Virtual-Channel Flow Control

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Presentation transcript:

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 If buffer is allocated to packet A, no packet except A can use associated channel A can block others needing same channel

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

Example 1 lane 2 lanes

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

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.)

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?

Throughput (constant storage)

Random vs. Deadline Scheduling Priority Traffic

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?