Network-on-Chip
Network Adapter and Network Issues
System-on-Chip Group, CSE-IMM, DTU
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NoC Overview Slide
Network AdapterRouting Node
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Overview
•Network Adapter Session / Transport Layer Plug and play interface Traffic encapsulation
•Network Topology Protocol
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Network Adapter
• Functions Encapsulation Service Management Interface multiple IPs to a single NoC
port
• Sockets OCP VCI
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Encapsulation
• Convert messages into packets Header to payload ratio Header is overhead
Routing information Control information (such as services, flit
number, etc) Possible error-correction
• Broadcast, narrowcast services
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Service Management
• Definition: Acquire, retain, use and relinquishany service in a predictable way!!
• Types of Services: BE: not guarantee, only correctness and
completion of transmission is guaranteed GS: provides bounded guarantees
Latency Bandwidth Power etc…
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Sockets
• Point-to-point connection (abstraction)• Abstract away the network details for
the IP cores• Examples: OCP, VCI, etc• Limitations (a bus-based view):
Broadcast, narrowcast, services are not supported
Easy means for GS service request, retention and teardown not supported
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Network
• Two characteristics Topology Protocol
• Flow-Control• Quality-of-Service
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Topology
• Form: relates to geometry Scalable with area and power Easy to lay out in 2D chip plane
• Nature of link: relates to unidirectional or bidirectional links
• Presence of IP core
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Forms of Topology
• preferred for easy of layout• better utilization of available bandwidth
K-ary 2-cube K-ary tree
• better hardware utilization for same bandwidth• good to exploit locality of traffic
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Nature of Topological Link
• Common variations: torus and mesh
Uni-directional Bi-directional
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IP Core of Topology
Direct network
Indirect network
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Irregular Topologies
• Hybrid, asymmetric and hierarchical
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Router Architecture
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Buffering Schemes
• Input Head-of-line blocking
• Output Expensive in terms of hardware
• Virtual-output Moderately buffer cost at very high
improvement in performance
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Crossbar and Arbitration Unit
• Crossbar connects input port to output port
• Arbitration is used to prioritize, setup and manage crossbar connections Possibly programmable for best-effort
and guaranteed service connections
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Protocol
• Many dimensions Circuit vs packet switched Connectionless or connection-
oriented Adaptive or deterministic Minimal or non-minimal Delay or loss Centralized or decentralized control
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Common Routing Mechanisms
• Store-and-forward• Virtual-cut through• Wormhole
Protocol
RouterStallingLatency Storage
Store-and-forward
Packet Packet At two nodes and link between them
Virtual-cut through
Header Header At all nodes and links spanned by the packet
Wormhole Header Packet At the local node
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Flow-Control
• Network-level: NA-to-NA In-order delivery Packet Acknowledgment Credit based injection schemes
• Link-Level: Node-to-Node Congestion look-ahead or stalling Virtual channel selection
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Quality-of-Service
• End-to-end Reserving virtual circuits from source to
destination One-way, round-trip or just reverse-way Connection management overhead!!
• Node-to-Node Logically independent resource allocation
(avoid contention) Division of link bandwidth!!
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Conclusion
• Each NoC level offers many parameters such as topology, packet size, buffereing, that to optimize the implementation
• Sockets are enable plug’n’play of IP cores, thus flexibility in placement anywhere within the network geometry
• Topology is influenced by placement of IP cores• Many protocol choices available, with wormhole
costing the least in terms of buffering• Buffers are most area consuming component
within the routers
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References
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• TAMIR, Y. and FRAZIER, G. L. 1988. High-performance multiqueue buffers for VLSI communication switches. In Proceedings of the 15th Annual International Symposium on Computer Architecture. IEEE Computer Society.