Pacer: Proportional Detection of Data Races

1. Michael Bond Katherine Coons Kathryn McKinley University ofTexas at Austin Pacer: Proportional Detection of Data Races

2. Detecting data races in production

4. Number of threads

5. Problem today Problem in future

6. Sampling rate

7. Sampling periods Non-sampling periods

9. Detect race  first access sampled

10. Thread A Thread B Non-sampling period Sampling period Non-sampling period Sampling period Non-sampling period

11. Thread A Thread B write x read y write y read x

12. Insight #1: • Stop tracking variable after • non-sampled access

13. Thread A write x unlock m Thread B

14. Thread A write x unlock m Thread B lock m

15. Thread A write x unlock m Thread B lock m write x

16. Thread A write x unlock m read x Thread B lock m write x

17. Thread A write x unlock m read x Thread B lock m write x Race!

18. Thread A write x unlock m read x Thread B lock m write x Race!

19. Thread A write x unlock m read x Thread B lock m write x A B A B Vector clocks 5 2 3 4

20. Thread A write x unlock m read x Thread B lock m write x A B A B Vector clocks 5 2 3 4

21. Thread A write x unlock m read x Thread B lock m write x A B A B Vector clocks 5 2 3 4

22. Thread A write x unlock m read x Thread B lock m write x A B A B 5 2 3 4

23. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 3 4

24. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 3 4 5 2

25. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 3 4 Increment clock 6 5 2 2

26. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 3 5 4 4 Join clocks 6 5 2 2

27. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 5 3 4 4 Happens before? 6 5 2 2

28. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 5 3 4 4 5 6 2 2

29. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 5 3 4 4 5 6 2 2

30. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 5 3 4 4 5 6 2 2 No work performed

31. Thread A write x unlock m read x Thread B lock m write x A B A B 5@A 5 2 5 3 4 4 5 6 2 2 Race uncaught

32. Thread A write x unlock m read x Thread B lock m write x A B A B 5 2 3 5 4 4 6 5 2 2 4@B

33. Thread A write x unlock m read x Thread B lock m write x A B A B 5 2 3 5 4 4 5 6 2 2 4@B Race! Happens before?

34. Insight #2: We only care whether • “A happens before B” • if A is sampled

35. Thread A Thread B Do these events happen before other events? We don’t care!

36. Thread A Thread B Don’t increment clocks Increment clocks Do these events happen before other events? We don’t care! Don’t increment clocks Increment clocks Don’t increment clocks

37. Thread A unlock m1 … unlock m2 Thread B lock m1 … lock m2 A B A B 5 2 3 4

38. Thread A unlock m1 … unlock m2 Thread B lock m1 … lock m2 A B A B No clock increment 5 2 5 3 5 4 4 4 5 2

39. Thread A unlock m1 … unlock m2 Thread B lock m1 … lock m2 A B A B 5 2 5 5 3 4 4 4 5 5 2 2

40. Thread A unlock m1 … unlock m2 Thread B lock m1 … lock m2 A B A B 5 2 3 5 5 4 4 4 5 5 2 2 Unnecessary join

41. Thread A unlock m1 … unlock m2 Thread B lock m1 … lock m2 A B A B 5 2 3 5 5 4 4 4 5 5 2 2 O(n) O(1)

42. Implementation http://jikesrvm.org/Research+Archive

43. Performance 1

44. Performance Qualitative improvement in time & space

45. Accuracy Probability (detecting any race) = r ?

46. Per-Race Accuracy(eclipse, r = 1%)

47. Related Work LiteRace[Marino et al. ’09] Cold-region hypothesis[Chilimbi & Hauswirth ’04] Full analysis at synchronization operations

48. Deployable Race Detection Accuracy, time, space  sampling rate Detect race  first access sampled

49. Deployable Race Detection Accuracy, time, space  sampling rate Detect race  first access sampled Qualitative improvement

50. Deployable Race Detection Accuracy, time, space  sampling rate Detect race  first access sampled Qualitative improvement Help developers fix difficult-to-reproduce bugs