Lies, Damn Lies, and Metrics (Distill 2014)

Metrics are great, and measuring things can provide tremendously useful insights. But there's a problem: metrics lie to you. Metrics just report the numbers that were measured. Analyzing those numbers is up to us, and that analysis can go wrong in so, so many ways. Learn how to arm yourself against human intuition, interpreter pauses, routing, instrumentation lag, and other issues. Don't get so caught up in instrumenting that you lose sight of why metrics exist! Make sure your metrics are telling you actionable information, instead of just accurate numbers.

Transcript

1. Lies, Damn Lies,
   and Metrics
   

   View Slide

2. André Arko
   @indirect
   

   View Slide

3. View Slide

4. Bundler
   

   View Slide

5. Metrics
   

   View Slide

6. Metrics
   are important
   

   View Slide

7. Metrics
   tell you what
   is happening
   

   View Slide

8. 
   you rn →
   

   View Slide

9. Metrics
   convince you
   you understand
   

   View Slide

10. 
    you later →
    

    View Slide

11. Averages
    convince you
    you understand
    

    View Slide

12. Averages
    are lie-candy
    for your brain
    

    View Slide

13. “Normal”
    5
    -5 -4 -3 -2 -1 0 1 2 3 4
    0
    0.1
    0.2
    0.3
    0.4
    

    View Slide

14. “Normal”
    5
    -5 -4 -3 -2 -1 0 1 2 3 4
    0
    0.1
    0.2
    0.3
    0.4
    

    View Slide

15. Real Life
    5
    -5 -4 -3 -2 -1 0 1 2 3 4
    0
    0.1
    0.2
    0.3
    0.4
    

    View Slide

16. brendangregg.com
    

    View Slide

17. brendangregg.com
    

    View Slide

18. 
    just heard
    “w
    e
    have
    a
    great average” →
    

    View Slide

19. Averages
    mask problems
    

    View Slide

20. 10
    0 1 2 3 4 5 6 7 8 9
    250
    0
    50
    100
    150
    200
    

    View Slide

21. Graph
    the median
    

    View Slide

22. 10
    0 1 2 3 4 5 6 7 8 9
    250
    0
    50
    100
    150
    200
    

    View Slide

23. Graph
    95th percentile
    

    View Slide

24. 10
    0 1 2 3 4 5 6 7 8 9
    250
    0
    50
    100
    150
    200
    

    View Slide

25. Graph
    99th percentile
    

    View Slide

26. 10
    0 1 2 3 4 5 6 7 8 9
    1000
    0
    250
    500
    750
    

    View Slide

27. Aggregate graphs
    another average
    

    View Slide

28. View Slide

29. Breakout graphs
    show each source
    

    View Slide

30. View Slide

31. Aggregate alerts
    more dead servers
    than alive servers
    

    View Slide

32. 
    site’s up if any
    servers are up!
    

    View Slide

33. Breakout alerts
    first dead server
    not all the servers
    

    View Slide

34. Servers
    

    View Slide

35. Servers
    you have no idea
    what is going on
    

    View Slide

36. really.
    

    View Slide

37. Runtime lag
    

    View Slide

38. Runtime lag
    how do you tell you
    lost consciousness?
    

    View Slide

39. Runtime lag
    you have it.
    

    View Slide

40. Runtime lag
    you have it.
    how bad is it?
    

    View Slide

41. VM lag
    

    View Slide

42. VM lag
    do you have it?
    

    View Slide

43. VM lag
    do you check for it?
    

    View Slide

44. VM lag
    do you know how
    to check for it?
    

    View Slide

45. Routing
    

    View Slide

46. Routing
    your app has this
    

    View Slide

47. Routing
    how does it work?
    

    View Slide

48. Development
    App
    You
    

    View Slide

49. Production
    People Router
    Server
    App
    App
    Router
    Server
    App
    App
    Router
    

    View Slide

50. Routing
    how slow is it?
    

    View Slide

51. Routing
    does it back up?
    

    View Slide

52. Request time
    

    View Slide

53. Request time
    not the time
    you measure
    

    View Slide

54. Request time
    wall-clock time
    from real clients
    

    View Slide

55. Request time
    make requests from
    around the world
    

    View Slide

56. metrics are good
    So, in the end
    

    View Slide

57. know what you
    are measuring
    So, in the end
    

    View Slide

58. @indirect
    andre@arko.net
    Questions?
    

    View Slide