netdev - Re: RFC: MTU for serving NFS on Infiniband

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Date:	Tue, 24 Aug 2010 13:49:14 -0600 (Mountain Daylight Time)
From:	Marc Aurele La France <tsi@...berta.ca>
To:	Ben Hutchings <bhutchings@...arflare.com>
cc:	Stephen Hemminger <shemminger@...tta.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: RFC: MTU for serving NFS on Infiniband

On Tue, 24 Aug 2010, Ben Hutchings wrote:
> On Tue, 2010-08-24 at 09:14 -0600, Marc Aurele La France wrote:
>> On Mon, 23 Aug 2010, Stephen Hemminger wrote:
>>> On Mon, 23 Aug 2010 08:44:37 -0600 (MDT)
>>> Marc Aurele La France <tsi@...berta.ca> wrote:
>>>> In regrouping for my next tack at this, I noticed that all stack traces go
>>>> through ip_append_data().  This would be ipv6_append_data() in the IPv6 case.
>>>> A _very_ rough draft that would have ip_append_data() temporarily drop down
>>>> to a smaller fake MTU follows ...

>>> Why doesn't NFS generate page size fragments?  Does Infiniband or your
>>> device not support this?  Any thing that requires higher order allocation
>>> is going to unstable under load.  Let's fix the cause not the apply bandaid
>>> solution to the symptom.

>> From what I can tell, IP fragmentation is done centrally.
> [...]

> Stephen and I are not talking about IP fragmentation, but about the
> ability to append 'fragments' to an skb rather than putting the entire
> packet payload in a linear buffer.  See
> <http://vger.kernel.org/~davem/skb_data.html>.

Any payload has to either fit in the MTU, or has to be broken up into 
MTU-sized (or less) fragments, come hell or high water.  That this is done 
centrally is a good thing.  It is the "(or less)" part that I am working 
towards here.

Marc.

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