Is Java Native Memory Faster than the heap?

Question

I'm exploring options to help my memory-intensive application, and in doing so I came across Terracotta's BigMemory. From what I gather, they take advantage of non-garbage-collected, off-heap "native memory," and apparently this is about 10x slower than heap-storage due to serialization/deserialization issues. Prior to reading about BigMemory, I'd never heard of "native memory" outside of normal JNI. Although BigMemory is an interesting option that warrants further consideration, I'm intrigued by what could be accomplished with native memory if the serialization issue could be bypassed.

Is Java native memory faster (I think this entails ByteBuffer objects?) than traditional heap memory when there are no serialization issues (for instance if I am comparing it with a huge byte[])? Or do the vagaries of garbage collection, etc. render this question unanswerable? I know "measure it" is a common answer around here, but I'm afraid I would not set up a representative test as I don't yet know enough about how native memory works in Java.

Answer 1

Direct memory is faster when performing IO because it avoid one copy of the data. However, for 95% of application you won't notice the difference.

You can store data in direct memory, however it won't be faster than storing data POJOs. (or as safe or readable or maintainable) If you are worried about GC, try creating your objects (have to be mutable) in advance and reuse them without discarding them. If you don't discard your objects, there is nothing to collect.

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Is Java native memory faster (I think this entails ByteBuffer objects?) than traditional heap memory when there are no serialization issues (for instance if I am comparing it with a huge byte[])?

Direct memory can be faster than using a byte[] if you use use non bytes like int as it can read/write the whole four bytes without turning the data into bytes. However it is slower than using POJOs as it has to bounds check every access.

Or do the vagaries of garbage collection, etc. render this question unanswerable?

The speed has nothing to do with the GC. The GC only matters when creating or discard objects.

BTW: If you minimise the number of object you discard and increase your Eden size, you can prevent even minor collection occurring for a long time e.g. a whole day.