Virtual-Channel%20Flow%20Control%20William%20J.%20Dally

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 interconnectn
  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 (b16, l4, pri0) 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?