Comment by lmm

> is it necessary for the performance boost and de-fluffing the objects?

Yes. The one part of the JVM GC that can't run concurrently is heap compaction; objects that can be moved by copying and then deleting would be a huge help for that. And it would be awkward to say the object has an identity but can't be wait/notify'd, at which point you need somewhere for the monitor to go.

> Does this happen in actuality? One would assume the allocator tries to put stuff sequentially on the heap?

Yes. Of course it tries, but semantically the pointers are just pointers and the prefetcher can guess but the system still has to chase them.