Luca's meaningless thoughts  

Lazy freeing RC

by Leandro Lucarella on 2008- 08- 19 02:31 (updated on 2008- 08- 19 02:31)
tagged d, dgc, en, lazy, lazy freeing, rc - with 0 comment(s)

The first optimization to analyze is a very simple one. What's the idea behind it lazy freeing? Just transfer some of the work of freeing unused cells to the allocation, making the collection even more interleaved with the mutator.

When you delete a cell, if it's counter drops to 0, instead of recursively free it, just add it to a free-list. Then, when a new cell has to be allocated, take it from the free-list, delete all its children (using the lazy delete, of course), and return that cell.

First drawback of this method: you loose finalization support, but as I said, most people don't care about that. So that's a non-problem. Second, allocation is not that fast anymore. But it's almost bounded. Why almost? Because it's O(N), being N the number of pointers to be deleted in that cell. This doesn't seems like a huge cost anyways (just decrement a counter and, maybe, add it to a free-list). Allocation is (usually) not bounded anyways (except for compacting collectors).

The big win? Bounded freeing. Really small pauses, with no extra costs.

Note

If you have a (simple) program that suffers from GC pauses that you think it could be easily converted to be reference counted (i.e. few pointer updates), please let me know if you want me to try to make it use lazy freeing RC to analyze the real impact on a real-life program.