Question

I have this weird problem with my (multithreaded) server when I get more than 500 players connected simultaneously, the PrinterWriter take more than 100 seconds or more (2 minutes) to finish flush() or print() sometimes.

Here is the code:

public static void send(Player p, String packet)
{   
    PrintWriter out = p.get_out();
    if(out != null && !packet.equals("") && !packet.equals(""+(char)0x00))
    {
        packet = Crypter.toUtf(packet);
        out.print((packet)+(char)0x00);
        out.flush();
    }
}

the printWriter is something like this:

_in = new BufferedReader(new InputStreamReader(_socket.getInputStream()));
_out = new PrintWriter(_socket.getOutputStream());

If I add the keyword synchronized to the send() method, the whole server starts to lag every 2 seconds, if I don’t then some random player starts to lag for no reason.

Anyone have any idea ? Where is this coming from? What should I do?

Answer 1

The print writer is wrapped around a socket’s output stream, so I’m going to guess and say that the socket’s output buffer is full, and so the write/flush call will block until the buffer has enough room to accommodate the message being sent.

The socket send buffer may become full if data is being written to it faster than it can be transmitted to the client (or faster than the client can receive it).

Edit:

P.S. If you’re having scalability problems, it may be due to using java.io (which requires one thread per socket) instead of java.nio (in which case a single thread can detect and perform operations on those sockets which have pending data). nio is intended to support applications which must scale to a large number of connections, but the programming model is more difficult.