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In our app running on Jdk 8 we use VisualVM to track the usage of loaded classes and the usage of the metaspace.
At some point in time while our app is running we see that the number of loaded classes don't increase any more but the metaspace still increases in it's size while our program is running. So what else apart from classes is stored in metaspace, that could cause that?
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Metaspace is a native memory region that stores metadata for classes. As a class is loaded by the JVM, its metadata (i.e. its runtime representation in the JVM) is allocated into the Metaspace. The Metaspace occupancy grows as more and more classes are loaded.
In the place of PermGen, a new feature called Meta Space has been introduced. MetaSpace grows automatically by default. Here, the garbage collection is automatically triggered when the class metadata usage reaches its maximum metaspace size. It is removed from java 8.
The default size of -XX:MetaspaceSize is platform-dependent and ranges from 12 MB to about 20 MB.
Simply put, Metaspace is a new memory space – starting from the Java 8 version; it has replaced the older PermGen memory space. The most significant difference is how it handles memory allocation. Specifically, this native memory region grows automatically by default.
While your program is running, some parts of your code may be determined as "hot" by HotSpot's JIT compiler. This will cause those parts to be transformed/compiled to native code, and also some other code may be inlined into it. This native code representation has to go somewhere, and it goes into the same place as other class metadata - the Metaspace.
It explains continuous growth you're seeing: hot parts are determined over time using a simple metric of how much times did that piece of code got executed. Over time more and more code pieces will be JIT'ed as they'll hit threshold set by -XX:CompileThreshold (defaults to 10000)
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