The permanent generation is used to hold reflective 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. Generally, sizing of this generation can be ignored because the default size is adequate. However, programs that load many classes may need a larger permanent generation.

PermSize is additional separate heap space to the -Xmx value set by the user. The section of the heap reserved for the permanent generation holds all of the reflective data for the JVM. You should adjust the size accordingly if your application dynamically load and unload a lot of classes in order to optimize the performance.

By default, MaxPermSize will be 32mb for -client and 64mb for -server. However, if you do not set both PermSize and MaxPermSize, the overall heap will not increase unless it is needed. When you set both PermSize and MaxPermSize, for example, 192mb, the extra heap space will get allocated when it startup and will stay allocated.