Multiple selects() within a single VM

Question

Is anyone aware of any contention issues when running multiple threads within a VM where each thread has it's own select statement?
I have an home grown app server that allows me to run multiple applications within a single VM. The applications are unrelated however they sometimes communicate with each other via TCP. All of them are very network intensive ( 500 messages a second where each message is around 200 bytes ). These apps are usually single threaded where the main thread is a select() loop.
Therefore, each application is a single threaded select() loop but there are multiple applications running within a single VM.
I am seeing performance issues when two apps running within the same VM try to send messages to one another via TCP. When the two apps are running on different boxes one app can send about 10,000 messages a second to the other app. However, when the apps are running within the same VM ( localhost TCP connection ) I can only transfer about 1000 messages a second.
Are 2 selectors running within the same VM a problem? Is it synchronized within the VM so that only one select can fire at a time?
I am running on a dual proc RH3 box with plenty of memory.
Any ideas?

Answer 1

Works for me, though I'm trying on Windows. Test program below. I get >10,000 replies and responses per second over loopback with both "java SelectPingPong both" (one VM) and running client and server on separate VMs.
Does this program get only 1000 messages/s on Linux? What VM version?
import java.util.*;
import java.net.*;
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
public class SelectPingPong
    private static final int MESSAGE_SIZE = 200;
    private String server_name;
    private Selector selector;
    private HashMap clients = new HashMap();
    static class Client
     ByteBuffer buf = ByteBuffer.allocate(MESSAGE_SIZE);
     long connect_time = System.currentTimeMillis();
     int number_of_messages;
    public SelectPingPong(int port, String server_name)
     throws IOException
     this.server_name = server_name;
     selector = Selector.open();
     ServerSocketChannel server_channel = ServerSocketChannel.open();
     server_channel.configureBlocking(false);
     server_channel.socket().bind(new InetSocketAddress(port));
     server_channel.register(selector, SelectionKey.OP_ACCEPT);
    public Socket connect(String host, int port)
     throws IOException
     SocketChannel channel = SocketChannel.open();
     Socket socket = channel.socket();
     socket.connect(new InetSocketAddress(host, port));
     configureSocket(socket);
     channel.configureBlocking(false);
     channel.register(selector, SelectionKey.OP_READ);
     clients.put(channel, new Client());
     return socket;
    private void configureSocket(Socket socket)
     throws IOException
     // Let's say we have a request-reply protocol with modest requirements
     socket.setReceiveBufferSize(1024);
     socket.setSendBufferSize(1024);
    public void mainLoop()
     while (true) {
         try {
          selector.select();
          for (Iterator iter = selector.selectedKeys().iterator(); iter.hasNext(); ) {
              SelectionKey key = (SelectionKey) iter.next();
              iter.remove();
              if (!key.isValid())
               continue;
              if (key.isAcceptable())
               acceptClient(key);
              if (key.isReadable())
               readFromClient(key);
         } catch (Exception e) {
          System.out.println(server_name + ": error in selector loop: " + e);
          e.printStackTrace();
    private void acceptClient(SelectionKey key)
     throws IOException
     ServerSocketChannel server_channel = (ServerSocketChannel) key.channel();
     if (server_channel == null)
         return;
     SocketChannel channel = server_channel.accept();
     if (channel == null)
         return;
     configureSocket(channel.socket());
     channel.configureBlocking(false);
     channel.register(selector, SelectionKey.OP_READ);
     clients.put(channel, new Client());
     System.out.println(server_name + ": got a new client; " +
                  clients.size() + " clients");
    private void readFromClient(SelectionKey key)
     throws IOException
     SocketChannel channel = (SocketChannel) key.channel();
     Client client = (Client) clients.get(channel);
     ByteBuffer buf = client.buf;
     int count;
     try {
         count = channel.read(buf);
     } catch (IOException e) {
         System.out.println(server_name + ": error reading, closing connection: " + e);
         clients.remove(channel);
         channel.close();
         return;
     if (count == -1) {
         clients.remove(channel);
         System.out.println(server_name + ": client disconnected; " +
                      clients.size() + " clients");
         channel.close();
         return;
     if (buf.position() == MESSAGE_SIZE) {
         if (++client.number_of_messages % 10000 == 0) {
          long now = System.currentTimeMillis();
          System.out.println(server_name + ": " + client.number_of_messages +
                       " messages in " + (now - client.connect_time) + " ms");
         buf.flip();
         // RFE write without blocking
         writeFully(channel, buf);
         buf.rewind();
    public static void writeFully(SocketChannel channel, ByteBuffer buf)
     throws IOException
     while (buf.remaining() != 0)
         channel.write(buf);
    private static void startClient()
     throws Exception
     SelectPingPong client = new SelectPingPong(6667, "client");
     Socket socket = client.connect("localhost", 6666);
     // Send initial message
     ByteBuffer buf = ByteBuffer.allocate(MESSAGE_SIZE);
     buf.put(new byte[MESSAGE_SIZE]);
     buf.flip();
     socket.getChannel().write(buf);
     client.mainLoop();
    public static void main(String args[])
     throws Exception
     if (args.length == 1 && args[0].equals("server")) {
         SelectPingPong server = new SelectPingPong(6666, "server");
         server.mainLoop();
     } else if (args.length == 1 && args[0].equals("client")) {
         startClient();
     } else if (args.length == 1 && args[0].equals("both")) {
         new Thread() { public void run() {
          try {
              SelectPingPong server = new SelectPingPong(6666, "server");
              server.mainLoop();
          } catch (Exception e) {
              System.err.println("error in server");
         } }.start();
         startClient();
     } else {
         System.err.println("usage: SelectPingPong [client | server | both]");
}