What is 'PermSize' in Java?


Java Problem Overview

I was going through the document in Java Memory Management and in that I came across PermSize which I couldn't understand. The document says that it stores, "JVM stores its metadata", but I couldn't exactly get what is meant by metadata. I was googling and somewhere I read it stores a value object (user defined object).

What kind of objects are stored there? An example with an explanation would be great.

Java Solutions

Solution 1 - Java

A quick definition of the "permanent generation":

> "The permanent generation is used to > hold reflective data of the VM itself > such as class objects and method > objects. These reflective objects are > allocated directly into the permanent > generation, and it is sized > independently from the other > generations." [ref]

In other words, this is where class definitions go (and this explains why you may get the message OutOfMemoryError: PermGen space if an application loads a large number of classes and/or on redeployment).

Note that PermSize is additional to the -Xmx value set by the user on the JVM options. But MaxPermSize allows for the JVM to be able to grow the PermSize to the amount specified. Initially when the VM is loaded, the MaxPermSize will still be the default value (32mb for -client and 64mb for -server) but will not actually take up that amount until it is needed. On the other hand, if you were to set BOTH PermSize and MaxPermSize to 256mb, you would notice that the overall heap has increased by 256mb additional to the -Xmx setting.

Solution 2 - Java

This blog post gives a nice explanation and some background. Basically, the "permanent generation" (whose size is given by PermSize) is used to store things that the JVM has to allocate space for, but which will not (normally) be garbage-collected (hence "permanent") (+). That means for example loaded classes and static fields.

There is also a FAQ on garbage collection directly from Sun, which answers some questions about the permanent generation. Finally, here's a blog post with a lot of technical detail.

(+) Actually parts of the permanent generation will be GCed, e.g. class objects will be removed when a class is unloaded. But that was uncommon when the permanent generation was introduced into the JVM, hence the name.

Solution 3 - Java

The permament pool contains everything that is not your application data, but rather things required for the VM: typically it contains interned strings, the byte code of defined classes, but also other "not yours" pieces of data.

Solution 4 - Java

lace to store your loaded class definition and metadata. If a large code-base project is loaded, the insufficient Perm Gen size will cause the popular Java.Lang.OutOfMemoryError: PermGen.


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