Noc Architecture

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Chapter 3

Architecture

This chapter describes the architecture of a RECONNECT router and its

congurations. Multiple conguration parameters provide an eas !a to

alter the behavior and functionalit of RECONNECT b either changing values

of constants "conguration #e$ibilit% or b replacing modules "module

#e$ibilit%. The e&ects of some of the parameters are not onl restricted

to RECONNECT or the 'abric( but also depend on other modules of the

architecture in !hich the 'abric is integrated into. )ince this architecture

is not *no!n at the time of !riting( it is henceforth referred to as target

architecture.

3.+ Architectural Overvie! of the Router

The tas* a router has to fulll( can be easil described, Ta*e the data from

the incoming -nput ort "-%( chec* !hich direction it needs to be for!arded

to( and pass it on to the ne$t router. A more detailed description is depicted

in gure 3.+.

Collection /ata from

neighboring -s

Crossbar Traversal -0O Arbitration

Relative Address

1pdate

2C )election for )torage

2C Arbitration

Route Calculation

ne$t Router previous Router

)toring of incoming 'lit in 2C

'igure 3.+, The processing steps a #it encounters !hile traversing a router.

+. The incoming data is stored in one of the 2irtual Channel "2C%. The 2C

has been calculated b the routing algorithm in the previous router. -t is

important to note that the 2C cannot be chosen arbitraril as described


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in detail in chapter . 4efore the #it is stored( it is chec*ed( if the ne!

#it is a header #it. -n this case( the ne$t Output ort "O% and 2C

+5 Architecture

number is calculated b the routing algorithm and stored along !ith

the #it.

6. -n the ne$t step the neighboring routers report their states of the 2Cs.

This step ensures that the - onl considers #its !hich have a chance

to be for!arded. 'or instance( if - 6 of the ne$t router reports that all

its 2Cs are full( all #its !hich !ant to be routed to!ards - 6 do not

need to be considered. This step creates a bit arra of 2Cs that contain

data and have a chance to be for!arded.

3. An arbiter residing inside the -( chooses one of the re7uesting 2Cs and

the - reports the desired O to the router.

. Multiple - might re7uest for the same route( hence another arbitration

step "-0O arbitration step% is re7uired to resolve this con#ict.

8. The - that !on the -0O arbitration( transmits its #it and deletes it

from the chosen 2C. -n case of multi#it environment the O is bound

to the - for the entire duration of the transmission of the pac*et to

prevent interleaving of #its from other -s to the same O.

9. The #it traverses through the crossbar and is received at the O in

!hich its relative address is updated in case it is a header #it. -n the

relative addressing scheme( the address tuple represents the distance

from the current node to the destination. )ince the distance changes

!hen the #it traverses the Net!or* on Chip "NoC%( it needs to be

updated at ever node the #it passes. -f all elements of the address

tuple e7ual :( the #it reached its destination and is e;ected from the

net!or*.

After the address has been updated( the #it is passed on to!ards the

ne$t router.

-n the follo!ing section the parameters for the conguration #e$ibilit


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are described. 'rom section 3.3 on!ards a point of vie! of the #it is ta*en

and the modules !hich a router consists of( are described in detail as it

traverses through the router.

3.6 Conguration arameters

This section describes the constants "


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the union. -f this dene is not set( the A1 is silent.

3.6.3 M1GT-'G-T)1ORT

-f the router are integrated in an environment that splits the pac*et into

multiple #its( this dene needs to be set. 4 doing so( the A1 is automaticall

added to the in;ection and e;ection ports of the router ensuring that the

pac*ets are divided into appropriate siFes of N>'G-T)-HE bits "see section

3.6.I%. /epending on this setting the bu&ers and the logic controlling the

#o! of pac*ets comprising of multiple #its( are ad;usted.

Currentl the router supports t!o #o! control mechanisms,

+. The !ormhole #o! control mechanism means that a header #it !ill

determine the path and can be immediatel for!arded even !ithout

!aiting for the tail #it +8D. The #its of the corresponding pac*et

traverse the routers similar to a train. The advantage of !ormhole

routing is that the bu&ers do not need to be able to hold the complete

pac*et once a congestion occurs. A pac*et comprising of several #it

can therefore be spread over multiple routers.

)ince the destination address is carried onl in the header #it and hence

onl !ith that #it the re7uired O can be calculated b the routing

6: Architecture

algorithm( the information !ere the follo!ing #its need to be sent to(

must be retained. Thus the 2C that ;ust got selected to serve the head

#it( must be bound to the output of the - module. 'urther the - needs

to be $ed to the O so that all successive #its can follo! their head #it.

?o!ever this binding cannot be bro*en( if the channel runs out of data

due to e.g. a congestion in previous hops. The route from the 2C to the

O !ithin the router cannot be restored( if data is available again. -f

the channel does not contain data( the resources are bloc*ed and other

#its are not able to utiliFe them( till a tail #it terminates the bound.

6. 2irtual cut=through 36D solves this problem be reassigning unused

resources resulting in a higher utiliFation of the available band!idth.


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?o!ever this might lead to pac*ets !hose #its got interleaved. The

destination is currentl not able to distinguish #its belonging to di&erent

pac*ets. )econdl the bu&er siFes re7uired for virtual cut=through

!ill be in an order of magnitude larger compared to !ormhole routing

+ID.

-f M1GT-'G-T)1ORT is set( the format of the #it is converted into a

union that provides header( headtail( paload and tail #its. ?eadtail #its are

used( if the header provides some bits for paload and the !hole paload

of the pac*et ts into it. -f M1GT-'G-T)1ORT is not set( all pac*ets become

headtail #its and hence the union becomes super#uous.

-n theor and for later re7uirements this mechanism of binding -0O

pairs could be e$ploited to provide virtual circuits bet!een a source and

destination( since the channel is never terminated as long as no tail #it is

sent.

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