netdev - Re: [PATCH v2 net-next] net: introduce gro_frag_list

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Date:	Wed, 30 Oct 2013 14:09:54 -0400
From:	Vlad Yasevich <vyasevic@...hat.com>
To:	Jerry Chu <hkchu@...gle.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Christoph Paasch <christoph.paasch@...ouvain.be>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Michael Dalton <mwdalton@...gle.com>
Subject: Re: [PATCH v2 net-next] net: introduce gro_frag_list_enable sysctl

On 10/30/2013 01:39 PM, Jerry Chu wrote:
> On Tue, Oct 29, 2013 at 9:42 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>>
>> On Tue, Oct 29, 2013 at 09:34:41PM -0700, Eric Dumazet wrote:
>>>
>>> What matters ?
>>>
>>> GRO ?
>>
>> What matters is that you should not treat the forwarding case
>> separately from the host case.
>>
>> For virtualisation the host case looks exactly like the forwarding
>> case.
>
>
> Not sure I agree - there are two different "forwarding" cases - forwarding
> to another physical NIC (to go out to the wire hence need to do GSO),
> and (for virtualization) forwarding to a virtual NIC and consumed internally
> (e.g., VM).

I don't think you can really differentiate these 2 case.  VM are
very commonly used as routers/forwarders.  In some cases, to get
better throughput,  VFs are assigned to the VMs as the externally
facing ports.  So, you still end up forwarding to another physical
NIC.

-vlad

> For the latter we should strive to push GSO pkts all the way
> to the VM stack w/o breaking them up. So for virtualization GRO is all
> goodness but not sure about the regular forwarding path. (From the
> perf perspective it boils down to if the cost of GSO/GRO will offset
> the benefit of GRO. Sure if one manages to get the cost close to zero
> than there is not reason to leave GRO always on. But it's still a big if for
> now.)
>
> Best,
>
> Jerry
>
>>
>>
>> IOW, if having a 64KB packet matters for the host, then it matters
>> for forwarding as well.
>>
>>> Before my patch, GRO packets were 17 MSS, and nobody complained packets
>>> were too small, so what are you saying exactly ?
>>
>> I'm not criticsing your mega-GRO patch at all.  That one is great
>> and means that we'll get aggregated packets up to 64K.  What we need
>> to do is just to patch up the GSO code so that it can handle these
>> mega-packets properly.
>>
>> Cheers,
>> --
>> Email: Herbert Xu <herbert@...dor.apana.org.au>
>> Home Page: http://gondor.apana.org.au/~herbert/
>> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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