1. A Survey of Architectural Design and Implementation Tradeoffs in Network on Chip Systems
Dan Marconett
Next-Generation Networking Systems Lab
University of California, Davis
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2. Overview Introduction SoC/NoC Architectures Routing Strategies
Energy Dissipation
Conclusion
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3. SoC What is System-on-Chip (SoC)?
Integration of multiple computer components (i.e. microcontroller, memory blocks, timers, etc.) onto a single silicon chip
Each on chip component referred to as a block
Block abstraction enables component-level design of SoC containing multiple proprietary elements
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4. NoC What is Network-on-Chip (NoC)?
Leveraging existing computer networking principles to improve inter-component intra-chip communications for SoC
Each on chip component connected by switch to a particular comm wire(s)
Improvement over standard bus based interconnections for SoC architectures in terms of throughput
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5. Overview Introduction SoC/NoC Architectures Routing Strategies
Energy Dissipation
Conclusion
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6. Architectures: CLICHE
CLICHÉ: Chip-Level Integration of Communicating Heterogeneous Elements
Two-dimensional mesh network layout for NoC design
All switches are connected to the four closest other switches and target resource block, except those switches on the edge of the layout
Connections are two unidirectional links
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7. Architectures: Folded Torus
Similar to mesh based architectures
Wires are wrapped around from the top component to the bottom and rightmost to leftmost
Smaller hop count
Higher bandwidth
Decreased Contention
Increased chip space usage
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8. Architectures: BFT BFT: Butterfly Fat Tree
Each node in tree model has coordinates (level, position) where level is depth and position is from left to right
Leaves are component blocks
Interior nodes are switches
Four child ports per switch and two parent ports
LogN levels, ith level has n/(2^i+1) switches, n = leaves (blocks)
Use traffic aggregation to reduce congestion
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9. Architectures: SPIN SPIN: Scalable, Programmable, Integrated Network
Leverages the Butterfly Fat Tree design
Now every level has same number switches
Network grows like (NlogN)/8
Trades area overhead and decreased power efficiency for higher throughput
Illustrative of performance vs. power consumption
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10. Architectures: Octagon
Standard model: 8 components, 12 interconnects
Design complexity increases linearly with number of nodes
Largest packet travel distance is two hops
High throughput
Shortest path routing easy to implement
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11. Overview Introduction SoC/NoC Architectures Routing Strategies
Energy Dissipation
Conclusion
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12. Routing: Circuit/Packet Switching
Circuit Switching
Dedicated path, or circuit, is established over which data packets will travel
Naturally lends itself to time-sensitive guaranteed service due to resource allocation
Reservation of bandwidth decreases overall throughput and increases average delays
Packet Switching
Intermediate routers are now responsible for the routing of individual packets through the network, rather than following a single path
Provides for so-called best-effort services
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13. Routing: Wormhole/Virtual Cut Through
Wormhole Switching
Message is divided up into smaller, fixed length flow units called flits
Only first flit contains routing information, subsequent flits follow
Buffer size is significantly reduced due to the limitation on the number of flits needed to be buffered at any given time
Virtual Cut Through Switching
Much like Wormhole switching
Header flit can travel ahead and undergo processing while remaining flits are still navigating the network
Higher acceptance rates and lower latencies than Wormhole
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14. Routing: Contention
Contention occurs when routers or IP blocks attempt to send data over the same link at the same time
For Circuit Switching, contention is resolved at the time of actual connection setup
For packet switching, contention resolution is handled at a much finer level, by the router buffering and scheduling individual packets of information
Better overall performance for packet switched networks at the cost of lack of service guarantee
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15. Overview Introduction SoC/NoC Architectures Routing Strategies
Energy Dissipation
Conclusion
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16. Energy Dissipation: Architectures
Two causes for dissipation, switches and wire segments
Many parameters in the architectural design phase which affect the key trade-off of performance vs. power dissipation
Length of physical wires
Switching techniques
Buffer allocation
Types of guaranteed service
The topology itself
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17. Energy Dissipation: Architectures (2)
Pande et al. [10] used a simulator to investigate various metrics, including energy dissipation, with respect to the five main architectures
Average dynamic energy dissipated per event, each layout containing 256 functional blocks
Energy dissipation increases linearly with the increase of virtual channels for all five architectures
Small number (4) of virtual channels will keep energy dissipation low without giving up throughput
When the traffic load was analyzed, it was found that the energy dissipation reached an upper limit when throughput was maximized
Architectures with more elaborate topologies, and therefore higher degrees of connectivity (such as SPIN and Octagon) have a higher much greater energy dissipation on average (~60 nj vs nj)
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18. Energy Dissipation: Switching
How to route information from block A to block B in such a way that the constraints on energy consumption are maintained
Banerjee et al. [9] address this issue through a modeling approach based on a 4x4 mesh layout
Virtual-cut Through Switching versus Wormhole Switching
For both routing techniques, energy dissipation rises linearly with the injection rate of data packets until the network is fully congested, after which it is constant
Both techniques yield same power consumption
Virtual-Cut Through switching produces higher acceptance rates and lower latencies than Wormhole Switching, therefore VCT is preferred
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19. Overview Introduction SoC/NoC Architectures Routing Strategies
Energy Dissipation
Conclusion
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20. Conclusion
More elaborate layouts with higher degrees of connectivity (SPIN and Octagon) were seen to have much higher rates of energy dissipation, however, they also yield increased throughput
Elaborate architectures also take up more space on the silicon chip
VCT is preferred to Wormhole due to decreased latency, though both have same energy dissipation for given traffic loads
Decide on priorities; communication reliability, energy efficiency, increased throughput, decreased latency….?
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21. References
[1] E. Rijpkema, K. Goossens, A. Radulescu, J. Dielssen, J. van Meerbergen, P. Wielage, and E. Waterlander, “Trade- offs in the Design of a Router with Both Guaranteed and Best-Effort Services for Networks on Chip,” IEE Proceedings Computers and Digital Techniques, vol. 150, no. 5, pp , Sept
[2] W. Dally, C. Seitz, “Deadlock-free Message Routing in Multiprocessor Interconnection Networks,” IEEE Transactions on Computers, vol. C-34, no. 10, pp , May 1987.
[3] S. Kumar, A. Jantsch, J. Soininen, M. Forsell, M. Millberg, J. Oberg, K. Tiensyrja, and A. Hemani, “A Network on Chip Architecture and Design Methodology,” Proceedings International Symposium VLSI (ISVLSI), pp ,
[4] W. J. Dally and B. Towles, “Route Packets, Not Wires: On-Chip Interconnection Networks,” Proceedings Design and Automation Conference (DAC), pp , 2001.
[5] P. P. Pande, C. Grecu, A. Ivanov, and R. Saleh, “Design of a Switch for Network on Chip Applications,” Proceedings International Symposium on Circuits and Systems (ISCAS), vol. 5, pp , May 2003.
[6] P. Guerrier and A. Greiner, “A Generic Architecture for On-Chip Packet-Switched Interconnections,” Proceedings Design and Test in Europe (DATE), pp , Mar
[7] F. Karim, A. Nguyen, and Sujit Dey, “An Interconnect Architecture For Networking Systems on Chips,” IEEE Micro, vol. 22, no. 5, pp , Sept./Oct
[8] Ateris, “A comparison of Network-on-Chip and Buses,”
[9] Nilanjan Banerjee, Praveen Vellanki, Karam S. Chatha, "A Power and Performance Model for Network-on-Chip Architectures," Proceedings of the Design, Automation, and Test in Europe Conference and Exhibition (DATE) , p , 2004.
[10] Partha Pratim Pande, Cristian Grecu, Michael Jones, Andre Ivanov, Resve Saleh, "Performance Evaluation and Design Trade-Offs for Network-on-Chip Interconnect Architectures," IEEE Transactions on Computers ,vol. 54, no. 8,  pp , August, 2005.
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