1. Using Packet Information for Efficient Communication in NoCs
Prasanna Venkatesh R, Madhu Mutyam
PACE Lab
IIT Madras

2. Agenda Motivation Existing techniques to handle multicasts at NoC
Dynamic Multicast Tree
VC as Cache
Packet Concatenation
IPC Results
Energy Analysis
Conclusion

3. Motivation
SPLASH and PARSEC benchmarks have upto 87% of nodes participating in a multicast. But the average is 7.5% only.

4. Motivation
SPLASH and PARSEC benchmarks have upto 87% of nodes participating in a multicast. But the maximum communication exists for < 4% of the time.

5. Multicasts: Solutions in the literature
Separate injections flood the network with redundant copies
Multicasts: Single copy till a common path and forks to multiple copies
Simplifies routing logic
Dynamic Multicast routing can make use of idle paths to avoid congestion.
But is it possible to meet timing constraints?

6. Our Proposals to achieve multicast efficiency
Dynamic multicast tree construction using redundant route computation units
Will penalize unicasts and create starvation?
Three optimizations on unicasts to enhance dynamic multicasting
VC as cache
Packet Concatenation
Critical word first

7. Critical Word First
Borrowed from Cache data transfer optimization technique
Make efficient use of the flit level split of a packet containing cache block
Send the requested word with the header flit

8. Dynamic Multicast Tree
Method
Compute Odd-Even route at each router for all multicast destinations
Takes one RC cycle per destination
Add a redundant RC unit to speed this process
No extra chip area because of the simplicity
Caveats
Bottlenecks unicasts
Slow when there is no congestion

9. VC as Cache: Scenario
A shared cache block is requested by more than one node at a given time frame
The owner sends a multicast of the block to all the requestors
A request arrives after this multicast
The owner resends the block after processing this request

10. Solution – Add the new requestor to the processed multicast midway!
Compare up to five multicast packets with an incoming request packet at the router
If matched,
Forward the request to the owner for coherence and book keeping with a time stamp of the previous message
Add this requestor to the multicast destinations

11. Packet Concatenation A request is a single flit packet
When RC units are busy, we can club single flit packets to the same destination to form a “super- packet”
This means it is going to take one RC cycle to compute multiple packet routes from there on.

12. Configuration for simulations
Simulators Multi2sim 4.0.1, Booksim 2.0, Orion 2.0
Real time simulation
64 Nodes with 32 cores + L1 nodes and 32 shared distributed L2 cache banks
1 Flit for request and coherence packets, 5 flits for cache block
Benchmarks:
SPLASH2 and PARSEC workloads with 32 threads
All high injection workloads are picked after an initial study on their injection rates

13. IPC Results Abbreviations: C – Critical Word first V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation

14. IPC Results Abbreviations: C – Critical Word first V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation

15. Scaling to 512 Nodes: IPC Results
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation

16. Fine Grained Energy Footprint of Barnes
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation

17. Conclusion and future extensions
Scalable solution for multicasts
Can fit with existing techniques
Easy to implement
Energy Efficient
Packet Concatenation can be switched on selectively depending on the load requirements
Other architecture level inputs can also be used for further performance.
Example: #Instructions waiting, memory level parallelism

18. Thank you