Re: [PATCH v3] net:Add sysctl_max_skb_frags

From	Hannes Frederic Sowa <hannes@stressinduktion.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3] net:Add sysctl_max_skb_frags
Date	2016-02-03 15:10 +0100
Message-ID	<qY6i8-2YF-31@gated-at.bofh.it> (permalink)
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On 03.02.2016 13:20, Herbert Xu wrote:
> On Wed, Feb 03, 2016 at 12:36:21PM +0100, Hannes Frederic Sowa wrote:
>>
>> Agreed that it feels like a hack, but a rather simple one. I would
>> consider this to be just a performance improvement. We certainly need
>> a slow-path when virtio drivers submit gso packets to the stack (and
>> already discussed with Hans). The sysctl can't help here. But without
>> the sysctl the packets would constantly hit the slow-path in case of
>> e.g. IPoIB and that would also be rather bad.
>
> So you want to penalise every NIC in the system if just one of
> them is broken? This is insane.  Just do the partial linearisation
> in that one driver that needs it and not only won't you have to
> penalise anyone else but you still get the best result for that
> driver that needs it.

Most normal Ethernet systems and drivers currently don't need tweating 
this knob at all, only some special kinds of installations. This patch 
referred to IPoIB as a possible user which drivers/firmware/cards seem 
to have this problem. Current behavior just leaves everything as-is.

If you use IPoIB you probably use it quite regular and linearizing an 
skbs *always* seems to be much more work than simply capping the number 
of frags globally.

> Besides, you have to implement the linearisation anyway because
> of virtualisation.

Yes, the slow-path is necessary. But instead of writing a new 
complicated linearizing function to just reduce the fragments we could 
also simply linearize it completely and ask the admin to also tune the 
vm guests.

I only see this tuning in kind in very specific environments where the 
admins now what they do.

Bye,
Hannes

Thread

[PATCH v3] net:Add sysctl_max_skb_frags Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-02-03 09:30 +0100
  Re: [PATCH v3] net:Add sysctl_max_skb_frags Herbert Xu <herbert@gondor.apana.org.au> - 2016-02-03 12:30 +0100
    Re: [PATCH v3] net:Add sysctl_max_skb_frags Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-02-03 12:40 +0100
      Re: [PATCH v3] net:Add sysctl_max_skb_frags Herbert Xu <herbert@gondor.apana.org.au> - 2016-02-03 13:30 +0100
        Re: [PATCH v3] net:Add sysctl_max_skb_frags Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-02-03 15:10 +0100
        Re: [PATCH v3] net:Add sysctl_max_skb_frags Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-03 15:40 +0100
        RE: [PATCH v3] net:Add sysctl_max_skb_frags David Laight <David.Laight@ACULAB.COM> - 2016-02-03 18:40 +0100
  Re: [PATCH v3] net:Add sysctl_max_skb_frags Alexander Duyck <alexander.duyck@gmail.com> - 2016-02-03 17:00 +0100
    Re: [PATCH v3] net:Add sysctl_max_skb_frags Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-03 17:10 +0100
      Re: [PATCH v3] net:Add sysctl_max_skb_frags Alexander Duyck <alexander.duyck@gmail.com> - 2016-02-03 18:50 +0100
        Re: [PATCH v3] net:Add sysctl_max_skb_frags Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-03 19:00 +0100
          Re: [PATCH v3] net:Add sysctl_max_skb_frags Alexander Duyck <alexander.duyck@gmail.com> - 2016-02-03 19:30 +0100
            Re: [PATCH v3] net:Add sysctl_max_skb_frags Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-03 20:30 +0100
              Re: [PATCH v3] net:Add sysctl_max_skb_frags Alexander Duyck <alexander.duyck@gmail.com> - 2016-02-03 22:10 +0100
  Re: [PATCH v3] net:Add sysctl_max_skb_frags David Miller <davem@davemloft.net> - 2016-02-09 10:40 +0100

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