Messages in this thread | | | Subject | Re: RFC: MTU for serving NFS on Infiniband | From | Ben Hutchings <> | Date | Tue, 24 Aug 2010 23:20:41 +0100 |
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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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