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Basic Garbage Collection Techniques

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An Khuong
An Khuong
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[email_address] Basic Garbage Collection Techniques
Contents ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Tracing
1. Garbage Collection ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Explicit Storage Management Hazards ,[object Object],[object Object],[object Object],[object Object],[object Object]
Explicit Storage Management Hazards ,[object Object],[object Object],[object Object],[object Object]
GC Complexity ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Garbage Collection – The Two-Phase Abstraction ,[object Object],[object Object],[object Object],[object Object]
2. Reference Counting ,[object Object],[object Object],[object Object]
Reference Counting ,[object Object],[object Object],[object Object]
The Cycles Problem ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
The Efficiency Problem ,[object Object],[object Object],[object Object]
Deferred Reference Counting ,[object Object],[object Object],[object Object],[object Object]
Reference Counting - Recap ,[object Object],[object Object],[object Object],[object Object]
3. Mark-Sweep Collection - [McCarthy 1960] ,[object Object],[object Object],[object Object],[object Object],[object Object]
Example Mark phase Sweep phase Before Heap root root root Free_list
Example p p Live object Marked object Garbage Free list r1 Free list r1 Free list r1
Basic Algorithm New (A)= if free_list is empty mark_sweep() if free_list is empty return (“out-of-memory”) pointer = allocate(A) return (pointer) mark_sweep ()= for Ptr in Roots mark(Ptr) sweep() mark (Obj)= if mark_bit(Obj) == unmarked mark_bit(Obj)=marked for C in Children(Obj) mark(C) Sweep ()= p = Heap_bottom while (p < Heap_top) if (mark_bit(p) == unmarked) then free(p) else mark_bit(p) = unmarked; p=p+size(p)
Properties of Mark & Sweep ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Standard Engineering Techniques ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
4. Mark-Compact Collection Compact after mark ,[object Object],[object Object],[object Object]
Mark-Compact Collection ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Object Ordering ,[object Object],[object Object],[object Object],1 2 3 4 Linearizing 4 1 2 3 Arbitrary Worst 1 3 4 2 Good 1 2 3 4 Sliding Best
The Two Finger Algorithm [Edwards 1974] ,[object Object],[object Object],[object Object],[object Object]
Pass I: Compact ,[object Object],[object Object],[object Object],[object Object],[object Object]
Two finger, pass I - Example
Pass II: Fix Pointers ,[object Object],[object Object],[object Object],Compacted area free area
[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Two finger –Properties
The Lisp2 Algorithm ,[object Object],[object Object],[object Object],[object Object],Free
The Lisp2 Algorithm ,[object Object],[object Object],[object Object],Free
Lisp 2 –Properties ,[object Object],[object Object],[object Object],Slower: 3 passes. Extra space required –a pointer per object.
5. Copying Garbage Collection ,[object Object],[object Object],[object Object]
Stop-and-Copy Collector ,[object Object],[object Object],[object Object]
Cheney breath-first copying
The algorithm Init()= Tospace=Heap_bottom space_size=Heap_size/2 top_of_space=Tospace+space_size Fromspace=top_of_space+1 free=Tospace New(n)= If free+n>top_of_space Collect() if free+n>top_of_space abort“Memoryexhausted” new-object=free free=free+n return(new-object) collect()= from-space,to-space= to-space,from-space//swap scan=free=Tospace top_of_space=Tospace+space_size forRinRoots R=copy(R) whilescan<free forPinchildren(scan) *P=copy(P) scan=scan+size(scan) copy(P)= ifforwarded(P) returnforwarding_address(P) else addr=free mem-copy(P,free) free=free+size(P) forwarding_address(P)=addr return(addr) 1 2 4 3
Efficiency of Copying Collection ,[object Object],[object Object],[object Object],[object Object]
6. Non-Copying Implicit Collection ,[object Object],[object Object],[object Object],[object Object],[object Object]
7. Choosing Among Basic Tracing Techniques ,[object Object],[object Object],[object Object],[object Object],[object Object]
8. Problems with Simple Tracing Collectors
9. Conservatism in GC ,[object Object],[object Object],[object Object]

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Basic Garbage Collection Techniques

  • 1. [email_address] Basic Garbage Collection Techniques
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  • 15. Example Mark phase Sweep phase Before Heap root root root Free_list
  • 16. Example p p Live object Marked object Garbage Free list r1 Free list r1 Free list r1
  • 17. Basic Algorithm New (A)= if free_list is empty mark_sweep() if free_list is empty return (“out-of-memory”) pointer = allocate(A) return (pointer) mark_sweep ()= for Ptr in Roots mark(Ptr) sweep() mark (Obj)= if mark_bit(Obj) == unmarked mark_bit(Obj)=marked for C in Children(Obj) mark(C) Sweep ()= p = Heap_bottom while (p < Heap_top) if (mark_bit(p) == unmarked) then free(p) else mark_bit(p) = unmarked; p=p+size(p)
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  • 25. Two finger, pass I - Example
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  • 34. The algorithm Init()= Tospace=Heap_bottom space_size=Heap_size/2 top_of_space=Tospace+space_size Fromspace=top_of_space+1 free=Tospace New(n)= If free+n>top_of_space Collect() if free+n>top_of_space abort“Memoryexhausted” new-object=free free=free+n return(new-object) collect()= from-space,to-space= to-space,from-space//swap scan=free=Tospace top_of_space=Tospace+space_size forRinRoots R=copy(R) whilescan<free forPinchildren(scan) *P=copy(P) scan=scan+size(scan) copy(P)= ifforwarded(P) returnforwarding_address(P) else addr=free mem-copy(P,free) free=free+size(P) forwarding_address(P)=addr return(addr) 1 2 4 3
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  • 38. 8. Problems with Simple Tracing Collectors
  • 39.

Editor's Notes

  1. Ưu điểm: Ko cần cấp thêm vùng nhớ để lưu trữ trường count field Các object ko cần di chuyển trong quá trình GC Loại bỏ được rác trong trường hợp có tham chiếu vòng (cyclic structures) Nhược điểm: Giới hạn giá trị lớn nhất có thể của ô nhớ (integer) Tăng khả năng làm phân mảnh vùng nhớ Chương trình phải tạm dừng khi GC thực thi Giai đoạn sweep tốn chi phí cao(thời gian)
  2. Ưu điểm: Giảm khả năng phân mảnh vùng nhớ heap. Thời gian thực hiện công việc thì tỉ lệ với các live object Có thể xử lý được rác chứa các tham chiếu vòng Không tốn bộ nhớ để lưu trường count field Nhược điểm: Vùng nhớ dùng cho chương trình bị chia đôi(fromspace – tospace) Các object phải di chuyển trong vùng nhớ trong suốt quá trình thu gom rác, do vậy các tham chiếu đến các đối tượng cũng phải cập nhật. Chương trình phải tạm dừng khi bộ thu gom rác thực thi.