Manual memory management is error-prone, but garbage collection (GC) frees programmers from this task by automatically freeing unused memory. GC determines unreachable objects using transitive closure from root objects, then frees them. Common GC algorithms include reference counting, mark and sweep, and generational collection, with tradeoffs between overhead and ability to handle different memory usage patterns.

2. The Purpose of Garbage Collection
Manual memory management is too tiresome and error-prone.
Memory leaks.
Dangling pointers.
GC frees programmer to focus on more important issues.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 2/9

3. A Few Definitions
Aim: to free objects that will not be used anymore.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 3/9

4. A Few Definitions
Aim: to free objects that will not be used anymore.
That is computationally impossible.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 3/9

5. A Few Definitions
Aim: to free objects that will not be used anymore.
That is computationally impossible.
Reachability: transitive closure of pointers starting with the root set
(all global and local variables).
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 3/9

6. Reference Counting
Each object has a counter.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 4/9

7. Reference Counting
Each object has a counter.
Reference assignment → decrease the counter of the old object and
increase the counter of the new one (if they exist).
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 4/9

8. Reference Counting
Each object has a counter.
Reference assignment → decrease the counter of the old object and
increase the counter of the new one (if they exist).
When the counter reaches zero, free the object.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 4/9

9. Reference Counting
Each object has a counter.
Reference assignment → decrease the counter of the old object and
increase the counter of the new one (if they exist).
When the counter reaches zero, free the object.
Disadvantages: fairly big overhead, can not deal with cycles.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 4/9

10. Mark and Sweep
Each object has a mark.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 5/9

11. Mark and Sweep
Each object has a mark.
Once in a while, go through transitive closure of the root set and
mark all objects along the way.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 5/9

12. Mark and Sweep
Each object has a mark.
Once in a while, go through transitive closure of the root set and
mark all objects along the way.
Then free all unmarked objects.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 5/9

13. Mark and Sweep
Each object has a mark.
Once in a while, go through transitive closure of the root set and
mark all objects along the way.
Then free all unmarked objects.
Disadvantages: fragmentation, locality of reference, cost is
proportional to the size of available memory.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 5/9

14. Mark and Compact, Mark and Copy
The same as mark and sweep except objects are moved (copied) so
that they are next to each other.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 6/9

15. Mark and Compact, Mark and Copy
The same as mark and sweep except objects are moved (copied) so
that they are next to each other.
Disadvantages: costly when there is a large number of survivors.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 6/9

16. Incremental Collectors
Stop the world algorithms disrupt the execution of the program for
too long.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 7/9

17. Incremental Collectors
Stop the world algorithms disrupt the execution of the program for
too long.
It is better to collect garbage by increments.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 7/9

18. Incremental Collectors
Stop the world algorithms disrupt the execution of the program for
too long.
It is better to collect garbage by increments.
However, incremental algorithms must accommodate for possible
changes in the object graph.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 7/9

19. Incremental Collectors
Stop the world algorithms disrupt the execution of the program for
too long.
It is better to collect garbage by increments.
However, incremental algorithms must accommodate for possible
changes in the object graph.
Two basic ways: read barrier and write barrier.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 7/9

20. Generational Collectors
Most objects live a very short time.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 8/9

21. Generational Collectors
Most objects live a very short time.
GC among younglings has much higher efficiency—i. e. it lasts
shorter and yields more free space.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 8/9

22. Generational Collectors
Most objects live a very short time.
GC among younglings has much higher efficiency—i. e. it lasts
shorter and yields more free space.
Therefore, the whole memory is divided into regions, objects are
propagated through regions based on their age.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 8/9

23. Generational Collectors
Most objects live a very short time.
GC among younglings has much higher efficiency—i. e. it lasts
shorter and yields more free space.
Therefore, the whole memory is divided into regions, objects are
propagated through regions based on their age.
The root set for a given region has to include references from regions
with more mature objects.
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 8/9

24. See
Paul Wilson. Uniprocessor Garbage Collection Techniques. Memory
Management. http://dx.doi.org/10.1007/BFb0017182
Michal P´se (CTU in Prague)
ıˇ Object Prgrmmng L. 11: Garbage Collection December 7, 2010 9/9