Incremental Garbage Collection

1. Incremental Garbage Collection Uwe Kern 23. Januar 2002 kern@cs.uni-sb.de

2. Incremental GC • Mutator runs while collecting is performed • Collecting is done in small, partial steps • Drastically reduced pause times • Extension of existing tracing GC's

3. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

4. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

5. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

6. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

7. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

8. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

9. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

10. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

11. A B Cells & GC state C completed found unvisited/garbage Problem: Intertwined mutation Root

12. Solution: Synchronization • Idea: • Intercept access to or modification of references in objects (barrier) • Do something in barrier to prevent race conditions

13. Example Barrier Which kind of mutations are fatal ? Reference to white object X written into one or more black objects. All references from white/grey objects to X deleted.

14. Example Barrier Which kind of mutations are fatal ? Reference to white object X written into one or more black objects. All references from white/grey objects toXdeleted. Idea: Eliminate black-white pointers by coloring black object grey again

15. Example Barrier Root A B C

16. Example Barrier Root A B C

17. Example Barrier Root A B C

18. Example Barrier Root A B C

19. Example Barrier Root A B C

20. Example Barrier Root A B C

21. Example Barrier Root A B C

22. Example Barrier Root A B C

23. Example Barrier Root A B C

24. Example Barrier Root A B C

25. Example Barrier Root A B C

26. Example Barrier Root A B C

27. Example Barrier Root A B C

28. Example Barrier Root A B C

29. Example Barrier Root A B C

30. Example Barrier Root A B C

31. Example Barrier Root A B C

32. Example Barrier Root A B C

33. Example Barrier Root A B C

34. Example Barrier Root A B C

35. Example Barrier Root A B C

36. Example Barrier Root A B C

37. Example Barrier Root A B C

38. Example Barrier Root A B C

39. Example Barrier Root A B C

40. Incremental GC Properties

41. Incremental GC Properties • Collector sees conservative approximation of reachability graph

42. Incremental GC Properties • Collector sees conservative approximation of reachability graph • Conservatism leads to floating garbage

43. Incremental GC Properties • Collector sees conservative approximation of reachability graph • Conservatism leads to floating garbage • Strong approximations decrease incrementality

44. Quality Criterias

45. Quality Criterias • Conservatism: level of floating garbage produced

46. Quality Criterias • Conservatism: level of floating garbage produced • Incrementality: more fine-grained is better

47. Quality Criterias • Conservatism: level of floating garbage produced • Incrementality: more fine-grained is better • Mutator overhead: processor and memory costs

48. Concurrent Mark-Sweep

49. Concurrent Mark-Sweep • [Steele, 1975]

50. Concurrent Mark-Sweep • [Steele, 1975] • mutator and collector running parallel