GC

Garbage collection is an interesting activity in JVM.

Following article explains about the Card Marking technique used by JVMs.
The JVM Write Barrier - Card Marking: http://psy-lob-saw.blogspot.co.uk/2014/10/the-jvm-write-barrier-card-marking.html, By Nitsan Wakart.
This technique is useful in case of generational garbage collection. It tracks of the location of the objects holding references for the other objects. Knowing the location of these objects helps in limiting the scanning of old generation to specific pages.