Feature Scattering in the Large: A Longitudinal Study of Linux Kernel Device Drivers (Modularity 2015) - Best Paper Award (slides and presentation by L. Passos)

Feature code is often scattered across wide parts of the code base. But, scattering is not necessarily bad if used with care—in fact, systems with highly scattered features have evolved successfully over years. Among others, feature scattering allows developers to circumvent limitations in programming languages and system design. Still, little is known about the characteristics governing scattering, which factors influence it, and practical limits in the evolution of large and long-lived systems. We address this issue with a longitudinal case study of feature scattering in the Linux kernel. We quantitatively and qualitatively analyze almost eight years of its development history, focusing on scattering of device-driver features. Among others, we show that, while scattered features are regularly added, their proportion is lower than non-scattered ones, indicating that the kernel architecture allows most features to be integrated in a modular manner. The median scattering degree of features is constant and low, but the scattering-degree distribution is heavily skewed. Thus, using the arithmetic mean is not a reliable threshold to monitor the evolution of feature scattering. When investigating influencing factors, we find that platform-driver features are 2.5 times more likely to be scattered across architectural (subsystem) boundaries when compared to nonplatform ones. Their use illustrates a maintenance-performance trade-off in creating architectures as for Linux kernel device drivers

Transcript

1. Feature Scattering in the Large: A Longitudinal
   Study of Linux Kernel Device Drivers
   Leonardo Passos
   lpassos@gsd.uwaterloo.ca
   University of Waterloo
   Canada
   Modularity’15 Research Track
   1
   Krzysztof Czarnecki
   kczarnec@gsd.uwaterloo.ca
   University of Waterloo
   Canada
   Thorsten Berger
   jpadillagaeta@gsd.uwaterloo.ca
   University of Waterloo
   Canada
   Sven Apel
   apel@uni-passau.de
   University of Passau
   Germany
   Jesús Padilla
   jpadillagaeta@gsd.uwaterloo.ca
   University of Waterloo
   Canada
   Marco Tulio Valente
   mtov@dcc.ufmg.br
   Federal University of Minas Gerais
   Brazil
   

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2. 2
   Feature = configuration option
   Feature CONFIG_ACPI is
   scattered across the
   IA-64 CPU code
   

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3. Hinders parallel development
   3
   

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5. Leads to code tangling, negatively affecting code
   understanding
   5
   

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6. Nonetheless, feature scattering is a popular
   mechanism to support new features
   6
   

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7. Quick solution understood by all developers
   7
   

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8. No modules, no interfaces, no design patterns, etc.
   8
   

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9. Allows overcoming modularity limitations in existing
   programming languages
   (not every feature can be modular)
   9
   

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10. 10
    Many large & long-lived software systems have shown that is possible
    to continuously-evolve in the face of feature scattering
    axTLS
    Coreboot SeaBIOS
    FreeBSD
    

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11. However, no empirical study has investigated
    feature scattering in the evolution of large
    and long-lived systems
    11
    

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12. Such kind of studies are key
    in creating a general theory on how to
    effectively manage feature scattering
    12
    

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13. 13
    Scattering
    How could a theory help?
    Scattering is harmful
    Scattering is not
    necessarily bad
    (easy & cheap solution)
    

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14. Many empirical works have to be performed
    before devising such a theory
    14
    

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15. “A journey of a thousand miles must begin
    with the first step”
    15
    

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16. Starting point: the Linux kernel
    16
    

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17. 17
    > 13,000 features
    feature-oriented system
    continuously evolving
    

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18. 18
    r = 0.996
    

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19. 19
    Scope: device-driver features
    

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20. 20
    In our analyses, we consider scattering of
    features in terms of referring ifdefs
    

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21. Scattering degree (SD)
    of a feature f
    Nbr. of ifdefs
    referring to f
    21
    

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22. 22
    #ifdef, #ifndef, #elif, #if
    ifdefs
    

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23. 23
    #ifdef CONFIG_ACPI || CONFIG_PM
    #ifndef CONFIG_ACPI
    #if defined(CONFIG_ACPI)
    ...
    A feature f is scattered if its SD(f) ≥ 2
    

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24. From the kernel evolution history,
    some limits clearly emerge...
    24
    

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25. % of scattered features is nearly constant
    (~ 18%)
    25
    

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26. Local vs global scattering
    26
    

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27. A feature is locally scattered when its referring
    ifdefs are restricted to files in the
    driver subsystem only
    27
    

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28. A feature is globally scattered when there is
    at least one referring ifdef in a file outside
    the driver subsystem
    28
    

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29. Stabilization
    (~ 43%)
    29
    % of globally scattered features is increasing,
    but ≤ 43% at all times
    

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30. What about the scattering degree
    of features?
    30
    

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32. For 50% (median) of scattered-driver
    features, SD ≤ 4
    32
    

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33. For 75%, SD ≤ 8
    33
    

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34. Non-outlier features: 8 < SD ≤ 55
    34
    

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35. 35
    Outliers: 35 ≤ SD ≤ 377
    

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36. There appears to exist different groups
    in Linux, with different SD-limits
    36
    

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37. Group 1 (low SD): SD ≤ 4
    37
    50% of scattered-driver features
    

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38. 38
    Group 2 (medium SD): 5 ≤ SD ≤ 8
    25% of scattered-driver features
    

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39. 39
    Group 3 (high SD): SD > 8
    Non-outliers: ~ 22.5%
    Outliers: ~ 2.5% (max SD = 377)
    

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40. 40
    … a single SD-limit controlling all features
    does not seem to apply
    

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41. 41
    In summary, …
    

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42. 42
    % of scattered-driver features ~ 18%
    % of globally scattered-driver features ≤ 43%
    SD is not defined by a single absolute value,
    although most features (75%) have SD ≤ 8
    

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43. 43
    75% of scattered-driver features have SD ≤ 8
    no more than 25% of features
    have SD > 8 (relative limit)
    

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44. What about possible factors influencing
    the observed scattering?
    44
    

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45. Platform-driver features: features whose
    drivers support devices that cannot be
    discovered by the CPU
    45
    Infrastructure-driver features: abstractions
    in the O.S domain (e.g., ACPI)
    

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46. The analyses of a random-sample
    shows statistically significant results :
    46
    Platform-driver features are 2.5x more
    likely to being globally scattered than
    non-platform ones
    

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47. 47
    In the sample, global scattering of
    platform-driver features occurs
    mostly in the arch subsystem
    Tight relationship between platform-
    driver features and CPU-dependent code
    (hard to modularize)
    

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48. 48
    In general, there is no relationship
    between infrastructure-driver features
    and global-/local-scattering
    

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49. 49
    In general, there is no relationship between
    being a platform-driver or infrastructure
    feature in scattering degree
    

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50. 50
    There is, however, a relationship
    between extreme scattering and
    infrastructure-related driver features
    

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51. 51
    9/15 are
    infrastructure
    extreme scattering
    

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52. 52
    Wrapping up…
    

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53. 53
    In the Linux kernel
    Most driver features are not scattered (~ 82%)
    C-language modularity constructs + the kernel
    plugin-based architecture are “good enough”
    

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54. 54
    In the Linux kernel
    When the existing solutions are not good
    enough, developers scatter features in code
    

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55. 55
    In the Linux kernel
    Scattering seems to respect some limits
    (consciously enforced???)
    

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56. 56
    Next steps
    

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57. 57
    Conduct interviews
    Are the observed limits consciously
    enforced in practice?
    If so, how, and how were they set-up?
    

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58. 58
    If not, why do they occur?
    Do they indirectly stem from some
    development practice or process?
    

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59. 59
    Investigate whether the limits we found
    also apply to other systems
    (ongoing collaborative work)
    

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60. 60
    Thanks for listening
    :)
    http://lpassos.bitbucket.org/modularity15/
    

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