Why make short and long-lived objects a difference in garbage collection?

Question

I've often read that in the Sun JVM short-lived objects ("relatively new objects") can be garbage collected more efficiently than long-lived objects ("relatively old objects")

• Why is that so?
• Is that specific to the Sun JVM or does this result from a general garbage collection principle?

Answer 1

Most Java apps create Java objects and then discard them rather quickly eg. you create some objects in a method then once you exit the method all the object dies. Most apps behave this way and most people tend to code their apps this way. The Java heap is roughly broken up into 3 parts, permanent, old (long lived) generation, and young (short lived) generation. Young gen is further broken up into S1, S2 and eden. These are just heaps.

Most objects are created in the young gen. The idea here is that, since the mortality rate of objects is high, we quickly create them, use them and then discard them. Speed is of essence. As you create objects, the young gen fills up, until a minor GC occurs. In a minor GC, all objects that are alive are copied over from eden and say S2 to S1. Then, the 'pointer' is rested on eden and S2.

Every copy ages the object. By default, if an object survives 32 copies viz. 32 minor GC, then the GC figures that it is going to be around for a lot longer. So, what it does is to tenure it, by moving it to the old generation. Old gen is just one big space. When the old gen fills up, a full GC, or major GC, happens in the old gen. Because there is no other space to copy to, the GC has to compact. This is a lot slower than minor GC, that's why we avoid doing that more frequently.

You can tune the tenuring parameter with

java -XX:MaxTenuringThreshold=16  

if you know that you have lots of long lived objects. You can print the various age bucket of your app with

java -XX:-PrintTenuringDistribution 

Answer 2

(see above explanations for more general GC.. this answers WHY new is cheaper to GC than old).

The reason eden can be cleared faster is simple: the algorithm is proportional to the number of objects that will survive GC in the eden space, not proportional to the number of live objects in the whole heap. ie: if you have an average object death rate of 99% in eden (ie: 99% of objects do not survive GC, which is not abnormal), you only need to look at and copy that 1%. For "old" GC, all live objects in the full heap need to be marked/swept. That is significantly more expensive.