Networks-on-Chip

Presentation Transcript

Ben Abdallah Abderazek

The University of Aizu,

Graduate school of Computer Science and Eng,

Adaptive Systems Laboratory,

E-mail: benab@u-aizu.ac.jp

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

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

Routing Algorithm Classifications

Three different criteria:

Where the routing decision are taken

Source routing
Distributed routing

How a path is defined

Static (deterministic)
Adaptive

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

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)

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.

Dynamic Routing

Packet

X

Adaptive routing method

More resources needed to monitor state of the network

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

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

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)

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

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

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

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

Clocking schemes

CMU

PE

NI

SYNC

SW

SYNC

Mesochronous

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

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

Basic NoC Building Blocks

Basic NoC Building BlocksPacket format

Message, Packet and Flit Formats

Basic NoC Building Blocks NoC Queuing Schemes

C

R

O

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

Basic NoC Building Blocks Flow Control Schemes

Stop threshold

Packet

enable

Receiver

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

Basic NoC Building Blocks Flow Control Schemes

Stop threshold

Packet

enable

Receiver

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

Basic NoC Building Blocks Queue and Buffer Design

Intermediate

Empty Bubble

Packet

IN

Packet

out

D Q

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.

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

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)

Basic NoC Building Blocks Bidirectional link

36 bits

Tag Data

2 * 32-bits data links
Asynchronous, credit based flow control

Easy floorplan routing & timing in DSM process

Basic NoC Building Blocks Router Design

Basic NoC Building Blocks Router design with cross point

4-bit

Output port Scheduler

request

1

2

3

4

Grant

Input

Queues

grant

IQ1

Switch

Fabric

ph-1:0

IQ1

phit-1:0

IQ1

Output Queues (Optional)

OQ1

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

Termo

encoder

Simple PE

P_enc

Gnt

T.E.

n

log n

n

Basic NoC Building BlocksPhit Size Determination

PE

NI

Buffers

phit size = packet size/SERR

SER/DES

Buffers

phit size = packet size

Operation freq = SERR* fNORM

Operation freq = fNORM

Switch

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

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