Networks-on-Chip

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Networks-on-Chip. Ben Abdallah Abderazek The University of Aizu, Graduate school of Computer Science and Eng, Adaptive Systems Laboratory, E-mail: [email protected] 03/01/2010.

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### Networks-on-Chip

Ben Abdallah Abderazek

The University of Aizu,

Graduate school of Computer Science and Eng,

E-mail: [email protected]

Hong Kong University of Science and Technology, March 2010

03/01/2010

Part IINoC topologies NoC Switching strategiesRouting algorithmsFlow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

NoC Routing Algorithms

Responsible for correctly and efficiently routing packets or circuits from source to destination

• They must prevent deadlock, livelock, and starvation
• Choice of a routing algorithm depends on:
• Minimizing power required for routing
• Minimizing logic and routing tables

Hong Kong University of Science and Technology, March 2010

Routing Algorithm Classifications

Three different criteria:

• Where the routing decision are taken
• Source routing
• Distributed routing
• How a path is defined
• Static (deterministic)
• The path length
• Minimal
• Nonminimal
• Routing schemes:
• Static,
• Dynamic,
• Distributed,
• Source routing,
• Minimal, and non-minimal routing

Adaptive Systems Laboratory, Univ. of Aizu

NoC Routing-Table
• The Routing Table determines for each PE the route via which it will send packets to other PEs.
• The routing table directly influences traffic in the NoC.
• Here we can also distinguish between 2 methods:
• Static routing
• Dynamic (adaptive) routing

Adaptive Systems Laboratory, Univ. of Aizu

Static Routing

The Routing Table is constant.

The route is embedded in the packet header and the routers simply forward the packet to the direction indicated by the header

The routers are passive in their addressing of packets (simple routers)

Hong Kong University of Science and Technology, March 2010

Dynamic Routing
• The routing table can change dynamically during operation
• Logically, a route is changed when it becomes slow due to other traffic
• Possibly out-of-order arrival of packets.
• Usually requires more virtual channels.
• In this method we can identify 2 systems:
• Routing altering decisions are made in the routers (smart routers)
• Routing altering decisions are made in a dedicated central unit that receives traffic information from all the routers and can decide to change the routing table.

Adaptive Systems Laboratory, Univ. of Aizu

Dynamic Routing

Packet

Packet

X

More resources needed to monitor state of the network

Hong Kong University of Science and Technology, March 2010

Routing Algorithms Requirements
• Routing algorithm must ensure freedom from deadlocks
• e.g. cyclic dependency shown below
• Routing algorithm must ensure freedom from livelocks and starvation

Hong Kong University of Science and Technology, March 2010

Part IINoC topologies Switching strategiesNoC Routing Flow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

Flow control schemesSTALL/GO
• Low overhead scheme
• Requires only two control wires
• One going forward and signaling data availability
• the other going backward and signaling either a condition of buffers filled (STALL) or of buffers free (GO)

Hong Kong University of Science and Technology, March 2010

Flow control schemesT-Error
• More aggressive scheme that can detect faults
• by making use of a second delayed clock at every buffer stage
• Delayed clock re-samples input data to detect any inconsistencies
• then emits a VALID control signal
• Resynchronization stage added between end of link and receiving switch

Hong Kong University of Science and Technology, March 2010

Flow control schemesACK/NACK

When flits are sent on a link, a local copy is kept in a buffer by sender

When ACK received by sender, it deletes copy of flit from its local buffer

When NACK is received, sender rewinds its output queue and starts resending flits, starting from the corrupted one

Implemented either end-to-end or switch-to-switch

Hong Kong University of Science and Technology, March 2010

Part IINoC topologies Switching strategiesRouting algorithmsFlow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

Clocking schemes
• Fully synchronous
• Single global clock is distributed to synchronize entire chip
• hard to achieve in practice, due to process variations and clock skew
• Mesochronous
• Local clocks are derived from a global clock
• Not sensitive to clock skew
• Pleisochronous
• clock signals are produced locally
• Asynchronous
• clocks do not have to be present at all

Hong Kong University of Science and Technology, March 2010

Clocking schemes

CMU

PE

NI

SYNC

SW

SYNC

SYNC

• Mesochronous
• Local clocks are derived from a global clock
• Not sensitive to clock skew

Hong Kong University of Science and Technology, March 2010

Part IINoC topologies Switching strategiesRouting algorithmsFlow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

Quality of Service (QoS)
• QoS refers to the level of commitment for packet delivery
• Three basic categories
• Best effort (BE)
• Only correctness and completion of communication is guaranteed
• Usually packet switched
• Guaranteed service (GS)
• makes a tangible guarantee on performance, in addition to basic guarantees of correctness and completion for communication
• Usually (virtual) circuit switched
• Differentiated service
• prioritizes communication according to different categories
• NoC switches employ priority based scheduling and allocation policies

Hong Kong University of Science and Technology, March 2010

Part IINoC topologies Switching strategiesRouting algorithmsFlow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks

Hong Kong University of Science and Technology, March 2010

Basic NoC Building BlocksPacket format

Message, Packet and Flit Formats

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks NoC Queuing Schemes

C

R

O

S

S

B

A

R

Input 0

Output 0

Input 1

Output 1

Input 3

Output 2

Input 3

Output 3

HOL blocking problem in Input Queuing

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Flow Control Schemes

Stop threshold

Packet

enable

Stop

Buffer is occupied

Transmitter

Packet

Buffer is released

Stop & Go Flow Control

Go threshold

Minimum Buffer Size = Flit Size x ( Roverhead + Soverhead + 2 x Link delay)

Roverhead: the required time to issue the stop signal at the received router

Soverhead : the required time to stop sending a flit as soon as the

stop signal is received

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Flow Control Schemes

Stop threshold

Packet

enable

Stop

Transmitter

Credit is decremented

A flit is transferred

Buffer is released

Credit is incremented

Credit Based (CB) Flow Control

• CB makes the best use of channel buffers
• Can be implemented regardless of the link length of the sender & receiver overhead

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Queue and Buffer Design

Intermediate

Empty Bubble

Packet

IN

Packet

out

D Q

D Q

D Q

D Q

D Q

sh

sh

sh

sh

sh

Conventional Shift Register Method

Effective bandwidth of Data link later is influenced by the traffic pattern and Q size

Q Buffers consume most of the area and power among all NoC building blocks.

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Network Interface
• Different interface shall be connected to the network
• The network uses a specific protocol and all traffic on the network has to comply to the format of this protocol

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Network Interface
• In order to allow for different resources to connect to the network, the network interface can be divided into
• A resource independent part (Network Interface)
• A resource dependent part (Resource Network Interface)

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Bidirectional link

36 bits

36 bits

Tag Data

• 2 * 32-bits data links
• Asynchronous, credit based flow control
• Easy floorplan routing & timing in DSM process

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Router Design

Hong Kong University of Science and Technology, March 2010

Basic NoC Building Blocks Router design with cross point

4-bit

Output port Scheduler

request

1

2

3

4

Grant

Input

Queues

grant

IQ1

IQ1

Switch

Fabric

ph-1:0

IQ1

phit-1:0

IQ1

Output Queues (Optional)

OQ1

OQ1

OQ1

OQ1

Hong Kong University of Science and Technology, March 2010

Basic NoC Building BlocksScheduler Design

n

Grant

AnyGnt

PPE

log n

Round-roubin Algorithm

circuits with 2 priority

ENC

INCI

anyGnt

Simple PE

Req

n

n

Termo

encoder

Simple PE

P_enc

Gnt

T.E.

n

log n

n

n

Hong Kong University of Science and Technology, March 2010

Basic NoC Building BlocksPhit Size Determination

PE

PE

NI

NI

Buffers

phit size = packet size/SERR

SER/DES

Buffers

phit size = packet size

Operation freq = SERR* fNORM

Operation freq = fNORM

Switch

Switch

The phit size is the bit width of a link and determines the switch area

Hong Kong University of Science and Technology, March 2010

NoC ExamplesÆthereal
• Developed by Philips
• Synchronous indirect network
• WH switching
• Contention-free source routing based on TDM
• GT as well as BE QoS
• GT slots can be allocated statically at initialization phase, or dynamically at runtime
• BE traffic makes use of non-reserved slots, and any unused reserved slots
• also used to program GT slots of the routers
• Link-to-link credit-based flow control scheme between BE buffers
• to avoid loss of flits due to buffer overflow

Hong Kong University of Science and Technology, March 2010

NoC ExamplesMore…

HERMES - Developed at the Faculdade de Informática PUCRS, Brazil

MANGO

Nostrum - Developed at KTH in Stockholm

Octagon - Developed by STMicroelectronics

QNoC - Developed at Technion in Israel

Xpipes Developed by the Univ. of Bologna and Stanford University

OASIS – Developed by the Adaptive Systems Lab, UoA, Japan (Our Group)

Hong Kong University of Science and Technology, March 2010

Part IINoC topologies Switching strategiesRouting algorithmsFlow control schemesClocking schemesQoSBasic Building Blocks Status and Open Problems

Hong Kong University of Science and Technology, March 2010

Status and Open Problems
• Power
• Complex NI and switching/routing logic blocks are power hungry
• Latency
• Additional delay to packetize/de-packetize data at NIs
• Flow/congestion control and fault tolerance protocol overheads
• Delays at the numerous switching stages encountered by packets
• Even circuit switching has overhead (e.g. SOCBUS)
• Lack of tools and benchmarks
• Simulation speed
• GHz clock frequencies, large network complexity, greater number of PEs slow down simulation

Hong Kong University of Science and Technology, March 2010

Trends
• Move towards hybrid interconnection fabrics
• NoC-bus based
• Custom, heterogeneous topologies
• New interconnect paradigms
• Optical
• Wireless
• Carbon nanotube

Hong Kong University of Science and Technology, March 2010

NoC research community

VLSI / CAD people

Computer system architects

Interconnect experts

Asynchronous circuit experts

Networking/Telecomm experts

Hong Kong University of Science and Technology, March 2010

Research Topics
• Speed enhancement
• New router architectures (e.g. faster arbitration)
• Different asynchronous protocols employment
• Support of link varying capacity
• Low-cost Serialization/De-serialization
• “Standardization” of Network Interface
• Packet construction/destruction
• Testing/Verification of A-NoC

Hong Kong University of Science and Technology, March 2010

Summary

NoC is a scalable platform for billion-transistor chips

Several driving forces behind it

Many open research questions

May change the way we structure and model VLSI systems

Hong Kong University of Science and Technology, March 2010

### Networks-on-Chip

Ben Abdallah Abderazek

The University of Aizu,

Graduate school of Computer Science and Eng,