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On the Data Path Performance of Leaf-Spine Datacenter Fabrics

On the Data Path Performance of Leaf-Spine Datacenter Fabrics. Mohammad Alizadeh Joint with: Tom Edsall. Datacenter Networks. Must complete transfers or “flows” quickly need very high throughput, very low latency. 1000s of server ports. w eb. a pp. c ache.

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On the Data Path Performance of Leaf-Spine Datacenter Fabrics

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  1. On the Data Path Performance of Leaf-Spine Datacenter Fabrics Mohammad Alizadeh Joint with: Tom Edsall

  2. Datacenter Networks • Must complete transfers or “flows” quickly need very high throughput, very low latency 1000s of server ports web app cache db map-reduce HPC monitoring

  3. Leaf-Spine DC Fabric Approximates ideal output-queued switch • How close is Leaf-Spine to ideal OQ switch? • What impacts its performance? • Link speeds, oversubscription, buffering ≈ Spine switches Leaf switches H7 H7 H8 H8 H9 H9 H1 H1 H2 H2 H3 H3 H4 H4 H5 H5 H6 H6

  4. Methodology • Widely deployed mechanisms • TCP-Reno+Sack • DropTail (10MB shared buffer per switch) • ECMP (hash-based) load balancing • OMNET++ simulations • 100×10Gbps servers (2-tiers) • Actual Linux 2.6.26 TCP stack • Realistic workloads • Bursty query traffic • All-to-all background traffic: web search, data mining Metric: Flow completion time

  5. Impact of Link Speed Three non-oversubscribed topologies: 20×10Gbps Uplinks 2×100Gbps Uplinks 5×40Gbps Uplinks 20×10Gbps Downlinks 20×10Gbps Downlinks 20×10Gbps Downlinks

  6. Impact of Link Speed • 40/100Gbps fabric: ~ same FCT as OQ • 10Gbps fabric: FCT up 40% worse than OQ Better

  7. Intuition Higher speed links improve ECMP efficiency 20×10Gbps Uplinks 2×100Gbps Uplinks Prob of 100% throughput = 3.27% 1 2 20 Prob of 100% throughput = 99.95% 1 2 11×10Gbps flows (55% load)

  8. Impact of Buffering Where do queues build up? 2.5:1 oversubscribed fabric with large buffers Spine port 60% load 24% load • Leaf fabric ports queues ≈ Spine port queues • 1:1 port correspondence; same speed & load Leaf fabric port Leaf front-panel port

  9. Impact of Buffering Where are large buffers more effective for Incast? Better • Larger buffers better at Leaf than Spine

  10. Summary • 40/100Gbps fabric + ECMP ≈ OQ-switch; some performance loss with 10Gbps fabric • Buffering should generally be consistent in Leaf & Spine tiers • Larger buffers more useful in Leaf than Spine for Incast

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