Mark and sweep algorithm(garbage collector)

1. Mark – Sweep Algorithm
 first algorithm for automatic storage
reclamation (McCarthy 1960).
 a stop and run algorithm.
 tracing garbage collection technique
 works in 2 Phases:
– mark all live nodes by traversal.
– sweep the heap by a linear scan.
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2. Mark-Sweep Algorithm
Benefits:
a) No overhead on pointer manipulations.
b) Low space cost: using a simple mark-
field/bit.
Drawbacks:
a) Computation halted while gc
b) Every cell is visited during marking
c) Cells are again re-examined by sweep
d) Recursive marking (time and space!)
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3. Mark -- Sweep Algorithm
 Each block or its corresponding book keeping
must contain bit (mark bit)
 Initially all blocks are unmarked
 Starting at each symbol perform a search marking
all blocks reachable (mark means in-use)
 Sweep through all blocks.
– If marked: Unmark
– If unmarked: move to free list ( i.e free it )
 Note: Algorithm must be only thing running
 Garbage collection is only done when necessary
– i.e. When free list is empty
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4. Characteristics of Mark and Sweep
Algorithm
 Can place an upper bound on the total amount of
time required to perform complete GC. Use this
to pace GC against ongoing allocation needs.
 No “time intrusive” overheads to interfere with
calculation of worst-case task execution times.
 But cannot guarantee to avoid fragmentation.
4

5. Consider the following
Arrangement of nodes
Internally
X ()
foo blarg
()
bar baz
Y ()
foo bar baz
5

6. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
6

7. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
7

8. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
8

9. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
9

10. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
10

11. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
11

12. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
12

13. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
13

14. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
14

15. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
15

16. Free
Mark
Free List
X
() ()
Y
() () Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
16

17. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
17

18. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
18

19. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
19

20. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
20

21. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
21

22. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
22

23. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
23

24. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
24

25. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
25

26. Sweep
Free
Free List
X
() ()
Y
()
Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
26

27. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
27

28. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
28

29. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
29

30. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
30

31. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
31

32. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
32

33. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
33

34. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
34

35. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
35

36. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
36

37. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
37

38. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
38

39. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
39

40. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
40

41. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
41

42. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
42

43. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
43

44. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
44

45. Sweep
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
() 45

46. Done
Free
Free List
X
() ()
Y
() Here , X and Y correspond to
the root Pointers and the
block pointed to by “Free”
Corresponds to the Free List.
() 46

47. As Simple as ......
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