The document discusses object graphs in object-oriented programming and the problem of referenced but unused objects consuming memory. It proposes "swapping" unused objects to disk rather than removing them, to avoid memory issues. This is done by grouping unused objects into "subgraphs" and replacing the roots with proxies. When an object is needed again, it can be loaded back from disk. The key aspects involve identifying used/unused objects at runtime, using proxies to represent swapped objects, and grouping objects intelligently into subgraphs to swap.
8. Problem
Use more memory than needed.
Make OOP languages unsuitable for memory
limited devices.
Existence of unused but referenced objects.
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9. The context
In OOP primary memory is represented by an object graph
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X
Y
A B C
D E F G H
I J K L
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10. Garbage Collector
Only collects objects that nobody else points to.
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X
Y
A B C
D E F G H
I J K L
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11. Garbage Collector
Only collects objects that nobody else points to.
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X
Y
A B C
D E F G H
I J K L
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12. Garbage Collector
Only collects objects that nobody else points to.
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X
Y
A B C
D E F G H
I J K L
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13. Garbage Collector
Only collects objects that nobody else points to.
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X
Y
A B C
D E F G H
I J K L
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14. But...what happens with referenced yet unused objects?
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15. Idea
Swap out (not remove) unused objects to
disk.
Automatically load them back when needed.
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16. Related work
Large object oriented memory (LOOM).
Melt - Supporting memory leaks.
ImageSegments.
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18. Main challenges
Not to use more memory than the one
released by swapping.
Low overhead penalty.
Group objects in an smart way.
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19. Key aspects
Mark and trace unused/used objects at
runtime.
The usage of proxies.
Group unused objects (subgraphs).
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20. Why we need to
group objects?
Because if we replace each object by a proxy,
we release little memory.
We want to replace a whole group by one or a
few proxies.
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21. Why subgraphs?
Group of objects that are used (or not used)
together.
We need few proxies (for the roots) for
several objects.
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22. Experiments done
Modify Smalltalk VM to mark and trace
objects usage.
Visualize objects and memory usage.
Take statistics from different scenarios.
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23. deployed web
application example
Used Unused Used Unused
Amount of objects Amount of memory
4%
15%
85%
96%
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24. Swapping steps and
challenges
1.Identify sets of objects and serialize them.
Problems: cycles, speed, etc.
2.Write the serialized objects into a file.
Problems: file format, encoding, speed, etc.
3. Load the objects from a file. Problems: class
reshape, avoid duplicates, speed, etc.
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25. Subgraphs
Roots Inner Shared External
X Z
Y
Subgraph to process
A B C
(Root) (Root) (Root)
D E F G H
I J K L
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26. More
problems
Should shared objects be
Roots Inner Shared External
included or not?
GC moves objects.
X Z
Y
Subgraph to process Pointers update.
A B C
(Root) (Root) (Root)
Class changes.
D E F G H Recreate and reinitialize
objects.
I J K L
Code executed after
18 loading.
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28. ImageSegment basis
Only write/swap roots and inner objects.
Shared objects are NOT swapped.
Keep an array in memory for the shared
objects.
Update object pointers to point to a relative
address inside the arrays (offset).
Roots are replaced by proxies.
Uses GC facilities to detect shared objects.
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29. Subgraph traverse
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
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30. Subgraph traverse
A’ B’ C’
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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31. Subgraph traverse
A’ B’ C’
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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32. Subgraph traverse
A’ B’ C’
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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33. Subgraph traverse
A’ B’ C’
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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34. Subgraph traverse
A’ B’ C’
D
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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35. Subgraph traverse
A’ B’ C’
D
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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36. Subgraph traverse
A’ B’ C’
D
Serialized objects WordArray
offset Shared objects Array
A B C
D E F G H
I J K L
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37. Subgraph traverse
A’ B’ C’ E’
D
Serialized objects WordArray
offset Shared objects Array
A B C
D E F G H
I J K L
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38. Subgraph traverse
offset
A’ B’ C’ E’
D
Serialized objects WordArray
offset Shared objects Array
A B C
D E F G H
I J K L
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39. Subgraph traverse
A’ B’ C’ E’
D
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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40. Subgraph traverse
A’ B’ C’ E’
D
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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41. Subgraph traverse
A’ B’ C’ E’ I’ H’ L’
D F G J
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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42. Offset Memory address
A’ B’ C’ E’ I’ H’ L’
D F G J
Serialized objects WordArray
Shared objects Array
A B C
D E F G H
I J K L
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43. anImageSegment
A’ B’ C’ E’ I’ H’ L’ D F G J
Serialized objects WordArray Shared objects Array
A B C
D E F G H
I J K L
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44. anImageSegment
A’ B’ C’ E’ I’ H’ L’ D F G J
Serialized objects WordArray Shared objects Array
P1 P2 P3
D E F G H
I J K L
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45. anImageSegment
A’ B’ C’ E’ I’ H’ L’ D F G J
Serialized objects WordArray Shared objects Array
P1 P2 P3
D E F G H
I J K L
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46. anImageSegment
A’ B’ C’ E’ I’ H’ L’ D F G J
Serialized objects WordArray Shared objects Array
P1 P2 P3
D F G
J K
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47. anImageSegment
D F G J
A’ B’ C’ E’ I’ H’ L’
Serialized objects WordArray Shared objects Array
Binary file
P1 P2
B P3
C
D F G
J K
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48. anImageSegment
D F G J
Shared objects Array
Binary file
A’ B’ C’ E’ I’ H’ L’ P1 P2
B P3
C
D F G
J K
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49. anImageSegment
NIL D F G J
Shared objects Array
Binary file
A’ B’ C’ E’ I’ H’ L’ P1 P2
B P3
C
D F G
J K
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50. ImageSegment
conclusions
Good speed.
Graph traverse is done in VM side.
Good use of GC facilities.
You have to be aware of shared objects.
Bad granularity level.
Implicit needed information in object graphs.
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51. Thanks!
Mariano Martinez Peck
marianopeck@gmail.com
Wednesday, September 15, 2010
