitecture of FireflyEvaluationConclusion

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Evaluation SetupCycle-accurate simulator (Booksim)Firefly vs. CMESH, Dragonfly and OP_XBARSynthetic traffic patterns and tracesElectricalHybridOpticalHybrid[ Kim et al, ISCA08]

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Load / Latency CurveThroughputUp to 4.8x over OP_XBARAt least +70% over Dragonfly

    Bitcomp, 1-cycleUniform, 1-cycle

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Energy BreakdownReduced hardware by partitioningReduced heatingThroughput impactLocality34% energy reduction over OP_XBAR with locality

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    Technology Sensitivity is heating ratio and is laser ratio.Firefly favors traffic locality.bitcomptaper_L0.7D7

    Motivation Architecture of Firefly Evaluation ConclusionISCA 2009Yan Pan*/25

    ConclusionTechnology impacts architectureNew opportunities in nanophotonicsLow latency, high bandwidth densityTailored architectures neededFirefly benefits from nanophotonics by providingPower Efficiency Hybrid signalingPartitioned R-SWMR crossbars Reduced hardware/powerScalabilityScalable inter-cluster bandwidthLow-radix routers/crossbars

    I am Yan Pan from Northwestern University.Glad to share with you our work: Firefly.This work was a collaboration with P,J,G,Y,A, carried out at Northwestern Univ.Prof. John Kim recently joined Korea Advanced Institute of Science and Technology.*With hundreds or even thousands of cores, how do we connect them?Traditional buses are probably not going to be efficient on a global scale.Many on-chip network topologies have been proposed. In addition to topology, recent development in silicon nanophotonics provides us with a new signaling technology.Compared to conventional electrical signaling or radio frequency, whats so attractive about nanophotonics?For one thing, bandwidth dendsityOther benefits include low latency, etc. This is especially true if optical switching can be achieved, as will be discussed in the Phastlane talk coming up next by Cianchetti et al. *Compare bandwidth density/ latency/ etc for Low Swing, RF, Optics*Components: Laser Source, coupler, waveguide, modulators and detectors. A common structure, ring resonator, is used to selectively couple laser energy of a specific wavelength.The resonant wavelength decided by, among other things, r, t and d.The detectors can be doped with Ge for optical energy absorbtion and O/E conversion. *Good news is, we know the resonant wavelength is sensitive to temperature.

    *However, nanophotonics is not all for free. There are various power components that are distance independent. To have an idea of the tradeoff here, lets look at a die with projected 256 cores connected in concentrated mesh.Neighboring links are 2.5 mm. For such links, conventional buffered RC lines will cost 100 fJ/bit, while the stacked power for nanophotonics will be around 400fJ/b.Intuition tells us, we probably do not want to use optical signaling for short links.***Take this 64 core chip multiprocessor for example. At the lowest level, we have a concentration of 4. *The shared routers are then connected in a mesh topology forming the CMESH clusters. Electrical signaling is used here, because the links are typically short and electrical signaling is more efficient.*On top of that, we connect the counter parts from each clusters using a crossbar topology. Here the inter-cluster links are long and nanophotonic signaling will give us low latency as well as better power efficiency.****Firefly is hierarchical. In short, we have clusters formed by electrically connected local concentrated mesh. And on top of that, we have assemblies connected by nanophotonic crossbars.Lets take a closer look..****

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