young generation and old generation. 1 eden and 2 survivor spaces.
minor GC, mark and copy, from eden and one survivor to the other survivor
full GC, mark, sweep and compact generations
both will stop the world.
- Serial GC, for small heap and single thread env
- Parallel GC, multiple thread to do GC, but stop application when do both minor GC and full GC. suitable for high throughput application due to the facts
- all cores are cleaning garbage during collection
- no cores are used for cleaning between collection cycles.
- CMS GC, low latency, but less throughput than parallel GC if it's cpu-bound application due to the fact that some CPU cores are cleaning garbage in most of time thus they are not working on application.
- parallel mark-copy stop-the-world collection in young generation (minor GC)
- mostly (two of phases stop the world) concurrent-mark-sweep in old generation (full GC)
- Initial Mark (STW), collecting all GC roots in old generation (direct GC roots or indirectly referenced from young generation)
- Concurrent Mark, marking all live objects (may not all of them due to application may mutate references at the same time) in old generation from the GC roots.
- Concurrent Preclean, accounting for references changed during previous marking phase.
- Concurrent Abortable Preclean, offloading as much work as possible from the final mark phase
- Final remark (STW)
- Concurrent sweep
- Concurrent reset
collection in young generation may occur when concurrent collection happens in old generation.
drawbacks are fragmentation in old generation and non-predictable pause duration especially in large heap.
- G1 GC (collect regions with more garbage)
heap is divided into regions, including eden regions, survivor regions and old regions.
- Young GC (STW, in parallel), coping to survivor regions or old regions
- GC in old generation
- Initial Mark (STW), mark survivor regions (root regions), it's piggybacked on young GC
- Root Region Scanning, find references to old generation
- Concurrent Marking, find live objects over entire heap
- Remark (STW), complete the marking of live objects. (snapshot-at-the-beginning). empty regions are removed and reclaimed. region liveness is calculated for all regions.
- Cleanup/copying, selecting regions with lowest liveness to be collected at the same time as young GC.