Flattened Butterfly Topology
for On-Chip Networks
John Kim, James Balfour, and William J.
Dally
Presented by Jun Pang
Motivation & Goal
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Most on-chip networks (2D mesh): low-radix
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High-radix networks
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Pros: simple & short wires
Cons: long network diameter & energy inefficiency
(many hops)
Intermediate routers: reduced a lot
Small latency & lower power
Goal: how does on-chip network use highradix routers to reduce latency & energy
On-chip network
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Plentiful bandwidth due to inexpensive wires
while buffers are expensive
lower cost: from smaller distance
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By reducing number of channels & buffers
Concentration: several terminal nodes share
resources (routers)
Latency:
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Reduce hop count at the expense of TS↑to get an
overall reduced latency
On-chip Flattened Butterfly
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Topology
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Fig. 3a
Radix=10(concentration factor:4; 3:d1; 3:d2)
2 hops
Longer wires-> deeper buffers
Non-minimal global adaptive routing (UGAL)
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Load balance & performance: path diversity
Routing minimally or non-minimally
Non-minimal: minimal Direction-ordered routing
(prevent deadlock)
Only 2 VCs
Bypass Channels & Microarchitecture
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Goal: reduce distance traveled by packets to reduce
latency and energy
Two types of muxes
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Yield arbiter to guarantee global fairness
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Input muxes: bypass inputs or direct inputs
Output muxes: direct outputs or bypass inputs
If primary input is idle, non-primary input is chosen
Control packet: prevent starvation
Combination of minimal and non-minimal routing
Bypass Channels (continue)
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Switch architecture
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Minimal: simplified crossbar switch
Non-minimal: more complexity
Non-minimal with bypass channels: less
complexity
Flow control & routing
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Buffers for non-primary inputs
Separate buffers for destination of control packets
Modify UGAL to support bypass channels
Evaluation
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Throughput: up to 50% throughput increase
compared to concentrated mesh
Power: about 38% power reduction
compared to mesh
Latency: about 28% latency reduction
compared to mesh
Scalability
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Lower channel increasing factor than
hypercube
Three ways to scale
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Concentrate factor
Dimension of the flattened butterfly
Hybrid approach
Future technology helps long wires
Increasing VCs will slightly reduce latency
Conclusion & Concerns
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Flattened-butterfly:
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interesting idea
Maximum distance between nodes=2
Non-minimal routing to balance load
Bypassing channel to reduce latency
Lower latency and power, high throughput compared to
mesh
Concerns:
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High channel count? (bigger than mesh & torus)
Low channel utilization? (due to high channel)
Control complexity? (arbitration, control packets)
Bypass channel: good idea? (How about just use nonminimal or minimal?)