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    SubjectRe: RFC: MTU for serving NFS on Infiniband
    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 <> 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
    > > <>.
    > 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


    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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