DatagramChannel.receive()

I use a selector to manage the unblocking DatagramChannel,
When I got the event that I can read from the channel
(key.isReadable()==true),
I call this method.

What I wanna know is - Is this method perform an I/O operation
at lower-layer or just copy bits from a place in the memory.

If it is the former, do I need to use multithread to invoke receive()
to make better performance???

this is part of my code:

 
channel.configureBlocking(false);
            selector = Selector.open();
            SelectionKey key = channel.register(selector, channel.validOps());

            while (run) {

                selector.select(250);

                if (key.isValid() && key.isReadable()) {

                    ByteBuffer buffer = ByteBuffer
                            .allocate(Constant.MAX_RECEIVE_BUFFER_SIZE);
                    buffer.order(ByteOrder.LITTLE_ENDIAN);
                    InetSocketAddress sock = (InetSocketAddress) channel
                            .receive(buffer);
                    if (sock == null)
                        continue;
                    receivedDatagramCount++;
                    log.info("received No." + receivedDatagramCount
                            + " datagram");
                    String ip = sock.getAddress().getHostAddress();
                    int port = sock.getPort();
                    if (!Crypt.decrypt(buffer.array(), buffer.position())) {
                        log.warn("checksum error$B!*(B");
                        return;
                    }

                    buffer.flip();
                    P2IHeaderInfo header = P2IHeaderInfo.getInstance(buffer);
                    buffer.rewind();

                    DispatchData data = new DispatchData(header, buffer, ip,
                            port, receivedDatagramCount);
                    Task task = new Task(Task.DISPATCH, data);
                    server.sendMessage(task);
                }
                if (key.isValid() && key.isWritable()) {

                    for(ToSendData data=resps.poll();data != null;data=resps.poll())
{
                                    String ip = data.getIp();
                        int port = data.getPort();
                        ByteBuffer buffer = data.getBuffer();
                        Crypt.encrypt(buffer.array(), buffer.remaining());
                        byte[] x = new byte[buffer.remaining()];
                        System.arraycopy(buffer.array(), 0, x, 0, buffer
                                .remaining());
                        sendDatagramCount++;
                        log.info("Server sends No." + sendDatagramCount
                                + " datagram: ");
                        StringUtil.printBytes(x);
                        channel.send(buffer, new InetSocketAddress(ip, port));
                    }
                }
                // }

            }
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (selector != null) {
                try {
                    selector.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            if (channel != null) {
                try {
                    channel.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }