Visualizing Garbage Collection in Rubinius, JRuby and Ruby 2.0

In this talk we’ll dive into Ruby internals and take a close look at an ugly topic: garbage collection. How do these Ruby VM’s allocate memory for new objects? How do they identify unused objects? How do they reclaim memory from garbage objects, allowing it to be used again?

You can learn a lot about someone from their garbage. Using a series of diagrams, we’ll visually compare and contrast the complex algorithms these very different Ruby implementations use. What computer science research is behind each garbage collector? We’ll also look at the GC changes planned for the upcoming Ruby 2.1 release.

Transcript

2. allocate identify reclaim
5. class Node def initialize(val) @value = val end end p Node.new(1) => #<Node:0x007fc72a17e5f0 @value=1> p Node.new(2) => #<Node:0x007fc72a17e4b0 @value=2>
6. allocate identify reclaim generational
8. n1 "ABC" n1 = Node.new("ABC")
9. n2 = Node.new("DEF") n1 "ABC" n2 "DEF"
10. n1 "ABC" n3 = Node.new("GHI") n3 "GHI" n2 "DEF"
11. Courtesy MIT Museum
12. Courtesy MIT Museum Mike Bernstein? from Code Climate
13. Courtesy MIT Museum John McCarthy
16. n1 = Node.new("ABC") n1 next "ABC"
17. n2 = Node.new("DEF") next n1 "ABC" n2 "DEF"
18. n3 = Node.new("GHI") next n1 "ABC" n2 "DEF" n3 "GHI"
19. n1 "ABC" n1 = Node.new("ABC") Node klass @value (RObject) (RString) (RClass) RVALUE
20. M n1 = Node.new("More than 23 chars here...") (RString) o r e t h a n etc...
21. n1 = Node.new("More than 23 chars here...") n1 next (RubyObject) Node (RubyClass) (RubyString) "More than 23 chars here..." (ByteArray)
22. allocate identify reclaim generational
23. n1 "ABC" n3 = Node.new("GHI") n3 "GHI" n2 "DEF"
24. n3 "GHI" n2 "DEF" "ABC" n1 "JKL" n1 = Node.new("JKL")
25. n3 "GHI" n2 "DEF" "ABC" n1 "MNO" "JKL" n1 = Node.new("MNO")
26. n3 "GHI" n2 "DEF" "ABC" "JKL" "MNO" n1 "PQR" n1 = Node.new("PQR")
28. n2 n3 n1
29. stop the world marking
30. n2 n3 n1 M M M
31. n2 n3 n1 1 1 1 0 0 0 0 0
32. n2 n3 n1 M M M
33. M M M M M M
34. concurrent marking
35. M M M M Collector
36. M M M M Collector New Object "Mutator"
37. M M M M not garbage! M M
40. Root Objects Remaining Objects Collector
41. Mark Stack Remaining Objects Collector M Marked Objects
42. M M Garbage Objects Live Objects M M
43. M New Object Mark Stack Remaining Objects Marked Objects Collector "Mutator"
45. M M New Object "Mutator" write barrier
46. allocate identify reclaim generational
47. n2 n3 n1 M M M
48. n2 n3 n1 M M M
49. M M M M M From-Space To-Space
50. M M M M M next allocation From-Space To-Space
51. next allocation From-Space To-Space
53. Eden Heap next allocation From-Space To-Space
54. allocate identify reclaim generational
55. M M M M M next allocation From-Space To-Space
56. n1 = Node.new("ABC")
57. weak generational hypothesis
58. From-Space To-Space promoted promoted
59. Young Generation Mature Generation ?
60. jruby -J-XX:+UseConcMarkSweepGC jruby -J-XX:+UseParallelGC jruby -J-XX:+UseSerialGC jruby -J-XX:+UseG1GC
64. MRI Ruby 2.1
65. Young Objects Mature Objects
66. Young Objects Mature Objects
67. Always marked Not marked again until a full GC
68. ? live or garbage? Young Mature
69. Write Barrier ? live or garbage? Young Mature