ElasticTree: Saving Energy in Data Center Networks 許倫愷2013/5/28

About the paperBrandon Heller, Srini Seetharaman, Priya Mahadevan, Yiannis Yiakoumis, Puneet Sharma, Sujata Banerjee, Nick McKeownNSDI’10 (USENIX conference on Networked systems design and implementation)Citation: 17416 pages

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

The motivation

The motivation

Very inefficient!!

Desired

Why wasting powerProvisioning for peak Time varying traffic demands Low efficiency at low loads

The goal of ElasticTree

The approach…Turn off unneeded links and switches

The challengePerformanceFault toleranceScalability

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

IntroductionWhat is ElasticTree:

ElasticTree is a system for dynamically adapting the energy consumption of a data center network

• What does it do:

Finding minimum-power network subsets across a range of traffic patternsTrade-off:

energy efficiency, performance and robustness

Introduction

Data center network(Traditional) 2N Tree:

One failure can cut the effective bisection bandwidth in half; two failures can disconnect servers

Data center networkFat tree: SIGCOMM 2008, A Scalable, Commodity Data Center Network Architecture

Data center networkprovision for peak workload Traffic varies daily, weekly, monthly, and yearly.

Energy Proportionality The strategy: turn off the links and switches that we don’t need

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

ElasticTreeElasticTree is a system for dynamically adapting the energy consumption of a data center network

ElasticTreeIf 0.2 Gbps of traffic per host ,1 Gbps link…

ElasticTree13/20 switches and 28/48 links stay active ElasticTree reduces network power by 38%

0.2

0.4

0.8

ElasticTreeThe optimizer: find the minimum- power network subset which satisfies current traffic conditions

OptimizerAs traffic conditions change, the optimizer continuously re-computes the optimal network subset 3 approaches:Formal Model , Greedy Bin-Packing , Topology-aware Heuristic

Optimizer comparison

Formal modelFinding the optimal flow assignment alone is an NP-complete problem for integer flows.Derived from standard multi-commodity flow (MCF) problem The model outputs a subset of the original topology, plus the routes taken by each flow to satisfy the traffic matrix O(n^3.5+)

Greedy Bin-PackingStrategy: choose the leftmost one with sufficient capacity O(n^2+)

1G link

Greedy Bin-Packing1G link

Topo-aware Heuristic1. does not compute the set of flow routes 2. assumes perfectly divisible flows => pack every link to full utilization and reduce TCP bandwidth => starter subset Decoupling power optimization from routing : => can be applied alongside any fat tree routing algorithm

Topo-aware HeuristicAn edge switch doesn’t care which aggregation switches are active, but instead, how many are active

Topo-aware HeuristicDecoupling power optimization from routing

Optimizer comparison

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

How to testK = 6, fat tree

OpenFlow

AnalysisTraffic pattern:

Near: servers communicate only with other servers through their edge switch Far: servers communicate only with servers in other pods

AnalysisRandom demand:

Individual aggregation/core switches turning on/off

Analysis 70% to outside, 30% inside DCN

Different traffic load

Analysis: redundancyIf only the MST is on => no redundancy => no fault tolerance

Analysis: redundancy+MST: additive cost, multiplicative benefit

Analysis: latency

0.25 0.33 0.5

Need safety margin!!

Ethernet overheads (preamble, inter-frame spacing, and the CRC) cause the egress buffer to fill up Packets either get dropped or significantly delayed

Analysis: latencySafety margin is the amount of capacity reserved at every link by the optimizer Traffic overload is the amount each host sends and receives beyond the original traffic matrix

Trade-off between Energy and Performance

OutlineThe big pictureIntroductionElasticTree systemAnalysisConclusion

Summary

Reference The paperThe slide (by the author)A youtube video (by the author, too)http://www.youtube.com/watch?v=G2_D-CH4tQk

Questions

Thank you!