Is CompressedClassSpaceSize area contains MaxMetaspaceSize area?

Question

What is CompressedClassSpaceSize?

What is the relationship between CompressedClassSpaceSize and MetaspaceSize?

Answer 1

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