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SubjectRe: RFC: MTU for serving NFS on Infiniband
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On Tue, 2010-08-24 at 13:49 -0600, Marc Aurele La France wrote:
> 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@ualberta.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.

Not necessarily. Offloading it to hardware, where possible, is usually
a performance win.

> It is the "(or less)" part that I am working towards here.

The inability to allocate large linear buffers is not a good reason to
generate packets smaller than the MTU. You are working around the real
problem.

Ben.

--
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.



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