Where is the memory of direct buffer allocated by netty, kernel space or user space?

Question

I write a simple http client using netty 4. As far as I know, if I use HeapBuffer, when I get the ByteBuffer in the ChannelInboundHandler

@Override
public void channelRead(ChannelHandlerContext ctx, final ByteBuf msg) throws Exception {
    // do nothing with the msg
}

The bytes from the website should be a process like (even if I don't call ByteBuf.read)

socket -> kernel space -> user space

The bytes should have been in the java heap, right?

I want to know if I use DirectBuffer, when the "channelRead" triggered, where are the bytes form the website if I don't call ByteBuf.read, are they in the kernel space, or in the user space? Is it one less memory-copy than I use HeapBuffer (because it won't copy to the user space)?

Answer 1

A DirectBuffer is allocated in user space, but outside the JVM heap. Note that when talking about socket I/O, this still involves a copy from the kernel buffers to the user space buffers. However, it does eliminate the need for intermediate user-space buffers that the JVM would need to use to copy to/from the heap. As such, I/O operations can happen directly on the native user-space buffer, as if you were running C code.

Re: your netty code snippet, by the time netty calls channelRead , data has already been read into the ByteBuf (in user-space). If you want the data to remain in socket buffers (for example, to implement backpressure), you can call channel.config.setAutoRead(false) to tell netty not to read from the socket buffers automatically.