Off-Heap Memory

1. What Is Off-Heap Memory?

It means memory that is not allocated on the Java heap, but in operating-system memory.

2. NIO Off-Heap Memory

Memory allocated by JDK NIO’s ByteBuffer.allocate method is on the Java heap. When data needs to be transferred, the data in that heap memory has to be copied unchanged into operating-system memory.

Memory allocated by Java NIO’s ByteBuffer.allocateDirect method is in operating-system memory, so this copy is not needed.

3. Netty Off-Heap Memory

When Netty allocates memory, it decides how much to allocate this time based on the size allocated previously. It stores values such as 16, 32, 48 … 496 and 512, 1024 … in an array. This array represents the amount of memory allocated each time.

At the same time, whether heap or off-heap memory is allocated is determined by whether the Unsafe class is available on the current classpath. If off-heap memory is used, the final call is NIO’s ByteBuffer.allocateDirect; otherwise a byte array is allocated.

4. References

• A Feast of Troubleshooting Netty Off-Heap Memory Leaks - Meituan Tech Team
• Troubleshooting Netty Off-Heap Memory Leaks: Everything Explained in One Article - Juejin
• Netty’s Three Levels of Zero Copy Implementation - Zhihu