1. A Programmable Adaptive Router for a GALS Parallel System Jian Wu APT Group University of Manchester May 2009

2. SpiNNaker System for Neural Simulation Massively-Parallel (1 million ARMs) Massive neural net simulations (1 billion neurons in real time)  GALS infrastructure Fault-tolerant Node = SpiNNaker CMP + large off- chip memory

3. SpiNNaker Chip

4. Router Requirements Operation requirements:  Route multicast, point-to-point and nearest-neighbour packets.  Reprogrammable at run-time.  Provide an external interface to system resources.  Fault-tolerant operation.  Power efficiency. Bandwidth Requirements: ~7.4Gb/s  On-Chip traffic: (20-1)procs x 1000neurons x 72bit x 1000Hz = 1.368Gb/s  Inter-chip traffic: 1Gb/s x 6 links = 6Gb/s Bandwidth Target = 72bit x 200MHz = 14.4Gb/s

5. Router architecture Packet checking: - Check packet for errors and enable appropriate routing engine Multicast (MC) router: - Route neural spikes according to their source address Point-to-Point (P2P) router: - Route system management and control information packets. Nearest-neighbor (NN) router: - Route system boot-up and debugging info - Provide external I/F to resources Adaptive routing: - Redirect blocked packets Router Interface to system NoC: - AHB Master and Slave Interfaces

6. Multicast Router

7. Default and Adaptive Routing Route packets “across chip” by default (save RT entries!)  Automatically re-route packets destined to congested or failed links

8. Interfacing with System NoC Nearest-Neighbour packets are diverted to the System NoC.  Programming data is sourced from the System NoC.

9. Elastic Buffering  The spiking rate for the great majority of neurons is low -just a few Hz: Pipeline “bubbles” between valid packets.  There can be more than one request to the datapath issued in the same clock cycle.  The adaptive routing mechanism stalls the pipeline to find an alternative path for the congested packet. Simple, synthezisable design:  Use ordinary flip-flops for data latching.  Use a global, combinatorial circuit to generate stall signals

10. Elastic Buffering Pipeline1Pipeline2Pipeline3 Pipeline Control Pipeline Control Pipeline Control Flag1Flag2Flag3 Disable Back Pressure

11. Input Interchangeable Buffer  Used for flow control at the head of the pipeline.  One register is used in normal operation  The second is used when a stall occurs in the next stage  The delay is re-introduced when the stall is removed

12. Parallel-Path Synchronizer Avoid 2-cycle penalty to increase throuhgput

13. Packet Drop Rate

14. Power vs. Traffic Load

15. Power Distribution Power distribution under full traffic load Power distribution under 10% traffic load

16. Thank you