Not more than 4 Worker threads

[Rajkumar S]

Hello all,

I am testing out netty for a project of mine and started out with the
DiscardServer in the user guide. To simulate a processing delay of 3
seconds and to observe how the system will behave when there is a
processing delay in messageReceived I added a sleep of 3 seconds to
messageReceived in DiscardServerHandler  as shown below:

   public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
        ChannelBuffer buf = (ChannelBuffer) e.getMessage();
        while(buf.readable()) {
            System.out.print((char) buf.readByte());
            System.out.flush();
        }
        try {
                Thread.sleep(3000);
        } catch(Exception ev){}
    }


The client program is a simple perl program that connects to the
server and transmit one line of text. This perl program is started 100
times by a loop in in a shell script to simulate 100 concurrent
connection.

I was expecting the DiscardServer to launch 100 worker threads where
each will print all the incoming message in one step and then sleep
for 3 seconds.

What I observed was that 4 messages are printed, then wait for 3
seconds, again 4 messages and 3 seconds wait till all the messages are
printed. During this time can see the connections that are not
processed in netstat.  They are are in CLOSE_WAIT and Recv-Q  has a
size of 6 (which is the number of characters in the line of text sent
by client) During every 3 seconds 4 connections will disappear from
netstat output as they are processed by DiscardServer. An example line
from netstart is as follows:

tcp6       6      0 192.168.3.18:8080       192.168.3.18:41671      CLOSE_WAIT

Now my questions are:

1. Is this the expected behavior?
2. Why didn't the number of threads increase to process the
connections that were available?
3. Why the magic number of 4? (I am using a Core2 Duo CPU with 2
cores) I tried to use newFixedThreadPool but the number of threads
started does not go above 4, but it does come down if a number < 4 is
used.
4. In a real world application if there is a heavy processing involved
for received messages (say takes three seconds to process each
message) is the correct approach to add that to messageReceived
method?
5. Am i missing any thing here? :)

I am just trying to get a hang of the natty library and event driven
programming in general. So please excuse me asking so many trivial
questions!

Thanks and regards,

raj