1. Exploring Concentration and Channel Slicing in On-chip Network Router
Prabhat Kumar1
Yan Pan1
John Kim2
Gokhan Memik1
Alok Choudhary1
1Northwestern University
2KAIST, South Korea

2. Contributions of the Work
Performance implication of concentration.
Integrated vs. external concentration.
47% reduction in area
36% reduction in energy
10% performance degradation
Channel Slicing
Virtual concentration for efficient resource utilization.
69% reduction in area
32 % reduction in energy

3. Outline Motivation Concentration Channel slicing Virtual concentration
Results
Conclusion

4. Motivation Limited Budget Efficiency Design Options Previous Work Area
Energy
Efficiency
Performance
Cost optimization
Design Options
Concentration
Channel Slicing
Previous Work
Concentration – CMESH, Flattened Butterfly, Firefly, Multidrop Express Channels (on-chip)
Channel Slicing – Dragonfly (off-chip)
On-chip communication is critical for CMPs as that what determines the performance of the system
Concentration and Slicing are previously proposed options whose trade-offs are not well investigated

5. Motivation
Firefly [Pan ISCA’09]
Simplified Router Microarchitecture

6. Typical Topology
2D Mesh
# Processor Nodes = # Routers

7. Solution: Concentration
Multiple cores share one router
Benefits
Resource sharing
Network Diameter decreases
Local communication cost decreases
Drawbacks
Router complexity increases significantly
You can say: An example, with concentration = 4 is shown on the right.
If we keep the bandwidth density constant then the channel width will be 2x the channel width in 2D MESH
Put a text below the figre: Concentration = 4, Router radix increases from 5->8, channel width -> 2x
C = 4
Radix = 8
Width = 2x

8. Issue : Router Complexity
Router components
Crossbar Switch ~ (radix)2
Arbitration logic
5x5 crossbar, 2D MESH
8x8 crossbar, 2D MESH, C = 4
(Integrated Concentration)
Put below the figures (Also say it is just an example of 2D mesh concentration): Typical crossbar, integrated crossbar (or something like what is an integrated crossbar), put the reference of the model
You can say: Crossbar Switch area increases quadratically with the radix, Higher radix leads to increased complexity in arbitration logic.
How can we reduce the complexity of crossbar switch?

9. Design Option: External Concentration
Multiplex injection ports
De-multiplex ejection ports
Benefits
Router radix decreases
Area decreases
Cons
Reduced switching capacity
You have to mention the philosophy of external concentration
You can say: The traffic going from the injection to the ejection ports still see an equivalent to a 8x8 crossbar, while the intermediate traffic (i.e., east, west, north, south) traffic sees a 5x5 crossbar
In few words, Put down the philosophy as it is the last slide of this section

10. Issue: Arbitration External Concentration Two levels of arbitration
Parallel Arbitration
Use router switch information for concentration arbitration
Add more details, examples or something

11. Outline Motivation Concentration Channel slicing Virtual concentration
Results
Conclusion

12. Issue: Wide Channels Constant bandwidth density => wider channels
Inefficient utilization
Cache lines ~ bits wide
Request, control, coherency packets much narrower
Router Area
Switch area ~ (channel width)2
Add the reference paper for the model
C = 4
Radix = 8
Width = 2x

13. Design Option: Channel Slicing
Slice wide channels
Pros
Complexity reduces further
Better channel utilization
Cons
Serialization latency increases (for long pkts)
Wide Channels imply larger area of components of the routers, how to put this statement in 2-3 words????
C = 4
Slicing Factor = 4

14. Outline Motivation Concentration Channel slicing Virtual concentration
Results
Conclusion

15. Combining Concentration and Slicing
Slicing + Concentration
Virtual Concentration
Nodes dedicated to a sliced layer
No sharing of input bandwidth
Explain clearly how the local traffic flows or something!!!

16. Outline Motivation Concentration Channel slicing Virtual concentration
Results
Conclusion

17. Evaluation Setup Simulation Environment Booksim simulator
Constant on-chip resources
Equal Bisection bandwidth for all configurations
Equal amount of buffer storage
Elaborate a little bit about holding on-chip resources constant!!!

18. External Concentration
Zero-load latency
21% reduction for UR
25% reduction for Bitcomp
Throughput
10% reduction for UR
No change for Bitcomp
Area
47% reduction compared to Integrated
Energy
36% reduction compared to Integrated

19. Virtual Concentration
Zero-load latency
No change compared to MESH
16% increase for UR compared to Integrated
12% increase for Bitcomp compared to Integrated
Throughput
No significant difference for UR
4.5% increase for Bitcomp
Area
69% reduction compared to MESH
Energy
32% reduction compared to MESH
Change the 16% and 12% according to the plots.

20. Area and Energy Consumption
69% reduction compared to MESH
88% reduction compared to Integrated concentration
Energy
32 % reduction compared to MESH
35% reduction compared to Integrated concentration

21. Conclusion
Combination of concentration and channel slicing provides efficient NoC design.
External concentration reduces complexity with some performance degradation.
Virtual Concentration saves 69% area and 32% energy compared to 2D MESH.
Make sure that the conclusion is in accordance to the previous slides, do not use any word which is not used earlier.

22. Thank you for your patience!!
Questions?