1. Design of a High-Throughput Distributed Shared-Buffer NoC Router
Rohit Sunkam Ramanujam*, Vassos Soteriou†,
Bill Lin*, Li-Shiuan Peh‡
*Dept. of Electrical Engineering, UCSD, USA
†Dept. of Electrical Engineering, CUT, Cyprus
‡Dept. of Electrical Eng. and Computer Science, MIT, USA

2. Chip Multiprocessors are a reality …
Power wall
Frequency wall
ILP wall
Non-Recurring Engineering costs
Time to market
Chip Multiprocessor
we were in the uniprocessor era until earlier this decade when chip makers started facing a number of obstacles that made it hard to improve the performance of uniprocessor systems.
 All these factors combined together to lead to the advent of the multicore era where parallelism started being leveraged in order to improve processor performance.
Sources: Intel Inc. and Tilera Inc.
Uniprocessor

3. The need for a Network on Chip (NoC)
Compute Unit
Router
In recent times computer architects have discovered that the design of heavily pipelined [monolithic] superscalar uni-processor cores that aim to achieve application execution speedup with ILP along with high operating frequencies as a means to yield greater performance, has reached a fundamental limit, threatening the escalation of performance gains from one generation to the next with diminishing returns as technology keeps scaling. Instead economies of scale and new design paradigms point to “divide and conquer” strategies, where applications are broken down into smaller concurrent operations and then distributed across many smaller processing units which reside on a single chip. These small and many processing units communicate with each other using a communication medium. As the number of these on-chip units increases core-interconnectivity is moving from fully connected crossbars (where is complexity is in the order of n^2 (n is the number of connected cores)) or bus architectures which connect a handful of cores to interconnection networks. An on-chip interconnection network is the communication medium of preference because in these networks wire connections are shorter than global wires therefore enabling wire delays to scale with the size of the architecture as single cycle link traversals between computation units at high bandwidth rates offering delay-predictable communication. Additional benefits of interconnection networks include resource re-use as various components/applications are routed over shared routers and connecting wiring that can be used to achieve traffic balancing and even fault tolerance and therefore enabling higher throughput rates and performance to keep scaling. Also application can be mapped in such a way to evenly distribute its load according to the network topology.
Scalable communication
Modular design
Efficient use of wires
A new way to organize and build VLSI systems

4. The Problem – Delivering high throughput in NoCs
Why Care?
NoCs in CMPs connect general-purpose processors.
Future applications unknown → traffic unknown.
Exploiting parallelism needs fine-grained interaction between cores.
Can expect high traffic volume for current and future applications running on many-core processors.
E.g. Cache coherence between large number of distributed shared L2 caches.
Parallel programming and parallel machines have been around for a while in the context of clusters, supercomputers or grids. However, the granularity of parallelism in these machines in quite coarse as off chip communication costs are quite high. With multi-core processors, the communication cost is drastically reduced because of the proximity of the cores within a chip and due to the abundance of on-chip wiring. So, we can expect to exploit parallelism at a much finer granularity. This in turn will greatly  increase the traffic volume  between cores and will require networks that can deliver high throughput.

5. An important design choice that affects throughput
Router microarchitecture
How well does a router multiplex packets onto its output links?

6. NoC routers – Current design
Input Buffered Routers (IBRs) – Flits buffered at the input ports
cycle = 3
cycle = 2
cycle = 1
Output 1
Input 1
Output 2
Input 2
Crossbar
Maximal Matching: Input 2 → Output 1
Maximal Matching: Input 1 → Output 1
Output 2 is unutilized in cycle 3 although there is a flit destined for output 2. Bottleneck: Maximal matching used for arbitration is not good enough.
(70-80% efficiency)

7. Output queueing to the rescue …
Output buffered router (OBR) – Flits buffered at the output ports
cycle = 3
cycle = 2
cycle = 1
Output 1
Input 1
Output 2
Input 2
Crossbar
Output links are always utilized when there are flits available.
Better multiplexing of flits onto output links ⇒ higher throughput.

8. How much difference does it make?
Uniform Traffic
A throughput gap of 18%!
OBR- Infinite buffering, 5 stage pipeline as described in the paper (matches DSB pipeline).
IBR- 200 flit buffering, 8VCs, 5flit/VC.

9. How much difference does it make?
Complement Traffic
A throughput gap of 12%!
OBR- Infinite buffering, 5 stage pipeline as described in the paper (matches DSB pipeline).
IBR- 200 flit buffering, 8VCs, 5flit/VC.

10. How much difference does it make?
Tornado Traffic
A throughput gap of 22%!
OBR- Infinite buffering, 5 stage pipeline as described in the paper (matches DSB pipeline).
IBR- 200 flit buffering, 8VCs, 5flit/VC.

11. Performance impact on real applications
Up to 98% reduction in average packet latency
Splash 2 benchmark applications

12. Output Buffering is great …
OBRs offer much higher throughput than IBRs.
OBRs have predictable delay.
Queuing delay modeled using M/D/1 queues.
Packet delays not predictable for IBRs.

13. So why aren’t OBRs used in NoCs ?
Input 1
Output 1
Input 2 . .
Input P-1
Output P-1
Crossbar
Implementing Output Buffering requires either:
Crossbar speedup of P, where P is the number of ports. Not practical for aggressively clocked designs.
Output buffers with P write ports and a PxP2 crossbar. Has huge area and power penalties.

14. Our approach: Emulate Output Queueing without any speedup
Current time = 5
Current time = 3
Current time = 1
Current time = 2
Current time = 6
Current time = 4
Step2: Find a conflict-free middle memory.
Step1: Timestamp the flits
Assign a future time at which a flit would depart the router assuming output buffering.
Step4: When current time == timestamp,
Read flit from middle memory to output port.
Step3: Move flits from input buffers to middle memories. 4
Input 1
Output 1
Input 2 5
Output 2
Input 3 6
Output 3
Crossbar 1
Middle Memories
Crossbar 2

15. Arrival and Departure Conflicts
Arrival Conflicts – With P input ports, a flit can have an arrival conflict with P-1 other flits.
Departure Conflicts – With P output ports, a flit can have a departure conflict with P-1 other flits.
By Pigeon hole principle, 2P-1 middle memories needed to avoid all arrival and departure conflicts.

16. The Distributed Shared-Buffer Router (DSB)
Aims at emulating the packet servicing scheme of an OBR with limited buffers and no speedup.
First-Come-First-Served servicing of flits.
Objectives:
Close the performance gap between OBRs with infinite buffers and IBRs (high throughput).
Make a feasible design → low power and area overhead.
Make packet delays more predictable for delay sensitive NoC applications.

17. DSB Router Innovations Router pipeline with new stages for:
Timestamping flits
Finding a conflict free middle memory
Complexity and delay-balanced pipeline stages for a high-clocked, high-performance implementation.
New flow control to prevent packet dropping when resources are unavailable.
Evaluate power-performance tradeoff of DSB architectures with fewer than 2P-1 middle memories.

18. Evaluation Cycle accurate flit level simulator.
Mesh topology – Each router has 5 ports, NSEW + Injection/Ejection.
Dimension Ordered Routing (DOR) – decouple effects of routing algorithm on network performance.

19. Evaluation – Traffic traces
3 Synthetic traffic traces:
Uniform
Bit Complement (Complement)
Tornado
Real traffic/memory traces from running multiple threads (49 threads ⇒ 7x7 Mesh) of eight SPLASH-2 benchmarks:
Complex 1D FFT, LU decomposition, Water-nsquared, Water-spatial, Ray tracer, Barnes-Hut, Integer Radix sort, Ocean simulation.

20. Performance on Uniform traffic
A throughput gap of just 9%
Same as 3. DSB flits, 4VCs, 5 flits/VC, 5 MM, 20 flits/MM

21. Performance on Complement traffic
A throughput gap of just 4%
Same as 3. DSB flits, 4VCs, 5 flits/VC, 5 MM, 20 flits/MM

22. Performance on Tornado traffic
A throughput gap of just 8%
Same as 3. DSB flits, 4VCs, 5 flits/VC, 5 MM, 20 flits/MM

23. Performance of DSB on SPLASH-2 benchmarks
Small difference in packet latency between OBR and DSB routers is mainly due to the limited buffering in the DSB router.
Raytrace, Barnes and Ocean traces have very little contention.
For these traces, IBR has lower latency because of a shorter pipeline.
Huge performance improvements over IBR in traces exhibiting high contention and demanding high bandwidth.
Performance of DSB is very close to an OBR with same number of pipeline stages.
64%
72%
Same as 3. DSB flits, 4VCs, 5 flits/VC, 5 MM, 20 flits/MM
97%

24. Input Buffered Router (IBR) pipelineRC ST LT VA SA
Input 1
utput 1
Input 2
Output 2
Crossbar
Virtual Channel Allocation
Reserve an output Virtual Channel (buffering) at the next hop router.
Route Computation
Determine the output port of the flit based on the destination coordinates.
Switch Arbitration
Acquire access to the output port through the crossbar.
Switch Traversal
Traverse the crossbar to reach the output link.
Link Traversal
Traverse the link to reach the input buffer of the next hop router.

25. Distributed Shared-Buffer Router pipeline
If CR or VA fails RC CR
XB1 +
MM_WR
MM_RD + XB2 LT VA TS
Input 1
Output 1
Input 2
Output 2
Crossbar 1
Middle Memory
Crossbar 2
Middle Memory Read + Crossbar 2
When the current time equals the timestamp, the flit is read from the middle memory and traverses the second crossbar.
Timestamp Allocation
Assign a timestamp to a flit for the output port requested.
Timestamp is the future time (cycle) at which the flit can depart the middle memory buffer.
Conflict Resolution + Virtual Channel Allocation
Conflict Resolution: Find a conflict free middle memory.
Virtual Channel Allocation: Reserve a virtual channel at the input of the next hop router.
Route Computation
Determine the output port of the flit based on the destination coordinates.
Crossbar 1 + Middle Memory Write
Flit traverses the first crossbar and gets written into the assigned middle memory.
Link Traversal
Flit traverses the output link to reach the input buffer of the next-hop router.

26. Higher throughput – At what cost?
Extra power !! RC CR
XB1 +
MM_WR
MM_RD + XB2 LT VA TS
Input 1
Output 1
Input 2
Output 2
Crossbar 1
Middle Memory
Crossbar 2
Two crossbars instead of one: With N middle memories, need one PxN and one PxN crossbar.
Middle memory buffers – Can have fewer input buffers to compensate for extra middle memory buffers.
TS stage instead of Switch Arbitration in IBRs
Extra stage for Conflict Resolution

27. Power-Performance tradeoff
Theoretically, 2P-1 middle memories needed to resolve all conflicts.
For a 5-port mesh router, need > 9 middle memories, a 5x9 and a 9x5 crossbar – large power overhead.
What is the impact of using fewer than 2P-1 middle memories?

28. Power and Area Comparison
If NoC consumes 20% of tile power, tile power overhead of only 7% for DSB-5 router
Router power overhead of 50% for DSB-5 router
If NoC consumes 10% of tile power, tile power overhead of only 3.5% for DSB-5 router

29. Thank you
Questions?