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Dynamic Evolution of Congestion Trees: Analysis and Impact on Switch Architecture

2. Outline. IntroductionCongestion trees and HOL blockingHOL blocking elimination techniquesTraditional view of congestion treesDifferent dynamics of congestion treesRECN improvementsPerformance evaluationConclusions. 3. Introduction. High-speed interconnection networks:Myrinet, Infiniba

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Dynamic Evolution of Congestion Trees: Analysis and Impact on Switch Architecture

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    1. Dynamic Evolution of Congestion Trees: Analysis and Impact on Switch Architecture P. J. García1, J. Flich2, J. Duato2, I. Johnson3, F. J. Quiles1, F. Naven3

    2. 2 Outline Introduction Congestion trees and HOL blocking HOL blocking elimination techniques Traditional view of congestion trees Different dynamics of congestion trees RECN improvements Performance evaluation Conclusions

    3. 3 Introduction High-speed interconnection networks: Myrinet, Infiniband, Quadrics, Advanced Switching… Main features: High bandwidth, Low latencies Additional features: Lossless networks, Flexible topology Cost and power consumption considerations recommend working close to the saturation point

    4. 4 Congestion trees and HOL blocking

    5. 5 Congestion trees and HOL blocking

    6. 6 Congestion trees and HOL blocking

    7. 7 Congestion trees and HOL blocking

    8. 8 Congestion trees and HOL blocking

    9. 9 Congestion trees and HOL blocking

    10. 10 Congestion trees and HOL blocking Congestion trees introduce HOL blocking, and this may degrade network performance dramatically

    11. 11 HOL blocking elimination/reduction techniques DAMQs and Virtual Channels Different buffers for different flows

    12. 12 RECN: Regional Explicit Congestion Notification RECN is a new efficient and scalable congestion management technique Basic ideas: The real problem is not the congestion, but its negative effects (HOL blocking) By eliminating HOL blocking, congestion becomes harmless Non-congested flows do not introduce significant HOL blocking HOL blocking elimination: Packets belonging to congested flows are stored in specific Set Aside Queues (SAQs) Packets belonging to non-congested flows are stored in a “common” queue Implementation requirements: Deterministic source routing A reduced number of SAQs per port, controlled by a CAM

    13. 13 How RECN works RECN basic procedure: Congested points are detected in any egress switch port of the network The routes to detected congested points are progressively notified to ingress and egress ports crossed by congested flows After receiving a notification, a port allocates a SAQ for the detected congested point A packet arriving to a port will be stored in a SAQ if it will pass through the congested point associated to that SAQ A packet arriving at a port will be stored in the common (“cold”) queue if its route does not match any SAQ SAQs can be deallocated, and later allocated for other congested points

    14. 14 How RECN Works

    15. 15 How RECN Works

    16. 16 How RECN Works

    17. 17 How RECN Works

    18. 18 How RECN Works

    19. 19 How RECN Works

    20. 20 How RECN Works

    21. 21 How RECN Works

    22. 22 How RECN Works

    23. 23 How RECN Works

    24. 24 How RECN Works

    25. 25 How RECN Works

    26. 26 How RECN Works

    27. 27 Traditional view of congestion trees Traditional ideas about congestion trees growth: Congestion propagates from the root to the leaves Congestion first appears at egress sides

    28. 28 Different dynamics of congestion trees Effect of switch architecture (I): Switch speedup may vary for different technologies Depending on switch speedup, congestion may appear at ingress or egress sides

    29. 29 Different dynamics of congestion trees Effect of switch architecture (II): Several congested points may appear both at ingress or egress sides along the branches of a congestion tree

    30. 30 Different dynamics of congestion trees Impact of traffic patterns (I): Depending on traffic patterns, the congestion tree root may “move” downstream

    31. 31 Different dynamics of congestion trees Impact of traffic patterns (II): Different congestion trees may merge, even when the involved packets have different destinations

    32. 32 Different dynamics of congestion trees Impact of traffic patterns (III): Different congestion trees may overlap without merging

    33. 33 Different dynamics of congestion trees Impact of traffic patterns (IV): A congestion tree root may also move upstream

    34. 34 Impact of congestion dynamics on RECN Original (“Basic”) RECN: Congestion is detected only at egress ports In order to keep in-order delivery of packets, no SAQ is allocated if it would be more specific than an existing one

    35. 35 Impact of congestion dynamics on RECN Basic RECN tree detection: Unique tree detected as several independent trees

    36. 36 RECN improvements Modified (“Enhanced”) RECN: Congestion is detected at ingress or egress ports Ingress cold queues are replaced by small “detection queues”, one per output port If a detection queue fills over a threshold, congestion is detected for the corresponding output port It is allowed the allocation of more-specific SAQs In order to keep in-order delivery of packets, a new allocated and more-specific SAQ is blocked until all the packets on the less-specific SAQ are forwarded A pointer to the new SAQ is placed on the less-specific SAQ in order to control the blocking

    37. 37 RECN improvements Enhanced RECN tree detection: Unique tree detected as unique tree (a single root)

    38. 38 Performance Evaluation Objective: Evaluation of RECN improvements Comparative evaluation based on simulation results Evaluation metric: Network throughput when using: Basic RECN Enhanced RECN VOQ at switch level (VOQsw)

    39. 39 Simulation Model Network configurations evaluated: 64 hosts connected by a 64x64 BMIN 512 hosts connected by a 512x512 BMIN 2048 hosts connected by a 2048x2048 BMIN Simulation assumptions: BMINs based on perfect shuffle scheme Deterministic routing 32 KB memories at ingress/egress ports Multiplexed crossbar (BW=8 or12 Gbps) Serial full-duplex pipelined links (BW=8 Gbps) 64-byte packets Credit-based and Xon-Xoff (for SAQs) flow control Maximum of 8 SAQs at ingress/egress ports (RECN)

    40. 40 Traffic Load Six different synthetic traffic patterns: Traces: From I/O activity at cello system disk interface A compression factor applied

    41. 41 Simulation Results Network throughput: Traffic cases 1 and 2 (single hot-spot incremental traffic) 64-endnodes networks Speedup: 1.5

    42. 42 Simulation Results Network throughput: SAN traffic (traces) 64-endnodes networks Traces compression factor: 40

    43. 43 Simulation Results Network throughput: Traffic cases 3 and 4 (single hot-spot sudden traffic) 64-endnodes networks No Speedup

    44. 44 Simulation Results Network throughput: Traffic cases 3 and 4 (single hot-spot sudden traffic) 64-endnodes networks Speedup: 1.5

    45. 45 Simulation Results Network throughput: Traffic cases 5 and 6 (four hot-spots sudden traffic) Uniform traffic injection rate 100% Speedup: 1.5

    46. 46 Conclusions Congestion trees producing HOL blocking may affect network performance We have shown that congestion trees may form and evolve in different ways We have analyzed the importance of considering congestion trees dynamics on the design of HOL blocking elimination techniques We have proposed some improvements for RECN, in order to manage HOL blocking independently of the way congestion trees form From the results of our experiments, these improvements were necessary

    47. Dynamic Evolution of Congestion Trees: Analysis and Impact on Switch Architecture P. J. García1, J. Flich2, J. Duato2, I. Johnson3, F. J. Quiles1, F. Naven3

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