netdev - Re: [patch v4, kernel version 3.2.1] net/ipv4/ip

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Date:	Fri, 27 Jan 2012 13:50:49 -0800
From:	Jesse Gross <jesse@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Joseph Glanville <joseph.glanville@...onvm.com.au>,
	David Miller <davem@...emloft.net>, steweg@...t.sk,
	kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v4, kernel version 3.2.1] net/ipv4/ip_gre: Ethernet
 multipoint GRE over IP

On Thu, Jan 26, 2012 at 9:59 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 27 janvier 2012 à 09:24 +1100, Joseph Glanville a écrit :
>> David is correct, the forwarding speed of Open vSwitch is at parity
>> with the Linux Bridging module and its tunneling speed is actually
>> slightly faster than the in kernel GRE implementation. I have tested
>> this across a variety of configurations.
>
> Thanks for this input ! When was this tested exactly, and do you have
> some "perf tool" reports to provide ?
>
> GRE is lockless since one year or so (modulo how is setup the tunnel as
> discovered recently)

The out-of-tree OVS GRE stack (which is separate for practical and
historical reasons but I would like to combine with the in-kernel one
in the future) uses a few dirty tricks to help performance.  I don't
know the parameters that Joseph used to test but one that he might be
benefiting from here is where we cache the full set of headers to be
pushed (GRE/IP/Ethernet) as well any routing/switching decisions.
This turns a GRE encapsulation into a simple memcpy if there is enough
headroom.  It's not something that I would want to propose for more
general usage but I think that GSO/GRO for GRE would get at least some
of that benefit and would also make things easier when NICs with
support for encapsulated offloads start showing up.
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