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
2. 2 Feature = configuration option Feature CONFIG_ACPI is scattered across the IA-64 CPU code
3. Hinders parallel development 3
5. Leads to code tangling, negatively affecting code understanding 5
6. Nonetheless, feature scattering is a popular mechanism to support new features 6
7. Quick solution understood by all developers 7
8. No modules, no interfaces, no design patterns, etc. 8
9. Allows overcoming modularity limitations in existing programming languages (not every feature can be modular) 9
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
11. However, no empirical study has investigated feature scattering in the evolution of large and long-lived systems 11
12. Such kind of studies are key in creating a general theory on how to effectively manage feature scattering 12
13. 13 Scattering How could a theory help? Scattering is harmful Scattering is not necessarily bad (easy & cheap solution)
14. Many empirical works have to be performed before devising such a theory 14
15. “A journey of a thousand miles must begin with the first step” 15
16. Starting point: the Linux kernel 16
17. 17 > 13,000 features feature-oriented system continuously evolving
18. 18 r = 0.996
19. 19 Scope: device-driver features
20. 20 In our analyses, we consider scattering of features in terms of referring ifdefs
21. Scattering degree (SD) of a feature f Nbr. of ifdefs referring to f 21
22. 22 #ifdef, #ifndef, #elif, #if ifdefs
23. 23 #ifdef CONFIG_ACPI || CONFIG_PM #ifndef CONFIG_ACPI #if defined(CONFIG_ACPI) ... A feature f is scattered if its SD(f) ≥ 2
24. From the kernel evolution history, some limits clearly emerge... 24
25. % of scattered features is nearly constant (~ 18%) 25
26. Local vs global scattering 26
27. A feature is locally scattered when its referring ifdefs are restricted to files in the driver subsystem only 27
28. A feature is globally scattered when there is at least one referring ifdef in a file outside the driver subsystem 28
29. Stabilization (~ 43%) 29 % of globally scattered features is increasing, but ≤ 43% at all times
30. What about the scattering degree of features? 30
32. For 50% (median) of scattered-driver features, SD ≤ 4 32
33. For 75%, SD ≤ 8 33
34. Non-outlier features: 8 < SD ≤ 55 34
35. 35 Outliers: 35 ≤ SD ≤ 377
36. There appears to exist different groups in Linux, with different SD-limits 36
37. Group 1 (low SD): SD ≤ 4 37 50% of scattered-driver features
38. 38 Group 2 (medium SD): 5 ≤ SD ≤ 8 25% of scattered-driver features
39. 39 Group 3 (high SD): SD > 8 Non-outliers: ~ 22.5% Outliers: ~ 2.5% (max SD = 377)
40. 40 … a single SD-limit controlling all features does not seem to apply
41. 41 In summary, …
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
43. 43 75% of scattered-driver features have SD ≤ 8 no more than 25% of features have SD > 8 (relative limit)
44. What about possible factors influencing the observed scattering? 44
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)
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
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)
48. 48 In general, there is no relationship between infrastructure-driver features and global-/local-scattering
49. 49 In general, there is no relationship between being a platform-driver or infrastructure feature in scattering degree
50. 50 There is, however, a relationship between extreme scattering and infrastructure-related driver features
51. 51 9/15 are infrastructure extreme scattering
52. 52 Wrapping up…
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”
54. 54 In the Linux kernel When the existing solutions are not good enough, developers scatter features in code
55. 55 In the Linux kernel Scattering seems to respect some limits (consciously enforced???)
56. 56 Next steps
57. 57 Conduct interviews Are the observed limits consciously enforced in practice? If so, how, and how were they set-up?
58. 58 If not, why do they occur? Do they indirectly stem from some development practice or process?
59. 59 Investigate whether the limits we found also apply to other systems (ongoing collaborative work)
60. 60 Thanks for listening :) http://lpassos.bitbucket.org/modularity15/