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Parallel

ProcessingLec26

Dr M Shamim Baig

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Multiprocessors CacheCoherence protocols

Snoopy based protocols: Broadcast oriented e.g

shared bus Directory based protocols

Selective communication

oriented e.g MIN

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Directory Based multiprocessor ccoherence protocol

snoopy CC-protocol isinherently

limited to busbased

as each coherence operation is broadcasted to all pr

When IN is not a bus e.gMIN!" broadcast is no

# requiresadditional hard$are # time delay to

as here point to point comm is %avored.

Why not send coherence re&uests to onlythose p

that need to be noti%ied'

It is done usinga directory"which maintains presenc

%or each data item cache line! along $ith its glo

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(rchitecture o% typical directory based systems:a a centrali)ed director b a distributed director

Directory Based CC-System

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Performance ofDirectory Based CC-Schemes

*he need %or a broadcast media is replace

the directory. *he underlying net$or+ must be able to ca

all the coherence re&uests.

*he additional bits to store the directory m

add signi%icant overhead. ,uanti%y memory re&uired %or the t$o Dire

based CC-schemes Normal vs %%icient!.

*he directory is a point o% contention" there

distributed directory schemes may be used

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Normal Directory based CC-schem Cache-bloc! "ntries structure

States bits

States bits

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ma# simultaneous cachingbounded

(r log2n + 2)

"$cient Directory based CC-schem Cache-bloc! "ntries structure

i% r n

States bits

/ M $here M 001

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"#ample %&ef class notes'Memory ()erhead for both

Directory based CC-schemes

NormalvsEfcient

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*opology "mbedding

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+hy *opologies Mapping, 2%ten" $e need to embed a +no$n

communication pattern into a giveninterconnection topology.

We may have an algorithm designed

%or one net$or+" $hich $e are portinto another topology.

For these reasons, it is useful to understand mapp

between graphs

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raphs. topology Mapping Me

When mapping a graph G(V,E)intoG(V,E)the

metrics are important:-

Dilation: Ma3imum number o% consecutive! lin

that any! one edge in Eis mapped onto is

dilation o% mapping. (!istance delay effect) Congestion:Ma3imum number o% edges (

consecuti"e)in that are mapped onto any on

in E is called congestion o% mapping. (#omm

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*opologies "mbedding *ypes Embedding Sparser networks in Denser n

4inear (rray into a Mesh

4inear (rray into a 5ypercube

Mesh into a 5ypercube

Simultaneous Multiple topology embeddin

Embedding Denser networks in Sparser n Mesh into a 4inear (rray

5ypercube in 4inear (rray 5ypercube into a Mesh