Why do we need to manually handle reference counting for Netty ByteBuf if JVM GC is still in place?

Question

According to the book Netty in Action v10, reference counting is used to handle the pooling of ByteBuf. But JVM is not aware of the n

Answer 1

Direct buffers are indirectly freed by the Garbage Collector. I will let you read through the answer to this question to understand how that happens: Are Java DirectByteBuffer wrappers garbage collected?

Heap buffers need to be copied to the direct memory before being handled by the kernel, when you're performing I/O operations. When you use direct buffers you save that copy operation and that's the main advantage of using direct buffers. A drawback is that direct memory allocation is reasonably more expensive then allocations from the java heap, so Netty introduced the pooling concept.

Pooling objects in Java is a polemic topic, but the Netty choice for doing so seems to have paid off and the Twitter article you cited shows some evidence of that. For the particular case of allocating buffers, when the size of the buffer is large, you can see that it really brings a benefit both in the direct and heap buffer cases.

Now for pooling, the GC doesn't reclaim the buffer when they are pooled, because either your application has one or several references to it, while you're using the buffer; or Netty's pool has a reference to it, when it has just been allocated and has not yet been given to your application or after your application used it and gave it back to the pool.

Leaks will happen when your application, after having used a buffer and not keeping any further reference to it, doesn't call release(), what actually means put it back into the pool, if you don't have any further reference to it. In such case, the buffer will eventually be garbage collected, but Netty's pool won't know about it. The pool will then grow believing that you're using more and more buffers that are never returned to the pool. That will probably generate a memory leak because, even if the buffers themselves are garbage collected, the internal data structures used to store the pool will not.

Answer 2

ByteBuf is using off heap memory , therefore its not visible to GC. Thats why you need to update the reference count ( otherwise netty will not know when to release the item ). Best regards