#Openjdk 16 jdk how to
#Openjdk 16 jdk free

The JVM always allocates through a class loader. In the general case, the JVM will have two instances of VirtualSpaceNode lists, one with a compressed space where class data is put, and one uncompressed that is not limited by address space and used for everything not needing compressed class pointers.

#Openjdk 16 jdk free

When the orange class loader is destroyed, the free 256 KiB chunk will merge with its buddy chunk twice, leaving big consecutive areas without external fragmentation. Orange classloader allocates 256 KiB note that both class loaders might allocate chunks from the same RootChunkArea.

Blue class loader allocates a 1 MiB block, the root chunk needs to be divided twice. When added back, the chunk will try to recursively merge into its neighbour buddy reducing external fragmentation.

These chunks can cheaply be removed or merged back using a buddy allocator scheme. A RootChunkArea is an area that can be recursively halved into smaller areas of logarithmic size, 4, 2, 1, ½, … MiB. This is the max size of a memory allocation. A JVM running with compressed class pointers will have two VirtualSpaceNodeLists, one for “normal” objects, one for objects that needs to be addressed by compressed class pointers.Įvery VirtualSpaceNode is divided into several, in address space consecutive RootChunkAreas, currently sized 4 MiB. Although the whole address space is reserved at once, memory is committed lazily only when being used.Įach VirtualSpaceNode will have a bitmap keeping track of which commit granules have been committed, more information on this further down. When allocating into compressed class space (orange), the list will contain precisely one VirtualSpaceNode reserving a 32-bit address space thus guaranteeing access with compressed pointers. When not allocating into compressed class space (blue), these will be stored in one linked list with many nodes. Memory is allocated from the operating system in VirtualSpaceNodes. Short walk-through Chunk allocation using a buddy allocator

#Openjdk 16 jdk how to

Metaspace can be seen as this special malloc that knows how to allocate to a small address space. So either all references to class metadata must go through an indirection table (requires compiler changes, with a possible performance impact) or we need a special malloc. The generic malloc does not know how to allocate in a restricted memory space (for compressed class pointers). Why not use an ordinary allocator such as malloc? This usage pattern allows us to tune the allocator somewhat in addition to support compressed class pointers. This feature is called compressed class pointers and works very similar to compressed oops.Ĭlass loading/unloading mostly follow the allocation pattern of allocating data without freeing until the classloader is discarded (and then everything is freed). With this feature, each Java object can reach class data using a 32-bit index instead of wasting a 64-bit pointer on each object header. Metaspace has one additional important feature in that you can make it allocate memory within a specified address space. Prior to JDK 8, metadata was allocated in a special part of the Java heap called the permanent generation, or PermGen.Īn allocator needs to be able to allocate and - most often - free memory. This memory is mostly used for class metadata, but it could be used as a general arena allocator as well. It allocates memory outside the Java heap. Metaspace is the name of a memory allocator (or the space used by the allocator). Take a look at the presentation by Thomas Stüfe, statistics starting at page 47. Even in normal applications such as Wildfly or Eclipse, memory reductions can be significant (>10%) in Metaspace. If you are doing lots of class loading and want to trim memory usage, this change should be of interest to you! Before this change, small class loaders wasted lots of memory due to inflexibility in chunk sizes for small class loaders huge savings can be made in Metaspace.

When memory is not used any more, there is a much higher probability that it can be returned to the operating system, which will reduce the memory footprint of the JVM process.

Fragmentation in Metaspace will be reduced, resulting in lower memory usage.

When you start using Java 16, you will benefit from the new Metaspace - without doing anything. This is a great example of an improvement made to OpenJDK by contributors outside of Oracle. Thomas Stüfe (working for SAP) has contributed a new Metaspace as described in JEP 387. Garbage? Blog - Metaspace in OpenJDK 16 Home About Metaspace in OpenJDK 16