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Last updated September 26, 2012 21:03, by Martijn Verburg
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Introduction

PermGen as we know it is being remove from the JVM, and its new (mostly native) replacement needs some thorough testing!

Generally speaking this change is invisible to the Java application. However, depending on the allocation behavior, more Java heap may be used. Also the amount of native memory used for the class metadata may be less than or more than the memory used previously by the permanent generation. Basically the message is that you should not see any differences but then again you may.

This change is in hsx 25 b2 and is expected to promote into jdk8 b58.

details

  • Most allocations for the class metadata previously done out of the permanent generation are now allocated out of native memory. Some miscellaneous data has been moved to the Java heap.
  • The permanent generation has been removed. The PermSize and MaxPermSize are ignored and a warning is issued if they are present on the command line.
  • The klasses that were used to described class metadata have been removed (klassKlass and it's derivatives).
  • By default class metadata allocation is only limited by the amount of available native memory. Use the new flag MaxMetaspaceSize to limit the amount of native memory used for class metadata. It is analogous to MaxPermSize.
  • In 64bit VM when compressed oops are used a special fixed size space is used for classes to set a compressed class pointer in object's header. The size of that

class's space is controlled by ClassMetaspaceSize flag with default value 2Mbytes.

  • A garbage collection may be induced to collect dead classloaders and classes. The first garbage collection will be induced when the class metadata usage reaches

MetaspaceSize (12Mbytes on the 32bit client VM and 16Mbytes on the 32bit server VM with larger sizes on the 64bit VM's). Set MetaspaceSize to a higher value to delay the induced garbage collections. After an induced garbage collection the class metadata usage needed to induce the next garbage collection may be increased.

  • Whereas class metadata was previously garbage collected as part of the permanent generation, without the permanent generation the storage for class metadata

is explicitly managed. C-heap allocation is not used.

  • jstat counter names for metadata allocations have not been updated. The old permanent generation names are still used but will be updated in the near future.

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