Virtual-Channel Flow Control William J. Dally

1. Virtual-Channel Flow ControlWilliam J. Dally Presented by: Nick Kirchem March 5, 2004

2. Motivation • Interconnection network is critical • Performance sensitive to network latency & throughput • Interconnect = large fraction of cost and power consumption • Interconnect throughput is limited to a fraction of capacity due to coupled resource allocation • Single buffers associated with physical channels • Blocks entire physical channel • True for circuit switching & wormhole routing

3. Solution: Virtual Channels • Add “lanes” for each physical channel • (lane = virtual channel)

4. VCs and Flow Control Background • Virtual Channels decouple physical channels from buffer memory • The most costly resources of interconnect’n network • Associate multiple virtual channels with single physical channel • Paper analyzes Flow Control • Determines how resources are allocated, and • How collisions over resources are resolved • Most beneficial to flow control strategies that block

5. Virtual Channels Structure • Each node contains set of buffers and a switch

6. Virtual Channels Structure • Organize flit buffers into several lanes

7. Virtual Channels State Logic • Status Register for Transmitting Node • Lane-is-free bit • Number of free flit buffers in lane • (Optionally) Priority of packet in lane • Status Register for Receiving Node • Input & Output pointers for each lane buffer • Channel state (free, waiting, active)

8. Virtual Channels State Logic

9. VC State Logic Storage Overhead • Number of bits of storage required for l lanes, b flit buffers, and pri priority bits: • Typical scenario (b=16, l=4, pri=0) requires: • 36 bits of overhead with virtual channels • 17 bits with no virtual channels • Small compared to total storage of 512 bits

10. VC Operation • Packet arrives at node • Assigned output channel by routing algorithm • Based on destination and output channel status • Assigned to any free virtual channel (lane) • Blocks if none are available • Flit advanced by flow control • Must gain access to a path through switch, and • Access to the physical channel to input of next node • Lane is deallocated when last flit leaves node

11. Allocation Policies • Allocate physical channel bandwidth for lanes that: • Have flit ready to transmit • Have room for flit at receiving end • Can use any arbitration algorithm • Random, round-robin, priority • Deadline scheduling (schedule by age)

12. VC Implementation Issues • Integration design changes • Replace FIFO buffers with multilane buffers • Modify switch for larger # of inputs and outputs • Flow control protocol modification • Switch Complexity • Added complexity to ACK when free buffer space opens up (identify lane = additional bits)

13. Virtual Channel Analysis • Some assumptions: • Packet destinations uniformly randomly distributed • Arriving packet is consumed without waiting • Single flit buffer for each lane • Packet blocking probabilities are independent • Lots of Math…

14. VC Analysis Results

15. Experimental Results • Simulator (C Program) • Various topologies and VC depth • Throughput and Latency Analysis match predicted performance • Better to have more lanes with less depth than vice versa • Scheduling Algorithms show possibilities of performance given priorities or deadlines

16. Experimental Results

17. Conclusion and Questions • Network throughput and latency improved by decoupling physical channels from buffers • Is it worth the added complexity? • Under which systems/network topologies would it be useful? Where would it not be so useful?