Answer»
- To properly understand how generational garbage collection works, it is important to first remember how Java heap is structured to facilitate generational garbage collection.
- The heap is divided up into smaller spaces or generations. These spaces are Young Generation, Old or Tenured Generation, and Permanent Generation.
- The young generation hosts most of the newly created objects. An empirical study of most applications shows that majority of objects are quickly short lived and therefore, soon BECOME eligible for collection. Therefore, new objects start their journey here and are only “promoted” to the old generation space after they have attained a certain “age”.
- The term “age” in generational garbage collection refers to the number of collection cycles the object has survived.
- The young generation space is further divided into three spaces: an Eden space and two survivor spaces such as Survivor 1 (s1) and Survivor 2 (s2).
- The old generation hosts objects that have lived in MEMORY longer than a certain “age”. The objects that survived garbage collection from the young generation are promoted to this space. It is generally larger than the young generation. As it is bigger in size, the garbage collection is more expensive and occurs less frequently than in the young generation.
- The permanent generation or more commonly called, PermGen, contains metadata required by the JVM to describe the CLASSES and methods used in the application. It also contains the string pool for storing interned strings. It is populated by the JVM at runtime based on classes in use by the application. In addition, platform library classes and methods may be stored here.
- First, any new objects are allocated to the Eden space. Both survivor spaces start out empty. When the Eden space fills up, a minor garbage collection is triggered. Referenced objects are moved to the first survivor space. Unreferenced objects are deleted.
- During the next minor GC, the same thing happens to the Eden space. Unreferenced objects are deleted and referenced objects are moved to a survivor space. However, in this case, they are moved to the second survivor space (S1).
- In addition, objects from the last minor GC in the first survivor space (S0) have their age incremented and are moved to S1. Once all surviving objects have been moved to S1, both S0 and Eden space are cleared. At this point, S1 contains objects with different ages.
- At the next minor GC, the same process is repeated. However this time the survivor spaces switch. Referenced objects are moved to S0 from both Eden and S1. Surviving objects are aged. Eden and S1 are cleared.
- After every minor garbage collection cycle, the age of each object is checked. Those that have reached a certain arbitrary age, for example, 8, are promoted from the young generation to the old or tenured generation. For all subsequent minor GC cycles, objects will continue to be promoted to the old generation space.
- This pretty much exhausts the process of garbage collection in the young generation. Eventually, a major garbage collection will be PERFORMED on the old generation which cleans up and compacts that space. For each major GC, there are several minor GCs.
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