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Methodology

Methodology. easy but important. ToC. What is performance evaluation about ? Metrics, Load and Goals Hidden Factors The Scientific Method Patterns. What is Performance Evaluation ?. Load. You need to define the load under which your system operates Make the difference between

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Methodology

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  1. Methodology easy but important 1

  2. ToC • What is performance evaluation about ? • Metrics, Load and Goals • Hidden Factors • The Scientific Method • Patterns 2

  3. Whatis Performance Evaluation ?

  4. Load • You need to define the load under which your system operates • Make the difference between • Intensity of the load (e.g. nb jobs per second) • Nature of the load • Statistical details that may matter: e.g. job sizes are heavy tailed or not • Benchmarks are artificial load generators; we will play with one of them

  5. Metric • Define a metric;examples • Response time • Power consumption • Throughput • Defineoperational conditions underwhichmetricismeasured (« Viewpoint », seeChapter 11)

  6. Compare Windows vs Linux 6

  7. Syscall Benchmark 7

  8. Memory Access Time 8

  9. Ghostscript 9

  10. Metrics are often Multidimensional 10

  11. A and D are non dominated 11

  12. Know your goals • A1 and A3 are comparisons, A2 is an absolute statement • E2 is an engineering rule

  13. 3. Hidden Factors • Factor: an element that may impact the performance • (desired factors): intensity of load, number of servers • (nuisance factors): time of the day, presence of denial of service attack 13

  14. TCP Throughput Increases with Mobility

  15. TCP ThroughputDecreaseswithMobility

  16. Why were we fooled ? • Hidden factor had a more important role than the factor we were interested in • We interpreted correlation as causality • Need to be aware of all factors and incorporate in the analysis • Or randomize experiment to reduce impact of hidden factors

  17. Simpson’sParadox • A wellknownphenomenon -- Special case of Hidden Factor paradoxwhenmetricissuccess rate and factors are discrete

  18. Berkeley Sex Case 1973 (source: wikipedia) 18

  19. Take Home Message • Pitfallnumber 1 is the presence of hidden factor • Anystudyis susceptible to it • Easy for opponents to find

  20. 4. Be Scientific • Joe measures performance of his Wireless Shop: • what would you conclude ?

  21. Scientific Method • Joe buys 2 more Access Points • improvement ? Before After 21

  22. Scientific Method • A conclusion can only proven to be wrong • Do not draw conclusions unless the experiment was designed to test the statement • Measurement 1 suggested that the wireless network was congested, but the experiment was not designed to test this statement • Joe should: design an experiment to validate:H1: “the wireless network is the bottleneck” • for example: measure the number of collisions / packet loss • result: collision · 1%; conclusion: H1 is not valid • hypothesis H2: the server is saturated • experiment: measure memory utilization : result ¼ 100% 22

  23. Performance After Doubling Server Memory 23

  24. Example from Nitin Vaidya, Mobicom 2000 Tutorial, slides 298-299 24

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  28. Use of Scientific Method TCP throughput may increase with mobility (1) Duration of link failure period is impacted by speed(2) It has a negative impact on TCP throughput Do more simulationsmeasure distrib of link failure period Verify (1) and (2) How ? • Recognize a fact • Pose a hypothesis • Verify the hypothesis on simulations / measurements designed to test it 28

  29. Is ATM-ABR better than ATM-UBR ? 29

  30. Take Home Message • You should not conclude from an experiment without trying to invalidate the conclusion • (Popper, 1934): you should alternate between the roles of • Proponent • Adversary

  31. 5. Patterns • These are common traits found in different situations • Knowing some of them may save a lot of time

  32. Bottlenecksmaybeyourenemy • Bottlenecks are like non invited people at a party – theymay impose their agenda • Previousexample: whatwe are measuringis the bottleneck, not the intended factor 33

  33. Bottlenecks are YourFriends

  34. Bottlenecks are YourFriends • Simplifyyour life, analyzebottlenecks ! • In many cases, youmay ignore the rest 35

  35. Behind a Bottleneck May HideAnotherBottleneck

  36. i i’ i” i” 37

  37. i i’ i” i” 38

  38. 39

  39. Congestion Collapse • Definition: Offeredloadincreases, workdonedecreases • Frequent in complexsystems • May be due to • cost per job increaseswithload • Impatience • Rejection of jobs beforecompletion • Designer must do something to avoid congestion collapse • Eg. Admission control in Apache servers • Eg. TCP congestion control • Analyst must look for congestion collapse 40

  40. Sources use TCP (= fair scheduling). Increase capacity of link 5 to 100 kb/s; what happens ? 41

  41. Competition Side Effect • System balances resources according to some scheduling • Apparent paradox: put more resources, some get less 42

  42. No TCP, users send as much can • Increase capacity of link 2 from 10 to 1000 kb/s 10 kb/s 43

  43. Competition Side Effect • Apparent paradox: put more resources, all get less 44

  44. Museum Audio Guide Lowspeed USB connections atdocking station High speed 45

  45. Latent Congestion Collapse • System is susceptible to congestion collapse • Low speed access prevents congestion collapse • Adding resources reveals congestion collapse 46

  46. Take Home Message • Watch for patterns, they are very frequent • Bottlenecks • Congestion collapse • Competition side effects • Latent Congestion collapse 47

  47. Now it’s your turn… 48

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