High Performance Embedded Computing © 2007 Elsevier Lecture 16: Interconnection Networks Embedded Computing Systems Mikko Lipasti, adapted from M. Schulte Based on slides and textbook from Wayne Wolf

© 2006 Elsevier Topics Interconnection Terms and Metrics Interconnection Models Routing and Flow Control Network-on-Chips

© 2006 Elsevier Interconnection networks Client: sender or receiver on a network. Port: connection to a network on a client Link: connection between two clients (full/half duplex) Topology: organization of network links. Network metrics:  Throughput.  Latency.  Energy consumption.  Area (silicon or metal). Quality-of-service (QoS) is important for multimedia applications.

© 2006 Elsevier Interconnection network models Source termination. Throughput T, latency D. Link transmission energy E b. Physical length L. Total link area A. Traffic models often use Poisson distribution  P(X = x) =  x e -   /(x)!x =0, 1, 2, …  E(x) = , Var(x) = . Streaming data is produced periodically with rate  and burstiness 

© 2006 Elsevier Network topologies Major choices for network topologies include.  Bus: common connection between a set of senders and receivers  Crossbar: fully connected network from every input port to every output port.  Buffered crossbar: add queues to a crossbar to enable multiple sources to share crossbar input  Mesh: network in which every node is connected to all of its neighbors.  Application-specific: topology is matched to the characteristics of the application.

© 2006 Elsevier Bus network Throughput:  T 1 = P/(1+C). – single word  T b = P*(n/(n + C)) – n word block Advantages:  Well-understood.  Easy to program.  Many standards. Disadvantages:  Contention.  Significant capacitive load.  Do not scale well.

© 2006 Elsevier Crossbar network Fully connected network Advantages:  No contention.  Simple design  Low latency  Broadcast. Disadvantages:  Expensive  Not feasible for large numbers of ports.

© 2006 Elsevier Buffered crossbar network Add queues shared by multiple sources Advantages:  Smaller than crossbar.  Can achieve high utilization. Disadvantages:  Requires scheduling. Clos networks  Connect multiple crossbars together in stages Xbar

© 2006 Elsevier Mesh network Every node connected to all of its neighbors Advantages:  Well-understood.  Regular architecture. Disadvantages:  Poor utilization  Variable latency.

© 2006 Elsevier Application-specific. network Topology is specific for application(s) Advantages:  Higher utilization.  Lower power. Disadvantages:  Must be designed.  Must carefully allocate data.

© 2006 Elsevier Network topology questions What type of network topology would you chose if you were building a 1,000 node system? Why would you use a buffered crossbar network instead of a regular crossbar network? What advantages and disadvantages does a 2D mesh have compared to a 3D mesh? What types of systems would you expect to use application-specific networks?

© 2006 Elsevier Routing and flow control Routing determines paths followed by packets.  Connection-oriented or connectionless.  Wormhole routing divides packets into flits and header flit determines route for remaining flits  Virtual cut-through ensures entire path is available before starting transmission.  Store-and-forward routing stores packets inside network. Flow control allocates links and buffers as packets move through the network.  Virtual channel flow control treats flits in different virtual channels differently.

© 2006 Elsevier Networks-on-chips Impact characteristics of MPSoC:  Energy.  Performance.  Cost. NoCs may not have to interoperate with other networks.  NoCs have to connect to existing IP, which may influence interoperability. QoS is an important design goal.

© 2006 Elsevier Nostrum Mesh network---switch connects to four nearest neighbors and local “resource” Each switch has queue at each input. Selection logic determines order in which packets are sent to output links. [Kum02] © 2002 IEEE Computer Society

© 2006 Elsevier Scalable, Programmable, Integrated Network (SPIN) Scalable network based on fat-tree.  Bandwidth of links is larger toward root of tree. All routing nodes use the same routing function.  Message goes up the tree until a common ancestor reached

© 2006 Elsevier Routing nodes in SPIN Packet consists of 32-bit words  One word header  Variable word packet  One word checksum trailer Network utilizes input queues and partial crossbars Outputs share buffers  Help with contention

© 2006 Elsevier Ye et al. energy model Assume: energy per packet is independent of data or packet address. Histogram captures distribution of path lengths. Energy consumption of a class of packet:  M = maximum number of hops.  h = number of hops.  N(h) = value of h th histogram bucket.  L = number of flits per packet.  E flit = energy per flit.

Ye et al. energy model © 2006 Elsevier Longer packets corresponds to longer block sizes Larger packets Decrease cache misses but increase the miss penalty Decrease number of packets but increase hops per packet Decrease cache and memory energy, but increase network energy

© 2006 Elsevier Goossens et al. NoC methodology Geared towards Application-specific SoCs QoS-intensive apps Network dimensioning – determine size of network and buffers NoC topology – determine connection between elements NoC configuration – set register values that control flow through the network

© 2006 Elsevier QNoC Designed to support QoS. Two-dimensional mesh, wormhole routing.  Fixed x-y routing algorithm. Four different types of service.  Each service level has its own buffers.  Next-buffer-state table records number of slots for each output in each class.  Transmissions based on next stage, service levels, and round-robin ordering. Can be customized for specific application.

QNoC Design Methodology © 2006 Elsevier

Xpipes and NetChip Xpipes is a library of soft IP macros for network switches and links. NetChip generates custom NoC designs using xpipes components.

© 2006 Elsevier Xu et al. H.264 network design Designed NoC for H.264 decoder. Process -> PE mapping was given. Compared RAW mesh to application-specific networks. [Xu06] © 2006 ACM Press

© 2006 Elsevier Application-specific network for H.264 [Xu06] © 2006 ACM Press

© 2006 Elsevier RAW/application-specific network comparison [Xu06] © 2006 ACM Press