In the case of a minor collection, you don't do a full mark-sweep. You only look at a small fraction of the live objects. Generally you only need to trace obje

In the case of a minor collection, you don't do a full mark-sweep. You only look at a small fraction of the live objects. Generally you only need to trace objects in the youngest generation which are reachable from GC roots. Since you expect most young generation objects to be eligible for collection, this is extremely fast. It is unusual for a lot of references to exist from older generations to the youngest generation, but when this occurs there are data structures for detecting and resolving these references very efficiently.