1. Virtual-Channel Flow Control William 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?