Flattened Butterfly Topology for On-Chip Networks John Kim, James Balfour, and William J. Dally Presented by Jun Pang

Motivation & Goal Most on-chip networks (2D mesh): low-radix  Pros: simple & short wires  Cons: long network diameter & energy inefficiency (many hops) High-radix networks  Intermediate routers: reduced a lot  Small latency & lower power Goal: how does on-chip network use high- radix routers to reduce latency & energy

On-chip network Plentiful bandwidth due to inexpensive wires while buffers are expensive lower cost: from smaller distance  By reducing number of channels & buffers  Concentration: several terminal nodes share resources (routers) Latency:  Reduce hop count at the expense of T S ↑to get an overall reduced latency

On-chip Flattened Butterfly Topology  Radix=10(concentration factor:4; 3:d1; 3:d2)  2 hops  Longer wires-> deeper buffers Non-minimal global adaptive routing (UGAL)  Load balance & performance: path diversity  Routing minimally or non-minimally  Non-minimal: minimal Direction-ordered routing (prevent deadlock)  Only 2 VCs Fig. 3a

Bypass Channels & Microarchitecture Goal: reduce distance traveled by packets to reduce latency and energy Two types of muxes  Input muxes: bypass inputs or direct inputs  Output muxes: direct outputs or bypass inputs Yield arbiter to guarantee global fairness  If primary input is idle, non-primary input is chosen  Control packet: prevent starvation Combination of minimal and non-minimal routing

Bypass Channels (continue) Switch architecture  Minimal: simplified crossbar switch  Non-minimal: more complexity  Non-minimal with bypass channels: less complexity Flow control & routing  Buffers for non-primary inputs  Separate buffers for destination of control packets  Modify UGAL to support bypass channels

Evaluation Throughput: up to 50% throughput increase compared to concentrated mesh Power: about 38% power reduction compared to mesh Latency: about 28% latency reduction compared to mesh

Scalability Lower channel increasing factor than hypercube Three ways to scale  Concentrate factor  Dimension of the flattened butterfly  Hybrid approach Future technology helps long wires Increasing VCs will slightly reduce latency

Conclusion & Concerns Flattened-butterfly:  interesting idea  Maximum distance between nodes=2  Non-minimal routing to balance load  Bypassing channel to reduce latency  Lower latency and power, high throughput compared to mesh Concerns:  High channel count? (bigger than mesh & torus)  Low channel utilization? (due to high channel)  Control complexity? (arbitration, control packets)  Bypass channel: good idea? (How about just use non- minimal or minimal?)