Eden is the region in memory where the objects are typically allocated when they are created. As there are typically multiple threads creating a lot of objects simultaneously, Eden is further divided into one or more Thread Local Allocation Buffer (TLAB for short) residing in the Eden space. These buffers allow the JVM to allocate most objects within one thread directly in the corresponding TLAB, avoiding the expensive synchronization with other threads.
When allocation inside a TLAB is not possible (typically because there’s not enough room there), the allocation moves on to a shared Eden space. If there’s not enough room in there either, a garbage collection process in Young Generation is triggered to free up more space. If the garbage collection also does not result in sufficient free memory inside Eden, then the object is allocated in the Old Generation.
When Eden is being collected, GC walks all the reachable objects from the roots and marks them as alive.
We have previously noted that objects can have cross-generational links so a straightforward approach would have to check all the references from other generations to Eden. Doing so would unfortunately defeat the whole point of having generations in the first place. The JVM has a trick up its sleeve: card-marking. Essentially, the JVM just marks the rough location of ‘dirty’ objects in Eden that may have links to them from the Old Generation. You can read more on that in Nitsan’s blog entry.
After the marking phase is completed, all the live objects in Eden are copied to one of the Survivor spaces. The whole Eden is now considered to be empty and can be reused to allocate more objects. Such an approach is called “Mark and Copy”: the live objects are marked, and then copied (not moved) to a survivor space.