PermGen elimination in JDK 8

Question

I have installed JDK 8 and trying to run Eclipse. I am getting following warning message:

Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=512m;
support was removed in 8.0 

What are the reasons for ignoring this VM option?

Answer 1

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.