Ruby's GC 20
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I will talk about some improvements of GC in Ruby 2.0.0. For instance, I will introduce about implementations of Bitmap Marking GC and so on, and show results of benchmarks after these are implemented. Animation version is here: https://gumroad.com/l/xWCR (premium version)
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Ruby's GC 20
- 1. #rubykaigi 03 nari/@nari3/authorNari Network Applied Communication Laboratory 2013/05/31 Ruby's GC 2.0
- 2. Self-introduction ➔nari, @nari3, authorNari ➔A CRuby committer ➔GC entertainer ➔“Nakamura” – is the most powerful clan in Ruby World
- 3. I went to Cebu in Philippines ➔I studied English a month. ➔But I can't speak English....
- 4. Because I was always alone
- 6. Today's Agenda ➔Non-recursive Marking ➔Bitmap Marking – My work in Ruby 2.0.0
- 7. What is GC ?
- 8. GC collects all dead objects
- 9. What is a dead object?
- 10. What is a dead 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? ➔Roots is a set of pointers that directly reference objects in the program. – e.g. Ruby's local variables, etc..
- 12. What is GC ?
- 13. GC collects objects that are unreachable from Roots.
- 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' Mark phase [] 'Leg' [] 'Body' mark mark mark mark all live(reachable) objects
- 18. [] Root @mami 'Leg' [] 'Body' [] 'Head' Sweep phase [] 'Leg' [] []'Body' unmark unmark unmark unmark free Free all dead(unreachable) objects Unmark all marked objects
- 19. You can buy a GC book at RubyKaigi!!
- 21. Please don't throw this away
- 23. Introduction of Recursive Marking (a traditional way in CRuby)
- 24. Recursive Marking An object graph Machine Stack gc_mark() gc_mark() gc_mark() Frame gc_mark() gc_mark() gc_mark() Recursive call
- 25. A bad case of a simple recursive call
- 26. Recursive Marking A deep object 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 to avoid a stack overflow, CRuby adopted ...
- 29. What's a Knuth's Algorithm? ➔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 marking buffer 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 objects of 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 you deal with an overflow of the marking buffer?
- 33. Rescanning all objects A B D E C F G ・・・ Marking buffer S O A CB overflow!! R A HIgnoring rescan rescan rescan D E F G It's very slow!!
- 35. 1. fail-safe system is slow ➔Rescanning is so slow. – If you have some deep object graphs, GC may be always slow with rescanning.
- 36. 2. We can't precisely 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't precisely 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 decided to say good bye to Knuth
- 40. Non-recursive Marking ➔Marking w/o the machine stack – w/ own Array based stack ➔Recursive => Iterative
- 41. Rescanning all objects A B D E C F G Stack chunk B A CB F G mark C mark F mark G mark mark Marking stack
- 42. Allocating new a stack chunk 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="-r 5" 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
- 46. In fact, Ruby 1.9.3 has backported this patch :-)
- 47. You can buy a GC book at RubyKaigi!!
- 48. Bitmap Marking
- 49. Bitmap Marking in CRuby ➔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) Page table 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) Page table Memory Space fork() Process 2 (P2) Page table write P1 private use P2 private use copy
- 52. If we have many 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 the old 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 CoW not friendly!! Memory SpaceHB1 HB2 HB3 Process 1 Process 2 GC.start!! write write write copy HB2HB1 HB3
- 56. Bitmap Marking Mark-bits are separated 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 CoW friendly!! Memory SpaceHB1 HB2 HB3 Process 1 Process 2 GC.start!! write copy BM Bitmap Bitmap decrease!!
- 59. e.g. Unicorn w/ marking in 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 Memory SpaceHB1 HB2 UP1(parent) UP2(child) GC.start!! read only write copy ・・・ Rails Rails app read/write app write Rails Bitmap Bitmap
- 61. How do you find an appropriate bit in Bitmap?
- 62. Finding an appropriate bit for 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 you allocate aligned memory?
- 64. Allocating aligned memory ➔Using posix_memalign() – For Unix-like OS ➔Using _aligned_malloc() – For Windows OS – mingw: __mingw_aligned_malloc()
- 65. Allocating aligned memory ➔Using malloc() ● 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 Ruby Heap was changed.
- 67. The structure of Heap in Ruby 1.8 Object heaps Object heap block
- 68. Object Object heap block header header slot ・ ・ ・ ・ ・ ・ The structure of Heap in Ruby 2.0 heaps slot freelist freelist Each slot has a freelist
- 69. Benchmark
- 70. skkzipcode https://github.com/authorNari/skkzipcode shared memory private memory 0 50 100 150 200 250 origin bmap (MB)
- 71. You can buy a GC book at RubyKaigi!!
- 72. Future
- 73. Other plans ➔Introduce new obj_(alloc/free) events to TracePont. ➔mmap()/munmap()
- 75. Conclusion
- 76. Conclusion ➔I implemented Non-recursive Marking and Bitmap Marking. ➔Rgengc is so cooooool!
- 77. Thank you!