Emerging Technologies of Computation

Montek Singh

COMP790-084Nov 10, 2011

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

Today: More on Network-on-Chip

Analogous to the network adapters for Wifi/Ethernet

Zooming in onto the n/w adapter or router

Generic router

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)

Adapter level

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◦ …

Adapter 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.

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.

Network level

Topologies: Regular

Topologies: Irregular

thin (spread) layout vs. concentrated

VLSI layout aspects

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)

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

Protocol questions

Central vs. distributed control◦ central: global routing decisions (e.g., bus

control)◦ distributed: decisions made locally

Protocol questions

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

Protocol: Flow Control

E.g.: VCs avoid stalls

Synchronization◦ different clock domains◦ maybe network is asynchronous

Challenges:◦ metastability◦ arbitration◦ synchronization

Link level questions

Globally Asynchronous Locally Synchronous◦ network is elastic, asynchronous◦ cores may be clocked

Link level: GALS

Why buffering?

How much buffering?

Link level: Buffering

Other than electrical? How about optical?

Link level: Physical mechanism

Other than electrical? How about optical?

Link level: Physical mechanism

Delay-insensitive encoding?◦ very robust

Energy-efficient encoding?◦ save energy

Handshaking?◦ two-phase vs. four-phase

Link level: data encoding