Emerging Technologies of Computation

1. Emerging Technologies of Computation Montek Singh COMP790-084 Nov 10, 2011

2. Today: More on Network-on-Chip • Design questions at various leves • Network Adapter design • Network level: topology and routing • Link level: synchronization and timing • Discussion • benefits and challenges • key research problems

3. Zooming in onto the n/w adapter or router • Analogous to the network adapters for Wifi/Ethernet

4. Generic router

5. Adapterlevel • Sockets abstraction • orthogonalize computation and communication • hide networking details • the core-side interface provides send/receive commands • Standardization • Open Core Protocols (OCP) • Virtual Component Interface (VCI) • Advanced eXtensible Interface (AXI) • Device Transaction Level (DTL)

6. Adapterlevel • Adapter responsibilities • encapsulation of traffic • for the underlying communication • management of services • that the network provides • Implementations: several • Muttersbach et al. • Bjerregaard et al. • Radulescu et al. • HERMES • Bhojwani/Mahapatra • …

7. Network level • Network responsibility • deliver messages from source to destination • hardware support for basic communication commands (send/receive) • well-built network should appear as a logical wire [Dally/Towles 2001] • Two main considerations • topology • layout and connectivity • protocol • how nodes and links are used, routing etc.

8. Network level • Network responsibility • deliver messages from source to destination • hardware support for basic communication commands (send/receive) • well-built network should appear as a logical wire [Dally/Towles 2001] • Two main considerations • topology • layout and connectivity • protocol • how nodes and links are used, routing etc.

9. Topologies: Regular

10. Topologies: Irregular

11. VLSI layout aspects • thin (spread) layout vs. concentrated

12. Protocol questions • Circuit vs. packet switching • circuit: the entire path is set up and reserved for the entire duration of data transport • packet: each packet is forwarded on a per-hop basis • Connection-oriented vs. connectionless • connection: dedicated logical path established prior to data transport • may or may not be circuit-switched (logically)

13. Protocol questions • Deterministic vs. adaptive routing • deterministic: path is determined by source and destination pair, alone • adaptive: dynamically determined, incl. arbitration, congestion, load balancing, etc. • Minimal vs. nonminimal routing • always shortest path or not? • Delay vs. loss model • delay model: data packets never dropped, but may be delayed • loss model: data packets may be dropped due to congestion, requiring retransmission

14. Protocol questions • Central vs. distributed control • central: global routing decisions (e.g., bus control) • distributed: decisions made locally

15. Protocol: Flow Control • Flow control: control the flow of data with some objective (delay, loss, etc.) • Virtual channels: 2 to 16 VCs per physical channel! • avoid deadlocks since they are mutually independent • optimize wire utilization by letting several VCs share a wire • improve performance because of fewer stalls • provide differentiated services for QoS

16. E.g.: VCs avoid stalls

17. Link level questions • Synchronization • different clock domains • maybe network is asynchronous • Challenges: • metastability • arbitration • synchronization

18. Link level: GALS • Globally Asynchronous Locally Synchronous • network is elastic, asynchronous • cores may be clocked

19. Link level: Buffering • Why buffering? • How much buffering?

20. Link level: Physical mechanism • Other than electrical? • How about optical?

21. Link level: Physical mechanism • Other than electrical? • How about optical?

22. Link level: data encoding • Delay-insensitive encoding? • very robust • Energy-efficient encoding? • save energy • Handshaking? • two-phase vs. four-phase