[jboss-jira] [JBoss JIRA] (JGRP-1476) TCP: double-write is useless as output stream is buffered

[Bela Ban (JIRA)]

Bela Ban created JGRP-1476:
------------------------------

             Summary: TCP: double-write is useless as output stream is buffered
                 Key: JGRP-1476
                 URL: https://issues.jboss.org/browse/JGRP-1476
             Project: JGroups
          Issue Type: Enhancement
            Reporter: Bela Ban
            Assignee: Bela Ban
            Priority: Minor
             Fix For: 3.0.11, 3.1


[Stanimir Simeonoff]

Now I'd like to report a relatively minor issue w/ TCPConnection.doSend and
'double-writes' per Unix Network Programming.
The body of the method is like follows:

        private void doSend(byte[] data, int offset, int length) throws
Exception {
            // we're using 'double-writes', sending the buffer to the
destination in 2 pieces. this would
            // ensure that, if the peer closed the connection while we were
idle, we would get an exception.
            // this won't happen if we use a single write (see Stevens, ch.
5.13).

            out.writeInt(length); // write the length of the data buffer
first
            *Util.doubleWrite(data, offset, length, out)*;// should be
just: out.write(data, offset, length);
            out.flush(); // may not be very efficient (but safe)
        }

The comment claims it uses 'double-writes' in order to capture a missing
connection close by the peer. Firstly, that misled me there were not
buffers involved, so it writes straight to the socket output stream but in
that case out.flush() is a NOP.
If that was the case there would be at least 6 packets(!) per message (when
naggling is disabled) - 4 separate calls to each byte of writeInt and then
2 more for Util.doubleWrite. That looked very suspicious and must have been
caught, so I  took a look at the out declaration and it's buffered via
BufferedOutputStream with default 8k buf size which is the better size on
linux (buffers less than 8k are allocated on the stack instead of
malloc/free).
However that means doubleWrite is totally not useful as the output is
buffered anyways.
Also double-write is not useful if there is a reading/polling as RST would
be caught immediately, closing the connection. It's useful only if there is
no reading on the socket and that's quite a corner case.


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