NETWORK-ON-CHIP (NOC): A New SoC Paradigm

1. NETWORK-ON-CHIP (NOC):A New SoC Paradigm Dr. Konstantinos Tatas

2. PRESENTATION OUTLINE • Introduction • Part A • Motivation – SoC Communication • Current Solutions • NoC Concept • Part B • Work@MicroLab • Summary

3. THE MANY CORES ERA Source: International Roadmap for Semiconductors 2007 edition (http://www.itrs.net/)

4. THE GROWING GAP: COMPUTATION VS. COMMUNICATION 2:1 9:1 Taken From ITRS, 2001

5. GROWING CHIP DENSITY 1998 ASIC - 0.35 mm 2012 SoC - 22nm Memory, I/O Future? P • Design complexity - high IP reuse • Efficient high performance interconnect • Scalability of communication architecture

6. Traditional SoC Nightmare System Bus DMA CPU DSP Mem Ctrl. Bridge The “Board-on-a-Chip” Approach The architecture is tightly coupled MPEG I o o C Control Wires Peripheral Bus • Variety of dedicated interfaces • Poor separation between computation and communication. • Design Complexity • Unpredictable performance

7. Computational demands of future multimedia applications - Memory bandwidth scalesproportional K. Uchiyama., “Power-Efficient Heterogeneous Parallelism for Digital Convergence”, VLSI Circuit Digest of Technical Papers, IEEE p 6-9, June 2008 Jian Li, “3D Integration opportunities and challenges”, ISCAS 2008 tutorial on 3D

8. Shared address space communications

9. System bus

10. Cross-bar

11. Multi-stages network on chip

12. An NoC example Source: ossum, Intel @ MPSoC’07

13. NOC Topologies • Regular topologies: general-purposed on-chip multiprocessors • Custom topologies:

14. NoC vs. “Off-Chip” Networks What is Different? Routers on Planar Grid Topology Short Point-To-Point Links between routers Unique VLSI Cost Sensitivity: • Area-Routers and Links • Power

15. NoC vs. “Off-Chip” Networks • No legacy protocols to be compliant with … • No software  simple and hardware efficient protocols • Different operating env. (no dynamic changes and failures)

16. NoC vs. “Off-Chip” Networks Custom Network Design – You design what you need! • No legacy protocols to be compliant with … • No software  simple and hardware efficient protocols • Different operating env. (no dynamic changes and failures)

17. Example1: Replace modules Replace NoC vs. “Off-Chip” Networks Custom Network Design – You design what you need! • No legacy protocols to be compliant with … • No software  simple and hardware efficient protocols • Different operating env. (no dynamic changes and failures)

18. Adapt Links NoC vs. “Off-Chip” Networks Custom Network Design – You design what you need! Example2: Adapt Links • No legacy protocols to be compliant with … • No software  simple and hardware efficient protocols • Different operating env. (no dynamic changes and failures)

19. NoC Cost Scalability vs. Alternatives • Compare the cost of: • NoC • Non-Segmented Bus (NS-Bus) • Segmented Bus (S-Bus) • Point-To-Point (PTP)

20. Why noc?

21. Which are the main challenges? • Communication infrastructure • Communication paradigm selection • Application mapping optimization • Programming model • Physical design • Design automation/tool-flow integration

22. Basic Switching Techniques • Circuit Switching A real or virtual circuit establishes a direct connection between source and destination. • Packet Switching Each packet of a message is routed independently. The destination address has to be provided with each packet. • Store and Forward Packet Switching The entire packet is stored and then forwarded at each switch. • Cut Through Packet Switching The flits of a packet are pipelined through the network. The packet is not completely buffered in each switch. • Virtual Cut Through Packet Switching The entire packet is stored in a switch only when the header flit is blocked due to congestion. • Wormhole Switching is cut through switching and all flits are blocked on the spot when the header flit is blocked.

23. Circuit Switching (are they noc?) • Phases: • Circuit Setup • Transmission • Tear Down • Disadvantages: • Exclusive allocation of resources • Long setup phase • Advantages: • High performance - throughput and latency • Low power consumption • Low overhead during transmission phase • Predictable transmission

24. Packet Switching vs Circuit Switching

25. NoC Router

26. NoC-based MPSoC • nodes • Processing Elements (PEs), such as CPUs, custom IPs, DSPs, etc. • storage elements (embedded memory blocks), • Routers • Links • Network Interfaces (NIs) • Often a switch together with its host node memory is referred to as a tile.

27. NoC Topologies • Regular/irregular • Direct/indirect • each node has a direct point-to-point link to a subset of other nodes in the system, called neighboring nodes

28. 2D Mesh • simplest and most popular topology for NoCs. • Every switch, except those at the edges, is connected to four neighboring switches and one node.

29. 2D Torus • layout of a regular mesh except that nodes at the edges are connected to switches at the opposite edge via wrap-around routing channels. • Every switch has five ports • The limitation of this topology affects the long end-around connections

30. Octagon • well-established direct topology found in NoCs. • ring of 8 nodes connected by 12 bi-directional links. • links provide two-hop communication between any pair of nodes in the ring • simple algorithms for fast yet efficient shortest-path routing. • In case a platform consists of more than eight nodes, the octagon is extended to multidimensional space

31. Fat-tree and butterfly fat-tree • nodes are connected to an architecture's external switch • switches have point-to-point links to other switches. • processing units and memory modules are assigned to the leafs of the trees, • switches are placed at the vertices, • communication involves climbing up and down some part of the tree. • A pair of coordinates is used to label each node, ($l$, $p$), where $l$ denotes a node's level and $p$ gives its position within this level.

32. Polygon • widely accepted topology • packets travel in a loop from one router to the next. • We can add chords to the circle • if chords are inserted only between opposite routers, the topology is called a spidergon.

33. Star • central router in the middle of the star, • computational resources, or subnetworks, in the spikes of the star. • The capacity requirements of the central router are quite large, • significant possibility of congestion in the middle of the star

34. Flow Control • intra-switch • switch-to-switch • Buffered • Bufferless • end-to-end

35. ACK/NACK • handshaking protocol • When a sender puts data on the link, it activates a VALID signal. • When the receiver is ready to consume the valid data, it activates the corresponding ACK signal. • If the data is corrupt or there is no buffer space to store them, a NACK signal is activated instead. • Upon receipt of a NACK, the sender starts resending flits starting from the not acknowledged one • inherently supports fault tolerance, • additional buffer space required to keep sent flits in case retransmission is required.

36. Stall/go • requires just two control wires • one going forward, signifying data availability, • one going backward and signaling either a condition of buffers filled ("STALL") or of buffers free ("GO")

37. Credit-based • transmitter has a "credit" counter • initialized to the value of empty buffer slots of the receiver • decrements it every time a flit is sent. • The credit counter must be updated in case the receiver consumes or forwards a flit and therefore increases its buffer space. • a credit value that is sent back to the transmitter to be added to the current value of the credit counter. • transmitter stalls when the credit value is zero and • resumes when its value increases again.

38. NI Design • logic required to connect the nodes to the NoC. • NIs can differ significantly depending on the nature of the node • Using a NI allows IPs and communication infrastructure to be designed independently • One end of a NI is connected to a router using the selected flow control protocol • the other to the node IP • Since most IPs are designed to communicate through a bus, the NI uses a bus interface • NI is not simply a protocol adapter from a processor bus to a router port. • Ideally, the NI must offer the processing cores the view of a shared memory system, and the network itself should be transparent.

39. NI services • adaptation services • packetization/depacketization • protocol conversion and clock domain crossing. • absolute minimum services required of the NI so that data can be sent and received on the NoC • transaction reordering services, • error and flow control services • error detection and/or correction • request retransmission when required • route computation services • Source routing • upper layer services • Cache coherence

40. Typical NoC Packet Format • Header • routing and network control information. • In the case of distributed routing the information required is the destination and source addresses • in the case of source routing the complete routing information is written • In the case of variable packet size a length field is required • Payload • Tail • sequence number • error control fields such as hamming code or CRC fields

41. Source vs Distributed Routing • In source routing the entire routing path is computed at the source and appended to the packet. • The routers do not make any routing decisions, • in distributed routing, the routing path is decided in a hop-by-hop basis at each router even for deterministic routing algorithms. • The only information required to be found in the packet is the destination address. • The advantage of source routing is that it requires simple routers and can easily support irregular architectures. Its disadvantage is that it does not provide adaptiveness and requires more complex NIs and packets.

42. Source vs Distributed Routing