PermGen and Heap, Difference and their significance

java memory jvm heap-memory
Punith Raj picture Punith Raj · Jun 13, 2012 · Viewed 19.9k times · Source

Friends,

Can you please give me significance, difference and uses for Heap and PermGen. Also it would be good to know what class are loaded in them respectively.

Explanation related to Java VM specification would be really helpful

Thanks Punith

Answer

amicngh picture amicngh · Jun 13, 2012

Memory(Heap) is managed in generations, or memory pools holding objects of different ages. Garbage collection occurs in each generation when the generation fills up. Objects are allocated in a generation for younger objects or the young generation, and because of infant mortality most objects die there.

When any new object is constructed it goes to Eden space which is a part of Young Generation.

If object is still alive after some time it goes to tenured generation where long lived objects lie.

If object is supposed to live until over process exist then object is moved to Perm Generation.Java classes are stored in the permanent generation.

Related questions

Java Refuses to Start - Could not reserve enough space for object heap

Background We have a pool of aproximately 20 linux blades. Some are running Suse, some are running Redhat. ALL share NAS space which contains the following 3 folders: /NAS/app/java - a symlink that points to an installation of a Java …

Read More
Increase heap size in java for weka

I'm trying to increase the heap size in java for weka which keeps crashing. I used the suggested line: > java -Xmx500m -classpath but I get the following error: -classpath requires class path specification I'm not sure what this …

Read More
What does the UseCompressedOops JVM flag do and when should I use it?

What does the HotSpot JVM flag -XX:+UseCompressedOops do and when should I use it? What sort of performance and memory-usage differences will I see when using it on a 64-bit Java instance (vs. not using it)?

Read More