difference between permgen and metaspace

 Metaspace by default auto increases its size (up to what the underlying OS provides), while PermGen always has a fixed maximum size. You can set a fixed maximum for Metaspace with JVM parameters, but you cannot make PermGen auto-increase.

It is worthwhile to mention that prior to Java 7, interned Strings used to be kept on the PermGen. That caused some serious problems with the infamous:

java.lang.OutOfMemoryError: PermGen space

Whenever there is a need to resize PermGen/Metaspace, JVM will do it as it does with the standard heap. Resizing those spaces requires a full GC, which is always an expensive operation. It can usually be observed during a startup when a lot of classes are being loaded. Especially if the application has dependencies on many external libraries. If there are a lot of full GCs during the startup, it’s usually because of that. If that case, increasing the initial size can boost the startup performance.

To increase PermGen, we have the following commands:

-XX:PermSize=N -  sets the initial (and minimum size) of the Permanent Generation space.

-XX:MaxPermSize=N  -  sets the maximum size of the Permanent Generation space.

In Java 8 and onwards, we can set the initial and maximum size of Metaspace using the following commands:

-XX:MetaspaceSize=N  - sets the initial (and minimum size) of the Metaspace.

-XX:MaxMetaspaceSize=N  - sets the maximum size of the Metaspace.