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

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Date:	Wed, 25 Aug 2010 14:17:43 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Marc Aurele La France <tsi@...berta.ca>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	"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

Le mercredi 25 août 2010 à 16:10 +0400, Alexey Kuznetsov a écrit :
> Hello!
> 
> > It is, but ip_append_data() is allocating a huge head if MTU is huge.
> 
> Hmm, strange, as I remember, it was supposed to work right.
> 
> If the device supports SG (which is required to accept non-linear skbs anyway),
> then ip_append_* should allocate skbs not rounded up to mtu and we should
> allocate small skb with NFS header only. Does not it work?
> 
> I can only guess one possible trap: people could do _one_ huge ip_append_data()
> (instead of "planned" scenario, when the header is sent with ip_append_data()
> and the following payload is appended with ip_append_page()). Huge ip_append_data()
> will generate huge skb indeed. Is this the problem?
> 
> 
> BTW this issue could be revisited and this "will generate huge" can be reconsidered.
> Automatic generation of fragmented skbs was deliberately suppressed, because it was
> found that all devices existing at the moment when this code was written
> are strongly biased against SG. Current code tries to _avoid_ generating
> non-linear skbs, unless it is intended for zero-copy, which compensated
> bias against SG. Modern hardware should work better.
> 
> Alexey

Hi Alexey,

Few hours ago, I privately asked to Marc Aurele if its infiniband device
was supporting NETIF_F_SG in its features ;)

Thanks !


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