![[]
Root
@mami
'Leg' []
'Body' []
@mami = nil
@mami = ['Leg']
@mami[1] = ['Body']
@mami[1][1] = ['Head']
'Head'
@mami[1].pop #=> ['Head']
GC.start](https://image.slidesharecdn.com/rubysgc20-130531030714-phpapp02/75/Ruby-s-GC-20-16-2048.jpg)
![[]
Root
@mami
'Leg' []
'Body' []
'Head'
Mark phase
[]
'Leg' []
'Body'
mark
mark
mark
mark all live(reachable) objects](https://image.slidesharecdn.com/rubysgc20-130531030714-phpapp02/75/Ruby-s-GC-20-17-2048.jpg)
![[]
Root
@mami
'Leg' []
'Body' []
'Head'
Sweep phase
[]
'Leg' []
[]'Body'
unmark
unmark unmark
unmark
free
Free all dead(unreachable) objects
Unmark all marked objects](https://image.slidesharecdn.com/rubysgc20-130531030714-phpapp02/75/Ruby-s-GC-20-18-2048.jpg)
Uploaded byNarihiro Nakamura
Ruby's GC 20
The document discusses the implementation of Ruby's garbage collection mechanism, focusing on non-recursive and bitmap marking techniques. It describes how non-recursive marking avoids stack overflow issues, while bitmap marking improves memory efficiency, particularly for forked processes. The presentation concludes with the future plans for enhancing Ruby's garbage collection features.
More Related Content
Ruby's GC 20
- 1. #rubykaigi 03 nari/@nari3/authorNari Network AppliedCommunication Laboratory 2013/05/31 Ruby's GC 2.0
- 2. Self-introduction ➔nari, @nari3, authorNari ➔ACRuby committer ➔GC entertainer ➔“Nakamura” – is the most powerful clan in Ruby World
- 3. I went toCebu in Philippines ➔I studied English a month. ➔But I can't speak English....
- 4. Because I wasalways alone
- 5.
- 6. Today's Agenda ➔Non-recursive Marking ➔BitmapMarking – My work in Ruby 2.0.0
- 7.
- 8. GC collects all deadobjects
- 9. What is adead object?
- 10. What is adead object? ➔A dead object is an object that is no longer referenced by the program ➔In GC terms, we say a that dead object is unreachable from Roots
- 11. What is Roots? ➔Rootsis a set of pointers that directly reference objects in the program. – e.g. Ruby's local variables, etc..
- 12.
- 13. GC collects objects thatare unreachable from Roots.
- 14.
- 15. CRuby's GC ➔Mark&Sweep ➔Mark phase –mark all live(reachable) objects ➔Sweep phase – free up all dead(unreachable) objects – Unmark all marked objects
- 16. [] Root @mami 'Leg' [] 'Body' [] @mami= nil @mami = ['Leg'] @mami[1] = ['Body'] @mami[1][1] = ['Head'] 'Head' @mami[1].pop #=> ['Head'] GC.start
- 17. [] Root @mami 'Leg' [] 'Body' [] 'Head' Markphase [] 'Leg' [] 'Body' mark mark mark mark all live(reachable) objects
- 18. [] Root @mami 'Leg' [] 'Body' [] 'Head' Sweepphase [] 'Leg' [] []'Body' unmark unmark unmark unmark free Free all dead(unreachable) objects Unmark all marked objects
- 19. You can buya GC book at RubyKaigi!!
- 20.
- 21. Please don't throwthis away
- 22.
- 23. Introduction of Recursive Marking (atraditional way in CRuby)
- 24. Recursive Marking An objectgraph Machine Stack gc_mark() gc_mark() gc_mark() Frame gc_mark() gc_mark() gc_mark() Recursive call
- 25. A bad caseof a simple recursive call
- 26. Recursive Marking A deepobject graph gc_mark() gc_mark() gc_mark() Frame gc_mark() gc_mark() gc_mark() gc_mark() ・ ・ ・ Max Suddenly SEGV Overflow!! Machine Stack
- 27. In order toavoid a stack overflow, CRuby adopted ...
- 28.
- 29. What's a Knuth'sAlgorithm? ➔To avoid a stack overflow ➔There is a fail-safe system which consists of two stages. – Using a marking buffer – Rescanning all objects
- 30. Using a markingbuffer A gc_mark() gc_mark() Frame gc_mark() ・ ・ ・ Max Machine Stack B D E C F G ・・・ Marking buffer B C A CB push push Avoding overflow!!
- 31. Marking all objectsof the marking buffer at the end of the mark phaseA Frame gc_mark() Machine Stack B D E C F G ・・・ Marking buffer B C A CB gc_mark() rescan rescan D E F G
- 32. How do youdeal with an overflow of the marking buffer?
- 33. Rescanning all objects A B DE C F G ・・・ Marking buffer S O A CB overflow!! R A HIgnoring rescan rescan rescan D E F G It's very slow!!
- 34.
- 35. 1. fail-safe systemis slow ➔Rescanning is so slow. – If you have some deep object graphs, GC may be always slow with rescanning.
- 36. 2. We can'tprecisely check stack overflow ➔There is a trade-off between speed and precision. – Marking will be slow if we check stack overflow in each gc_mark(). – So we checked it at the appropriate time. – But, it's not precise.
- 37. 2. We can'tprecisely check stack overflow ➔This causes SEGV in the worst case scenario – For instance, Fiber sometimes fails unexpectedly. – Fiber uses small machine stack(128 KB) – At times, checking for stack overflows doesn't work well with Fiber.
- 38. So I decidedto say good bye to Knuth
- 39.
- 40. Non-recursive Marking ➔Marking w/othe machine stack – w/ own Array based stack ➔Recursive => Iterative
- 41. Rescanning all objects A B DE C F G Stack chunk B A CB F G mark C mark F mark G mark mark Marking stack
- 42. Allocating new a stackchunk A B D E C F G Marking stack X A CB F G X mark X X X X DStack chunk E Allocate!
- 43. Pros and Cons ➔Pros –Good-bye complex fail-safe systems – Good-bye SEGV! ➔Cons – Fast enough? – There is a risk of allocating a stack chunk during GC
- 44. mark benchmark OPTS="-r5" https://gist.github.com/authorNari/3806667 vm3_gc 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 origin non-recursive
- 45.
- 46. In fact, Ruby1.9.3 has backported this patch :-)
- 47. You can buya GC book at RubyKaigi!!
- 48.
- 49. Bitmap Marking inCRuby ➔Mark-bits separate from object headers – for CoW friendly ➔REE has adopted this approach – Since 2008 – But we can't import this patch
- 50. What's CopyOnWrite? (Unix) Process 1 (P1) Pagetable Memory Space fork() Process 2 (P2) Page table At first, P1 and P2 use same memory space.
- 51. What's CopyOnWrite? (Unix) Process 1 (P1) Pagetable Memory Space fork() Process 2 (P2) Page table write P1 private use P2 private use copy
- 52. If we havemany forked processes Process 1 Shared Process 2 P1 P2 Process 3 Process 4 ・・・ copy P3 P4 write ・・・ Increase memory usage of all forked processes
- 53. Marking in theold way … 16KB …Object … 16KB …Object ・ ・ ・ mb mb mb mb mb mb mb Ruby Heap HeapBlock 1 (HB) HeapBlock 2
- 54. GC.start … 16KB …Object …16KB …Object ・ ・ ・ Ruby Heap mb mb mb mb mb mb mb write write write write HeapBlock 1 (HB) HeapBlock 2
- 55. This Marking is CoWnot friendly!! Memory SpaceHB1 HB2 HB3 Process 1 Process 2 GC.start!! write write write copy HB2HB1 HB3
- 56. Bitmap Marking Mark-bits areseparated from the heap … 16KB …Object … 16KB …Object ・ ・ ・ mb mb mb mb mb mb mb Ruby Heap HeapBlock 1 HeapBlock 2 Bitmap header header
- 57. This Marking is CoWfriendly!! Memory SpaceHB1 HB2 HB3 Process 1 Process 2 GC.start!! write copy BM Bitmap Bitmap decrease!!
- 58.
- 59. e.g. Unicorn w/ markingin the old way Memory SpaceHB1 HB2 UP1(parent) UP2(child) GC.start!! read only write copy ・・・ Rails Rails app read/write app write write Rails Rails
- 60. e.g. Unicorn w/ BitmapMarking MemorySpaceHB1 HB2 UP1(parent) UP2(child) GC.start!! read only write copy ・・・ Rails Rails app read/write app write Rails Bitmap Bitmap
- 61. How do youfind an appropriate bit in Bitmap?
- 62. Finding an appropriate bitfor an object … 16KB …HB 1 mark Bitmap Header 16KB align (low 13 bits must be 0) Allocate a heap block using memory align & ~0x3fff HB1 mark …
- 63. How do youallocate aligned memory?
- 64. Allocating aligned memory ➔Usingposix_memalign() – For Unix-like OS ➔Using _aligned_malloc() – For Windows OS – mingw: __mingw_aligned_malloc()
- 65. Allocating aligned memory ➔Usingmalloc() ● Thanks to yugui-san's help! – For other environments – For instance, Max OS X Lion and so on – It allocates 32KB and returns an address which is a multiple of 16KB ● 16KB memory space is wasted ● We should use mmap(), but ....
- 66. The structure of RubyHeap was changed.
- 67. The structure ofHeap in Ruby 1.8 Object heaps Object heap block
- 68. Object Object heap block header header slot ・ ・ ・ ・ ・ ・ The structureof Heap in Ruby 2.0 heaps slot freelist freelist Each slot has a freelist
- 69.
- 70. skkzipcode https://github.com/authorNari/skkzipcode shared memory privatememory 0 50 100 150 200 250 origin bmap (MB)
- 71. You can buya GC book at RubyKaigi!!
- 72.
- 73. Other plans ➔Introduce newobj_(alloc/free) events to TracePont. ➔mmap()/munmap()
- 74.
- 75.
- 76. Conclusion ➔I implemented Non-recursive Markingand Bitmap Marking. ➔Rgengc is so cooooool!
- 77.