Virtual-Channel%20Flow%20Control

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Virtual-Channel Flow Control

• William J. Dally
• Presented by John Calandrino

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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

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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

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Example

• 1 lane
• 2 lanes

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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

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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.)

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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?
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Throughput (constant storage)
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• Random vs.
• Deadline
• Scheduling

Priority Traffic
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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?