netdev - Re: [PATCH] sendfile() and UDP socket

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Date:	Sun, 21 Sep 2008 19:07:15 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	johnpol@....mipt.ru
Cc:	johaahn@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] sendfile() and UDP socket

From: Evgeniy Polyakov <johnpol@....mipt.ru>
Date: Mon, 22 Sep 2008 05:08:34 +0400

> On Sun, Sep 21, 2008 at 05:44:50PM -0700, David Miller (davem@...emloft.net) wrote:
> > > > Applications which work over datagram protocols must perform their own
> > > > segmentation.  It is not like doing a send over a stream protocol like
> > > > TCP, where you can use whatever length you want for send calls and
> > > > segmentation is done for the application.
> > > 
> > > But isn't the whole idea of the sendfile() is to send a file no matter
> > > what underlying media is?
> > 
> > It's a way to fabricate a send() directly from the page cache.
> 
> And to send exactly required number of bytes (or size of the cache)?
> To send a single page (combined to several other pages) we have simple
> ->sendpage() callback, which should not return error when it is asked to
> send a data and it can do it by actually submitting two packets without
> special tcp-like processing of the segments.

You're basically throwing away the difference between datagram and stream
socket semantics.

I don't see what else I can explain if you cannot see that this is
significant.
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