G1's goals in the default configuration are neither maximum throughput nor lowest latency, but to provide relatively small, uniform pauses at high throughput.

evacuating

On a high level, the G1 collector alternates between two phases. The young-only phase contains garbage collections that fill up the currently available memory with objects in the old generation gradually. The space-reclamation phase is where G1 reclaims space in the old generation incrementally, in addition to handling the young generation. Then the cycle restarts with a young-only phase.

G1 garbage collection pauses can reclaim space in the young generation as a whole, and any additional set of old generation regions at any collection pause. During the pause G1 copies objects from this collection set to one or more different regions in the heap. The destination region for an object depends on the source region of that object: the entire young generation is copied into either survivor or old regions, and objects from old regions to other, different old regions using aging.