A simple rule of when I should use direct buffers with Java NIO for network I/O?

Question

Can someone with the natural gift to explain complex things in an easy and straightforward way address this question? To acquire the best performance when should I use direct ByteBuffers versus regular ByteBuffers when doing network I/O with Java NIO?

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For example: Should I read into a heap buffer and parse it from there, doing many get() (byte by byte) OR should I read it into a direct buffer and parse from the direct buffer?

Answer 1

To acquire the best performance when should I use direct ByteBuffers versus regular ByteBuffers when doing network I/O with Java NIO?

Direct buffers have a number of advantages

• The avoid an extra copy of data passed between Java and native memory.
• If they are re-used, only the page used are turning into real memory. This means you can make them much larger than they need to me and they only waste virtual memory.
• You can access multi-byte primitives in native byte order efficiently. (Basically one machine code instruction)

Should I read into a heap buffer and parse it from there, doing many get() (byte by byte) OR should I read it into a direct buffer and parse from the direct buffer?

If you are reading a byte at a time, you may not get much advantage. However, with a direct byte buffer you can read 2 or 4 bytes at a time and effectively parse multiple bytes at once.

[real time] [selectors]

If you are parsing real time data, I would avoid using selectors. I have found using blocking NIO or busy waiting NIO can give you the lowest latency performance (assuming you have a relatively small number of connections e.g. up to 20)

Answer 2

A direct buffer is best when you are just copying the data, say from a socket to a file or vice versa, as the data doesn't have to traverse the JNI/Java boundary, it just stays in JNI land. If you are planning to look at the data yourself there's no point in a direct buffer.