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Virtual-Channel Flow Control

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

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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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