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

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Date:	Fri, 24 May 2013 14:57:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesse Gross <jesse@...ira.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, 2013-05-24 at 14:23 -0700, Jesse Gross wrote:
> 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.
> 

GSO is fully supported in nfnetlink, I see no reason why Open vSwitch
would not allow that.

> 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.

I really doubt checksumming a SYN/ACK packet is that a performance
issue. Do you have performance numbers ?

You could always provide a patch to restore this copy/checksum if it
really gives a benefit, and if people still use NIC not doing this
checksum.



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