What is CompressedClassSpaceSize?
What is the relationship between CompressedClassSpaceSize and MetaspaceSize?
Metaspace is the memory area for storing class metadata - internal JVM structures created while parsing .class files.
Class metadata includes:
- Internal representation of Java classes
- Methods with their bytecode
- Field descriptors
- Constant pools
- Symbols
- Annotations
- etc.
-XX:MaxMetaspaceSize is unlimited by default.
When -XX:+UseCompressedClassPointers option is ON (default for heaps < 32G), classes are moved from Metaspace to the separate area called Compressed Class Space. This is to allow addressing VM class structures with 32-bit values instead of 64-bit.
So, Compressed Class Space contains internal representation of Java classes, while Metaspace holds all the rest metadata: methods, constant pools, annotations, etc.
The size of Compressed Class Space is limited by -XX:CompressedClassSpaceSize, which is 1G by default. The maximum possible value of -XX:CompressedClassSpaceSize is 3G.
Non-class Metaspace and Compressed Class Space are two disjoint areas. MaxMetaspaceSize limits the committed size of both areas:
committed(Non-class Metaspace) + committed(Compressed Class Space) <= MaxMetaspaceSize
If MaxMetaspaceSize is set smaller than CompressedClassSpaceSize, the latter is automatically decreased to
CompressedClassSpaceSize = MaxMetaspaceSize - 2*InitialBootClassLoaderMetaspaceSize