Young space collection algorithm
Almost all new objects are allocated in Eden space. To be more effective HotSpot is using thread local allocation blocks (TLAB) for allocation of new objects, but TLAB themselves are allocated in Eden. Once Eden becomes full minor GC is triggered. Goal of minor GC is to clear fresh garbage in Eden space. Copy-collection algorithm is used (live objects are copied to another space, and then whole space is marked as free memory). But before start collecting live objects, JVM should find all root references. Root references for minor GC are references from stack and all references from old space.
Normally collection of all reference from old space will require scanning through all objects in old space. That is why we need write-barrier. All objects in young space have been created (or relocated) since last reset of write-barrier, so non-dirty pages cannot have references into young space. This means we can scan only object in dirty pages.
Once initial reference set is collected, dirty cards are reset and JVM starts coping of live objects from Eden and one of survivor spaces into other survivor space. JVM only need to spend time on live objects. Relocating of object also requires updating of references pointing to it.
While JVM is updating references to relocated object, memory pages get marked again, so we can be sure what on next young GC only dirty pages has references to young space.
Finally we have Eden and one survivor space clean (and ready for allocation) and one survivor space filled with objects.
Object promotion
If object is not cleared during young GC it will be eventually copied (promoted) to old space. Promotion occurs in following situations:
-XX:+AlwaysTenure makes JVM to promote objects directly to old space instead of survivor space (survivor spaces are not used in this case).
once survivor space is full, all remaining live object are relocated directly to old space.
If object has survived certain number of young space collections, it will be promoted to old space (required number of collections can be adjusted using –XX:MaxTenuringThreshold option and –XX:TargetSurvivorRatio JVM options).
Almost all new objects are allocated in Eden space. To be more effective HotSpot is using thread local allocation blocks (TLAB) for allocation of new objects, but TLAB themselves are allocated in Eden. Once Eden becomes full minor GC is triggered. Goal of minor GC is to clear fresh garbage in Eden space. Copy-collection algorithm is used (live objects are copied to another space, and then whole space is marked as free memory). But before start collecting live objects, JVM should find all root references. Root references for minor GC are references from stack and all references from old space.
Normally collection of all reference from old space will require scanning through all objects in old space. That is why we need write-barrier. All objects in young space have been created (or relocated) since last reset of write-barrier, so non-dirty pages cannot have references into young space. This means we can scan only object in dirty pages.
Once initial reference set is collected, dirty cards are reset and JVM starts coping of live objects from Eden and one of survivor spaces into other survivor space. JVM only need to spend time on live objects. Relocating of object also requires updating of references pointing to it.
While JVM is updating references to relocated object, memory pages get marked again, so we can be sure what on next young GC only dirty pages has references to young space.
Finally we have Eden and one survivor space clean (and ready for allocation) and one survivor space filled with objects.
Object promotion
If object is not cleared during young GC it will be eventually copied (promoted) to old space. Promotion occurs in following situations:
-XX:+AlwaysTenure makes JVM to promote objects directly to old space instead of survivor space (survivor spaces are not used in this case).
once survivor space is full, all remaining live object are relocated directly to old space.
If object has survived certain number of young space collections, it will be promoted to old space (required number of collections can be adjusted using –XX:MaxTenuringThreshold option and –XX:TargetSurvivorRatio JVM options).
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