Network-on-chip is critical for performance. Network-on-chip is critical for performance

Network-on-chip is critical for performance.

• Network-on-chip is critical for performance.

Electrical signaling

• Electrical signaling

• Repeater insertion needed
• Bandwidth density (up to 8 Gbps/um) [Chang HPCA‘08]

• Nanophotonics

• Bandwidth density ~100 Gbps/ μm !!! [Batten HOTI’08]
• Generally distance independent power consumption
• Speed of light  low latency
• Propagation
• Switching [Cianchetti ISCA’09]

Basic components

• Basic components

Selective

• Selective

• Couple optical energy of a specific wavelength

Modulation & detection

• Modulation & detection

• ~100 Gbps/μm bandwidth density [Batten HOTI’08]

Power Cost

• Power Cost

• Ring heating
• Laser Power
• E/O & O/E conversions
• Distance insensitive

• For short links (2.5mm)

• Nanophotonics
• Electrical
• RC lines with repeater insertion

Design an architecture that differentiates traffic.

• Design an architecture that differentiates traffic.

• Use electrical signaling for short links.
• Use nanophotonics only for long range traffic.

• What do we gain?

• Low latency
• High bandwidth density
• High power efficiency
• Localized arbitration
• Scalability

Motivation

• Motivation

• Architecture of Firefly

• Evaluation

• Conclusion

Concentration

• Concentration

• 4 cores share a router
• 16 routers

Concentration

• Concentration

• Clusters

• Electrically connected
• Mesh topology
• 4 routers per cluster
• 4 clusters

Concentration

• Concentration

• Clusters

• Assemblies

• Routers from different clusters
• Optically connected
• Logical crossbars

Clusters

• Clusters

• Electrical CMESH

• Assemblies

• Nanophotonic crossbars

Single-Write-Multiple-Read (SWMR) [Kirman’06] (CMXbar†)

• Single-Write-Multiple-Read (SWMR) [Kirman’06] (CMXbar†)

• Dedicated sending channel
• Multicast in nature
• Receiver compare & discard
• High fan-out  laser power

Multiple-Write-Single-Read (MWSR)[Vantrease’08] (DMXbar†)

• Multiple-Write-Single-Read (MWSR)[Vantrease’08] (DMXbar†)

• Dedicated receiving channel
• Demux to channel
• Global arbitration needed!

Goal

• Goal

• Avoid global arbitration
• Reduce power

• Proposed design

• Reservation channels
• Narrow
• Multicast to reserve
• Destination ID
• Packet length
• Uni-cast data packet

Virtual-channel router

• Virtual-channel router

• Added optical link ports and extra buffer.

Routing

• Routing

• Intra-cluster routing
• Traversing optical link

Clusters

• Clusters

• Short electrical links
• Concentrated mesh

• Assemblies

• Long nanophotonic links
• Partitioned crossbars

• Benefits

• Traffic locality
• Reduced hardware
• Localized arbitration
• Distributed inter-cluster bandwidth

Motivation

• Motivation

• Architecture of Firefly

• Evaluation

• Conclusion

Cycle-accurate simulator (Booksim)

• Cycle-accurate simulator (Booksim)

• Firefly vs. CMESH, Dragonfly† and OP_XBAR

• Synthetic traffic patterns and traces

Throughput

• Throughput

• Up to 4.8x over OP_XBAR
• At least +70% over Dragonfly

Reduced hardware by partitioning

• Reduced hardware by partitioning

• Reduced heating

• Throughput impact

• Locality

• 34% energy reduction over OP_XBAR with locality

α is heating ratio and β is laser ratio.

• α is heating ratio and β is laser ratio.

• Firefly favors traffic locality.

Technology impacts architecture

• Technology impacts architecture

• New opportunities in nanophotonics
• Low latency, high bandwidth density
• Tailored architectures needed

• Firefly benefits from nanophotonics by providing

• Power Efficiency
• Hybrid signaling
• Partitioned R-SWMR crossbars  Reduced hardware/power
• Scalability
• Scalable inter-cluster bandwidth
• Low-radix routers/crossbars