Generational Hypothesis

As we have mentioned before, doing a garbage collection entails stopping the application completely. It is also quite obvious that the more objects there are the longer it takes to collect all the garbage. But what if we would have a possibility to work with smaller memory regions? Investigating the possibilities, a group of researchers has observed that most allocations inside applications fall into two categories:

  • Most of the objects become unused quickly
  • The ones that do not usually survive for a (very) long time

These observations come together in the Weak Generational Hypothesis. Based on this hypothesis, the memory inside the VM is divided into what is called the Young Generation and the Old Generation. The latter is sometimes also called Tenured.

object age in java

Having such separate and individually cleanable areas allows for a multitude of different algorithms that have come a long way in improving the performance of the GC.

This is not to say there are no issues with such an approach. For one, objects from different generations may in fact have references to each other that also count as ‘de facto’ GC roots when collecting a generation.

But most importantly, the generational hypothesis may in fact not hold for some applications. Since the GC algorithms are optimized for objects which either ‘die young’ or ‘are likely to live forever’, the JVM behaves rather poorly with objects with ‘medium’ life expectancy.