netdev - Re: [PATCH net-next 2/2] openvswitch: Use zerocopy if applicable when performing the upcall

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Message-ID: <CAEP_g=898=iOWV15Y_bcjvS9sGSfDQSP1zQLNkffoXB5imdU7g@mail.gmail.com>
Date:	Fri, 24 May 2013 14:23:51 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Graf <tgraf@...g.ch>, netdev <netdev@...r.kernel.org>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>
Subject: Re: [PATCH net-next 2/2] openvswitch: Use zerocopy if applicable when
 performing the upcall

On Fri, May 24, 2013 at 11:58 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2013-05-24 at 10:24 -0700, Jesse Gross wrote:
>
>> Does this have any impact on small packets? Those are usually the
>> common case (i.e. TCP SYN) and I think this is slightly less optimal
>> for those.
>
> No difference at all, small packets are copied anyway in skb->head

Yes, but it makes the Open vSwitch code slightly worse - for example,
currently checksumming and copying are done in a single step but this
prevents that. Actually, I'm also curious about the test case that was
used for large packets and the full profile output since checksumming
and GSO aren't listed in the one that Thomas gave.

My guess is that there isn't a real different for small packets since
everything will be in the cache but it seems worth checking given that
this is optimizing a rare case at the expense of the common one.
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