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While learning about java memory profiling, I keep seeing the term "perm space" in addition to "heap." I know what the heap is - what's perm space?
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A java. lang. OutOfMemoryError: PermGen Space is a runtime error in Java which occurs when the permanent generation (PermGen) area in memory is exhausted. The PermGen area of the Java heap is used to store metadata such as class declarations, methods and object arrays.
PermGen stands for Permanent Generation. It is a special type of heap space. It is separate from the main memory (heap). JVM uses PermGen to keep track of loaded class metadata. All the static content is stored by the JVM to this memory section.
In Java 8, PermGen method area replaced with MetaSpace. They have moved permGem to the separate memory in the native OS and that is called MetaSpace. It can by default auto increases its size. In MetaSpace, classes can load and unload during the lifespan of the JVM.
Metaspace is memory the VM uses to store class metadata. Class metadata are the runtime representation of java classes within a JVM process - basically any information the JVM needs to work with a Java class. That includes, but is not limited to, runtime representation of data from the JVM class file format.
It stands for permanent generation:
The permanent generation is special because it holds meta-data describing user classes (classes that are not part of the Java language). Examples of such meta-data are objects describing classes and methods and they are stored in the Permanent Generation. Applications with large code-base can quickly fill up this segment of the heap which will cause
java.lang.OutOfMemoryError: PermGen no matter how high your -Xmx and how much memory you have on the machine.
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Perm space is used to keep informations for loaded classes and few other advanced features like String Pool(for highly optimized string equality testing), which usually get created by String.intern() methods. As your application(number of classes) will grow this space shall get filled quickly, since the garbage collection on this Space is not much effective to clean up as required, you quickly get Out of Memory : perm gen space error. After then, no application shall run on that machine effectively even after having a huge empty JVM.
Before starting your application you should java -XX:MaxPermSize to get rid of this error.
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