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

[Bela Ban (JIRA)]

     [ https://issues.jboss.org/browse/JGRP-1476?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Bela Ban resolved JGRP-1476.
----------------------------

    Resolution: Done

    
> 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:
> {code}
>         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)
>         }
> {code}
> 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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