linux-kernel - Re: sendfile() expert advice sought

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Message-ID: <23986fd91002171847l749e71eh6a71259b1ff0a2df@mail.gmail.com>
Date:	Wed, 17 Feb 2010 18:47:07 -0800
From:	"Patrick J. LoPresti" <lopresti@...il.com>
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: sendfile() expert advice sought

On Wed, Feb 17, 2010 at 6:19 PM, Bryan Donlan <bdonlan@...il.com> wrote:
>
> sendfile() always copies pages

Really?  So the description in the following article (see "Figure 4") is wrong?

http://www.linuxjournal.com/article/6345?page=0,1

(To be clear, I want to replace the step where the kernel buffers get
populated from disk with a step where I populate them programatically.
 That is what I mean by "sendfile() for anonymous pages".)

According to that article, among others, if your network device
supports scatter/gather and the pages are already resident, then
sendfile() is a true zero-copy operation.  That is, it takes page
cache pages and arranges to DMA them to the network card without
creating any additional copies in system RAM.

My questions make a lot more sense if that is how it works, anyway :-).

 - Pat
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