Search code examples
javajava-8jvmpermgen

PermGen elimination in JDK 8

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?

Solution

  • 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