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

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Oracle s JVM implementation for Java 8 got rid of the PermGen model and replaced it with Metaspace

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Because the PermGen space was removed  Memory management has changed a bit   java-8-permgen-metaspace

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PermGen space is replaced by MetaSpace in Java 8  The PermSize and MaxPermSize JVM arguments are ignored and a warning is issued if present at start-up    Most allocations for the class metadata are now allocated out of native memory    The classes that were used to describe class metadata have been removed   Main difference between old PermGen and new MetaSpace is  you don t have to mandatory define upper limit of memory usage  You can keep MetaSpace space limit unbounded  Thus when memory usage increases you will not get OutOfMemoryError error  Instead the reserved native memory is increased to full-fill the increase memory usage    You can define the max limit of space for MetaSpace  and then it will throw OutOfMemoryError   Metadata space  Thus it is important to define this limit cautiously  so that we can avoid memory waste

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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

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This is one of the new features of Java 8  part of JDK Enhancement Proposals 122      Remove the permanent generation from the Hotspot JVM and thus the need to tune the size of the permanent generation    The list of all the JEPs that will be included in Java 8 can be found on the JDK8 milestones page

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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  The JDK 8 HotSpot JVM is now using native memory for the representation of class metadata and is called Metaspace  Read More

[java] PermGen elimination in JDK 8

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