Reasons of ignoring these argument is permanent generation has been removed in HotSpot for JDK8 because of following drawbacks
- Fixed size at startup – difficult to tune.
- Internal Hotspot types were Java objects : Could move with full GC, opaque, not strongly typed and hard to debug, needed meta-metadata.
- Simplify full collections : Special iterators for metadata for each collector
- Want to deallocate class data concurrently and not during GC pause
- Enable future improvements that were limited by PermGen.
The Permanent Generation (PermGen) space has completely been removed and is kind of replaced by a new space called Metaspace.
The consequences of the PermGen removal is that obviously the PermSize and MaxPermSize JVM arguments are ignored and you will never get a java.lang.OutOfMemoryError: PermGen error.
Advantages of MetaSpace
- Take advantage of Java Language Specification property : Classes and associated metadata lifetimes match class loader’s
- Per loader storage area – Metaspace
- Linear allocation only
- No individual reclamation (except for RedefineClasses and class loading failure)
- No GC scan or compaction
- No relocation for metaspace objects
Metaspace Tuning
The maximum metaspace size can be set using the -XX:MaxMetaspaceSize flag, and the default is unlimited, which means that only your system memory is the limit. The -XX:MetaspaceSize tuning flag defines the initial size of metaspace If you don’t specify this flag, the Metaspace will dynamically re-size depending of the application demand at runtime.
Change enables other optimizations and features in the future
- Application class data sharing
- Young collection optimizations, G1 class unloading
- Metadata size reductions and internal JVM footprint projects
There is improved GC performace also. More Details
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