Internals of Java Garbage Collector

GC Responsibilities
Garbage Detection: Garbage are objects that are no longer reachable
Garbage Reclamation:Make space available to the running program again

HotSpot VM Heap Layout

SS#1: Survivor Space 1
SS#2: Survivor Space 2

How HotSpot’s GC Works

Empirical Statistics
Most objects are very short lived
– 80 – 98% of all newly allocated objects die within a few million instructions
– 80 – 98% of all newly allocated objects die before another megabyte has been allocated

Assumptions
Weak generation hypothesis
– Most new objects die young
– Concentrate effort on managing the young generation
– Make allocate/manage/deallocate cycle fast and efficient
For older objects, manage as little as possible
Keep young and old objects separate
Use different GC algorithms for each generation
– Different requirements for each group

Object Allocation