Off-Heap memory

Java GC can only run in JVM heap which is called on-heap memory. However, to control the rubbish collection size of Java GC to realse some pressure of it . we can also open some memory space outside JVM heap in the Operating system level. This part of memory is called off-heap memory.

How do we manipulate off-heap memory ?

Normally there exits 3 opensource library for us to use , Chronicle Queue、Chronicle Map and Thread Affinity . These 3 can help us manipulate off-heap memory flexibly.

When do we use off-heap memory ?

1. When Some Java NIO related APIs are called,  we may use off-heap memory indirectly. Operating system's kernel(eg: linux kernel) will directly write data into the off-heap memory rather than do like normal API caching one copy of data in the linux kernel then copying another one into the application when the application want to read. 
2. When Java JVM management can't fullfil the project's requirement, we need to use the off-heap memory and manage it flexbily ourselves. 

For example : 

One incoming connection is to ask for 2M to create 2000 small Objects in Java and to release this 2M memory immediatly  when connection is finished/broken. For Java GC, this is a extra burden to manage those extra 2000 objects  and GC doesn't know those 2000 Objects' memory can be released immediatly after being used (Connection is finsihed). But we , programmers, know this logic and we know better what will happen in the system memory and JVM, so in that case , we can write code ourselves to manage the memory in the most efficient way.

How do we create a off-heap memory space?

One of those methods is to use DirectBuffer :

ByteBuffer.allocateDirect(size)

Another way is to use JNI to write a C/C++ extension in JAVA, in the extension, we don't create memory space in JVM.

But normally, I don't think people may need write in this very backend way.
If you are focus on normal business logic , I think chronicleMap will fullfil your requirement.
If you are writing some frameworks in Java related to NIO, the first method may apply(eg: Netty , ActiveMQ).
But if you don't limit your language in Java, write a C extension to control the whole system. This is the best solution, isn't it.