An introduction to Network-On-chips Sayed Amirhossein Mirhosseini - PowerPoint PPT Presentation

An introduction to Network-On-chips Sayed Amirhossein Mirhosseini

Fully Crossbar Interconnection  Commercial Example  Niagara multiprocessor SoC from SUN

BUS  Buses are the simplest and most widely used SoC interconnection networks  Definition: A collection of signals (wires) to which one or more IP components (which need to communicate data with each other) are connected  Only one IP component can transfer data on the shared bus at any given time

Shared Bus  Commercial Example  DaVinci family of digital video processing SoCs from Texas Instruments

Hierarchical Shared Bus  Improves system throughput  Multiple ongoing transfers on different buses  Commercial example:  Customizable multiprocessor ARM PrimeXsys SoCs

Towards NOC  Buses are not scalable

Network Topologies Torus Hypercube Binary Tree

Topology Selection Factors  Diameter :  Maximum shortest path distance between two nodes in the network. Networks with small diameters are preferable.  Number of Links :  A topology with large number of links can support high bandwidth.  Average Distance :  Average among the distances between all pairs of nodes of a graph. A topology having lesser average distance is preferable.

Topology Selection Factors (cont.)  Node Degree :  Numbers of channels connecting the node to its neighbors. The lower this number, the easier to build the network.  Bisection Width :  Minimum number of wires removed in order to bisect a network. A larger bisection width enables faster information exchange, and preferable.

NOC Elements  NoCs use packets to route data from the source to the destination PE via a network fabric that consists of :  switches (routers)  interconnection links (wires)

Router Architecture

Switching Strategies  Determine how data flows through routers in the network  Flit :  an elementary packet on which link flow control operations are performed  Phit :  physical unit of data that is transferred on a link in a single cycle

Circuit Switching  Physical path between the source and the destination is reserved prior to the transmission of data  Message’s header flit traverses the network from the source to the destination, reserving links along the way  When the probe reaches the destination, a complete path has been set up and an acknowledgment is transmitted back to the source.  Circuit switching is generally advantageous when messages are infrequent and long

Packet switching  packets are transmitted from source and make their way independently to receiver  zero start up time, followed by a variable delay due to contention in routers along packet path

virtual cut-through switching  The router can start forwarding the header and following data bytes as soon as routing decisions have been made and the output buffer is free.

Wormhole Switching  flit from a packet is forwarded to receiving router if space exists for that flit  parts of the packet can be distributed among two or more routers  more susceptible to deadlocks due to usage dependencies between links

Deadlock  One or more packets in a network become blocked, and remain blocked for an indefinite amount of time waiting for an event that cannot occur.  Example :

Deadlock (cont.)  Deadlock Avoidance : Techniques that request resources in such a way that no deadlock can arise.  Deadlock Recovery : These techniques require a mechanism to detect and resolve potential deadlock situations.

A Deadlock Free Router

Livelock  Livelocks are similar to deadlocks, except that states of the resources involved constantly change with regard to one another, without making any progress  occurs especially when dynamic (adaptive) routing is used  e.g. can occur in a deflective routing if a packet is bounced around over and over again between routers and never reaches its destination

Routing Algorithm  Routing algorithms establish the path followed by each message or packet .  Many properties of the interconnection network are a direct consequence of the routing algorithm used :  Connectivity :  Ability to route packets from any source node to any destination node.  Adaptivity :  Ability to route packets through alternative paths in the presence of contention or faulty components.  Deadlock and livelock freedom :  Ability to guarantee that packets will not block or wander across the network forever.

Routing Algorithms Taxonomy

Deterministic Routing  fixed paths are used to transfer data between a particular source and destination  Does not take into account current state of the network  Advantages :  easy to implement  in-order packet delivery if single path is used  Manageable to be deadlock and livelock free

Adaptive Routing  routing decisions are made according to the current state of the network  Considering factors such as availability and load on links  Path between source and destination may change over time  More resources needed to monitor state of the network and dynamically change routing paths  Able to better distribute traffic in a network

Example : Odd-Even Routing (cont.)  The odd-even turn model is governed by the following two rules: Any packet is not allowed to take an EN turn at 1. any nodes located in an even column, and it is not allowed to take an NW turn at any nodes located in an odd column. 2. Any packet is not allowed to take an ES turn at any nodes located in an even column, and it is not allowed to take an SW turn at any nodes located in an odd column.

Example : Odd-Even Routing (cont.)  Theorem : Any routing algorithms that follow the rules of the odd-even turn model are deadlock free as long as 180-degree turns are prohibited.  Proof :  By Contradiction : NOT Possible !

Example : Odd-Even Routing (cont.)

Virtual channel  Buffers at physical channels are commonly operated as FIFO queues. Therefore, once a message occupies a buffer for a channel, no other message can access the physical channel.  Each virtual channel is realized by an independently managed pair of message buffers.

A Virtual Channeled Router

Many ways to gain bonus marks   Example : Pipelined Router

References  Principles and Practices of Interconnection Networks by William James Dally and Brian Pattrick Towles [2004]  Interconnection Networks: An Engineering Approach by Jose Duato, Sudhakar Yalamanchili and Lionel M. Ni [1997]  Computer Architecture, Fifth Edition: A Quantitative Approach by John L.Hennessy and David A. Patterson [2011]  Chiu, Ge-Ming. "The odd-even turn model for adaptive routing." Parallel and Distributed Systems, IEEE Transactions on 11.7 (2000): 729-738.