Transcript Lec16_Interconnection_Networks

Lecture 16: Interconnection
Networks
Embedded Computing Systems
Mikko Lipasti, adapted from M. Schulte
Based on slides and textbook from Wayne Wolf
High Performance Embedded Computing
© 2007 Elsevier
Topics
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Interconnection Terms and Metrics
Interconnection Models
Routing and Flow Control
Network-on-Chips
© 2006 Elsevier
Interconnection networks
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Client: sender or receiver on a network.
Port: connection to a network on a client
Link: connection between two clients (full/half duplex)
Topology: organization of network links.
Network metrics:
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Throughput.
Latency.
Energy consumption.
Area (silicon or metal).
Quality-of-service (QoS) is important for multimedia
applications.
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Interconnection network models
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Source <- line -> termination.
Throughput T, latency D.
Link transmission energy Eb.
Physical length L.
Total link area A.
Traffic models often use Poisson distribution
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P(X = x) = m xe-m /(x)!
E(x) = m, Var(x) = m.
x =0, 1, 2, …
Streaming data is produced periodically with
rate s and burstiness r
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Network topologies
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Major choices for network topologies include.
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Bus: common connection between a set of
senders and receivers
Crossbar: fully connected network from every
input port to every output port.
Buffered crossbar: add queues to a crossbar to
enable multiple sources to share crossbar input
Mesh: network in which every node is connected
to all of its neighbors.
Application-specific: topology is matched to the
characteristics of the application.
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Bus network
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Throughput:
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Advantages:
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T1 = P/(1+C). – single word
Tb = P*(n/(n + C)) – n word block
Well-understood.
Easy to program.
Many standards.
Disadvantages:
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Contention.
Significant capacitive load.
Do not scale well.
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Crossbar network
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Fully connected network
Advantages:
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No contention.
Simple design
Low latency
Broadcast.
Disadvantages:
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Expensive
Not feasible for large
numbers of ports.
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Buffered crossbar network
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Add queues shared by
multiple sources
Advantages:
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Disadvantages:
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Smaller than crossbar.
Can achieve high utilization.
Requires scheduling.
Clos networks
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Connect multiple crossbars
together in stages
© 2006 Elsevier
Xbar
Mesh network
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Every node connected to all of its neighbors
Advantages:
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Well-understood.
Regular architecture.
Disadvantages:
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Poor utilization
Variable latency.
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Application-specific. network
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Topology is specific for
application(s)
Advantages:
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Higher utilization.
Lower power.
Disadvantages:
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Must be designed.
Must carefully allocate
data.
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Network topology questions
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What type of network topology would you chose
if you were building a 1,000 node system?
Why would you use a buffered crossbar network
instead of a regular crossbar network?
What advantages and disadvantages does a 2D
mesh have compared to a 3D mesh?
What types of systems would you expect to use
application-specific networks?
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Routing and flow control
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Routing determines paths followed by packets.
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Connection-oriented or connectionless.
Wormhole routing divides packets into flits and header flit
determines route for remaining flits
Virtual cut-through ensures entire path is available before
starting transmission.
Store-and-forward routing stores packets inside network.
Flow control allocates links and buffers as packets
move through the network.
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Virtual channel flow control treats flits in different virtual
channels differently.
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Networks-on-chips
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Impact characteristics of MPSoC:
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NoCs may not have to interoperate with other
networks.
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Energy.
Performance.
Cost.
NoCs have to connect to existing IP, which may
influence interoperability.
QoS is an important design goal.
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Nostrum
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Mesh network---switch
connects to four nearest
neighbors and local “resource”
Each switch has queue at
each input.
Selection logic determines
order in which packets are sent
to output links.
[Kum02] © 2002 IEEE Computer Society
© 2006 Elsevier
Scalable, Programmable, Integrated Network
(SPIN)
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Scalable network based on fat-tree.
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Bandwidth of links is larger toward root of tree.
All routing nodes use the same routing function.
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Message goes up the tree until a common ancestor reached
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Routing nodes in SPIN
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Packet consists of
32-bit words
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One word header
Variable word packet
One word checksum
trailer
Network utilizes input
queues and partial
crossbars
Outputs share buffers
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Help with contention
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Ye et al. energy model
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Assume: energy per packet
is independent of data or
packet address.
Histogram captures
distribution of path lengths.
Energy consumption of a
class of packet:
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M = maximum number of
hops.
h = number of hops.
N(h) = value of hth
histogram bucket.
L = number of flits per
packet.
Eflit = energy per flit.
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Ye et al. energy model
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Longer packets corresponds to longer block sizes
Larger packets
 Decrease cache misses but increase the miss penalty
 Decrease number of packets but increase hops per
packet
 Decrease cache and memory energy, but increase
network energy
© 2006 Elsevier
Goossens et al. NoC methodology
Geared towards
 Application-specific SoCs
 QoS-intensive apps
 Network dimensioning –
determine size of network
and buffers
 NoC topology – determine
connection between
elements
 NoC configuration – set
register values that control
flow through the network
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© 2006 Elsevier
QNoC
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Designed to support QoS.
Two-dimensional mesh, wormhole routing.
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Four different types of service.
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Fixed x-y routing algorithm.
Each service level has its own buffers.
Next-buffer-state table records number of slots for
each output in each class.
Transmissions based on next stage, service
levels, and round-robin ordering.
Can be customized for specific application.
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QNoC Design Methodology
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Xpipes and NetChip
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Xpipes is a library of soft IP macros for network
switches and links.
NetChip generates custom NoC designs using
xpipes components.
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Xu et al. H.264 network design
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Designed NoC for
H.264 decoder.
Process -> PE mapping
was given.
Compared RAW mesh
to application-specific
networks.
[Xu06] © 2006 ACM Press
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Application-specific network for H.264
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[Xu06] © 2006 ACM Press
RAW/application-specific network
comparison
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[Xu06] © 2006 ACM Press