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Virtual-Channel Flow Control

Learn about virtual channels and their benefits in interconnection networks, including increased throughput, resource utilization, and flexibility. Understand the operation overview and analytical models for optimizing network performance.

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Virtual-Channel Flow Control

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  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?

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