memory – What is PermSize in Java?

memory – What is PermSize in 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.

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, heres 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.

memory – What is PermSize in 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.

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