This document summarizes research on recovering token-level authorship information from Git. It analyzes the Linux kernel source code repository to determine statistics on code changes at the token level, such as the percentage of commits that modify small amounts of code. The analysis finds that while line-level and token-level authorship information are equivalent for large changes, tokens provide a finer-grained view of small code changes and the evolution of the code.

1. cregit:
Who Authored the Kernel?
Recovering Token-Level Authorship
Information from Git
Daniel M German
University of Victoria
dmg@uvic.ca
Kate Stewart
Linux Foundation
Bram Adams
Polytecnique of Montreal

3. Cincinnati Library
Image in the public domain

4. diff-able

5. Image in the public domain

8. “History is writen by
the winners”
By Y. Karsh. Image in the public domain in
Canada. Copyrighted in the US

10. “Archeology is the search for fact, not truth.
If it's truth you're interested in,
Dr. Tyree's Philosophy class is right down the hall.”
-- Indiana Jones
Image Copyright Walt Disney Company

11. The history in git is likely to be incomplete

12. Yet, what can we do with it?

13. The Dream, by H. Rousseau. In the public domain.

24. Image in the public domain

25. Evolutionary Views of VC Repos

31. Per Line
Per Token

34. Linux History

35. Warning
● The author (git parlance) of a commit is not
necessarily the author of the code
– Code imported from another source
– Refactorings
– Moving code

36. Up to 4.7
Persons in blame:
Line: 12,005
Token: 12,087

43. Token Line
Linux

44. Token Line
kernel/

45. Many small changes
Non-merges that modified C and H files with respect to total of all commits
● 9.5 % of commits added 3 or less c-tokens and removed 3 or less c-
tokens
● 7% of commits did not add any c-tokens but removed c-tokens
● 3.8% of commits added one c-token and removed one c-token
● 22.4% of commits added 10 or less c-tokens and removed 10 or less c-
tokens
● 50% of commits added 60 or less c-tokens and removed 60 or less c-
tokens
● 2 commits added at least 1M c-tokens and removed at least 1M c-
tokens

46. C-Churn
● Churn = C Tokens added – C tokens removed in non-
merge commits
Non-merges that modified C and H files with respect to total
of all commits
● 10% of commits had c-churn == 0
● 48% had c-churn <= 10
● 26% had negative c-churn
● 2 commits had c-churn >= 1M

47. Conclusion
● On the large
– Token and Line are
equivalent
● On the small
– Provide a fine grained
view of the evolution
of the code