| GC name | Flags | Availability | Details | Further reading |
|---|---|---|---|---|
| SerialGC | ||||
| ParallelGC | ||||
| ParallelOldGC | ||||
| ConcMarkSweepGC | ||||
| - incremental mode | ||||
| G1GC | ||||
| ShenandoahGC | ||||
| - incremental mode | ||||
| ZGC |
naive algorithms never used on their own, but may be used by other GCs Understanding them is required to understand most used GC algorithms
Basic algos, never used on their own as GC fires when a heap space use watermark is reached has to stop every thread while mark run mark-sweep - as simple as it gets: mark used objects, delete what isn’t marked This would cause OOMError when you try to allocate more than the largest gap between existing objects mark-sweep-compact - de-fragment empty memory by moving objects to the start to alleviate the OOMError problem compact stops threads too mark-copy - needs 2x the RAM, copy live objects to the other half, empty first half Switching the semi-spaces still halts all threads
CMS, ConcMarkSweepGC - three generations, young, old, permanent; new allocations go into young mark-copy on young periodically and on high water mark, multiple parallel threads; stops affected only mark-sweep on old & permanent, triggered only by high water mark, concurrent threads https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/cms.html iCMS, CMSIncrementalMode - a mode of CMS; only does a little bit of m&s on old/permanent every time; depr in SE 8 https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/cms.html Shenandoah - backported to SE 8, requires build-time flag in SE 11, so may or may not be available in your flavor of older OpenJDK versions; enabled by default in current releases https://wiki.openjdk.java.net/display/shenandoah/Main