PermGen Vs Metaspace

As you saw in Java 8 PermGen space has been decommission. In JDK 8, classes metadata is now stored in the native heap and this space is called Metaspace.

I recommend that you read the PermGen removal summary and comments from Jon on this subject. 

In summary:

PermGen space situation

• This memory space is completely removed.
• The PermSize and MaxPermSize JVM arguments are ignored and a warning is issued if present at start-up.

Metaspace memory allocation model

• Most allocations for the class metadata are now allocated out of native memory.
• The klasses that were used to describe class metadata have been removed.

Metaspace capacity

• By default class metadata allocation is limited by the amount of available native memory (capacity will of course depend if you use a 32-bit JVM vs. 64-bit along with OS virtual memory availability).
• A new flag is available (MaxMetaspaceSize), allowing you to limit the amount of native memory used for class metadata. If you don’t specify this flag, the Metaspace will dynamically re-size depending of the application demand at runtime.

Metaspace garbage collection

• Garbage collection of the dead classes and classloaders is triggered once the class metadata usage reaches the “MaxMetaspaceSize”.
• Proper monitoring & tuning of the Metaspace will obviously be required in order to limit the frequency or delay of such garbage collections. Excessive Metaspace garbage collections may be a symptom of classes, classloaders memory leak or inadequate sizing for your application.

Java heap space impact

• Some miscellaneous data has been moved to the Java heap space. This means you may observe an increase of the Java heap space following a future JDK 8 upgrade.

Metaspace monitoring

• Metaspace usage is available from the HotSpot 1.8 verbose GC log output.
• Jstat & JVisualVM have not been updated at this point based on our testing with b75 and the old PermGen space references are still present.

How to set the Metaspace in Java 8

There are some new flags added for Metaspace in JDK 8:

• -XX:MetaspaceSize=

where is the initial amount of space(the initial high-water-mark) allocated for class metadata (in bytes) that may induce a garbage collection to unload classes. The amount is approximate. After the high-water-mark is first reached, the next high-water-mark is managed by the garbage collector

• -XX:MaxMetaspaceSize=

where is the maximum amount of space to be allocated for class metadata (in bytes). This flag can be used to limit the amount of space allocated for class metadata. This value is approximate. By default there is no limit set.

• -XX:MinMetaspaceFreeRatio=

where is the minimum percentage of class metadata capacity free after a GC to avoid an increase in the amount of space (high-water-mark) allocated for class metadata that will induce a garbage collection.

• -XX:MaxMetaspaceFreeRatio=

where is the maximum percentage of class metadata capacity free after a GC to avoid a reduction in the amount of space (high-water-mark) allocated for class metadata that will induce a garbage collection.