how the live objects are figured out in young generation collection?

Question

I understand that time taken by YGC is proportional to number of live objects in Eden. I also understand that how the live objects are figured out in Major collections (All the

Answer 1

To trace just the youngest generation, the garbage collector scans the same root set (stacks and registers) and ALSO all the older (non-collected) generations that have been modified since the previous young generation scan. Only those objects that have been modified can possibly point at young generation objects, as unmodified objects cannot possibly point at objects that were created after their last modification.

So the tricky part is, how does the GC know which objects have been modified since the last GC? There are a number of techniques that can be used, but they basically come down to keeping track of writes to old generation objects. This can be done by trapping writes (write barriers) or just keeping track of all write targets (write buffers, card marking), all of which add overhead to the execution of the program while the GC is not running (so it doesn't show up as GC pause time, but does show up in the total elapsed time). Hardware support helps a lot, if its available. The tracking need not be exact, as long as every modified older generation object is scanned (scanning unmodified objects is a waste of time, but won't hurt anything).