[Twisted-Python] Sending a long string/buffer without copying it

[Brian Granger]

But a producer will just make sure the whole thing isn't copied at the
same time right?  It still does many smaller copies - while the memory
is saved there is still the performance hit.

I just wanted to make sure that I wan't missing something obvious.  I
think the right way of doing this is to use a true rw buffer, such as
those created by numpy.

On 9/28/06, Jean-Paul Calderone <exarkun at divmod.com> wrote:
> On Thu, 28 Sep 2006 00:11:41 -0600, Brian Granger <ellisonbg.net at gmail.com> wrote:
> >HI,
> >
> >In one of my Twisted based applications, I need to send large string
> >and buffers.  They can be 100's of MB's long (they come from large
> >numpy arrays).  I would like to be able to send them *without making
> >any copies* in the process.
> >
> >This seems to be dificult with the way that certain parts of Twisted
> >are written:
> >
> >in protocols.basic many of the sendString/sendLine method having
> >things that make a copy of the string or line to be send:
> >
> >    def sendLine(self, line):
> >        """Sends a line to the other end of the connection.
> >        """
> >        return self.transport.write(line + self.delimiter)
> >
> >If line is 100MB, this just made a second 100MB string.  To make
> >things worse, in my case a server needs to send this line to many
> >clients that are connected.  The line gets copied for each client!  If
> >I have 10 clients, I have nearly a GB worth of extra memory allocated
> >for this temporary copy.
> >
> >This problem is easy solve at the protocol level: you just do separate
> >writes for the delimiter and the line.  Or if you are using a length
> >prefixed protocol, write the length bytes and the string separately.
> >
> >BUT....
> >
> >Even if I do that, it appears that Twisted is making copies elsewhere
> >- like in FileDescriptor.doWrite.  So, how can I send something
> >without making a copy?  I don't mind making copies of slices, just not
> >the whole thing.
>
> Don't pass the entire thing to a single call to transport.write() (or
> LineReceiver.sendLine).  Instead, write a producer.
>
> Jean-Paul
>
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