netdev - Re: [RFC] [PATCH] Optimize TCP sendmsg in favour of fast devices?

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Date:	Fri, 15 Jan 2010 01:04:50 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	simon.kagstrom@...insight.net
Cc:	krkumar2@...ibm.com, ilpo.jarvinen@...sinki.fi,
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [RFC] [PATCH] Optimize TCP sendmsg in favour of fast devices?

From: Simon Kagstrom <simon.kagstrom@...insight.net>
Date: Fri, 15 Jan 2010 10:00:11 +0100

> On Fri, 15 Jan 2010 00:52:24 -0800 (PST)
> David Miller <davem@...emloft.net> wrote:
> 
>> > The same thing goes for via-velocity.c, it's turned on via ethtool
>> > though (ethtool_op_set_sg).
>> 
>> Indeed, see my reply to Krishna's ethtool_op_set_sg() patch.
>> 
>> I think it's a cruddy way to do things, SG ought to be on by
>> default always unless it is defective.  And if it's defective
>> support should be removed entirely.
> 
> I kept it off by default since I didn't see any big improvement in my
> tests (negative in some, positive in some). But I suppose you're
> right though.

Well, it has to provide significantly better performance for
sendfile() (especially wrt. cpu utilization) because we avoid the copy
out of the page cache pages entirely.
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