1. Virtual-Channel Flow Control
William J. Dally
Presented by John Calandrino

2. Motivation Two types of resources in interconnection networks
Buffers: hold flits
Channels: transport flits
Typically, these resources are coupled
Single buffer associated with single channel
If buffer is allocated to packet A, no packet except A can use associated channel
A can block others needing same channel

3. Virtual Channels Consist of a flit buffer + other state
Multiple virtual channels per physical channel
Or, multiple buffers per channel
Decouples allocation of buffers and channels
Highway metaphor
Virtual channels are “lanes”
Multiple virtual channels allow for blocked packets to be “passed”
Very compatible with wormhole routing

4. Example
1 lane
2 lanes

5. Benefits Increased network throughput
Greater utilization of network capacity
More freedom in allocation of resources
Extra dimension: which VC do we service?
Minimal hardware changes
FIFO buffers replaced with “multilane” buffers
Minimal additional hardware complexity

6. Operation Overview Virtual channels allocated to packets
Can be reassigned when last flit of packet exits
If no virtual channel available, packet blocks
Flits travel across physical channel
Physical channel shared by multiple packets from different virtual channels
Physical channel allocated according to some scheduling policy (FIFO, round-robin, packet priorities, packet age, etc.)

7. Analytical Model Latency Throughput (2-ary n-cube)
Claim: “4 to 8 lanes per physical channel is adequate for most networks.” Why is 60+% of capacity adequate?

8. Throughput (constant storage)

9. Random vs.
Deadline
Scheduling
Priority Traffic

10. Questions
Can we use the analytical model to make guarantees about the time a packet will take to reach a destination?
Probably not in the general case – assumptions of model may be too simplistic
What if packets needing such guarantees are given highest priority?
Are there applications where the benefits of virtual channels will not be realized?
What if multiple packets on the same path can frequently become blocked?