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

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Message-Id: <20131029.223331.1745589461091527596.davem@davemloft.net>
Date:	Tue, 29 Oct 2013 22:33:31 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hkchu@...gle.com
Cc:	herbert@...dor.apana.org.au, eric.dumazet@...il.com,
	christoph.paasch@...ouvain.be, netdev@...r.kernel.org,
	mwdalton@...gle.com
Subject: Re: [PATCH v2 net-next] net: introduce gro_frag_list_enable sysctl

From: Jerry Chu <hkchu@...gle.com>
Date: Tue, 29 Oct 2013 19:13:50 -0700

> On Tue, Oct 29, 2013 at 7:05 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>> On Tue, Oct 29, 2013 at 10:02:53PM -0400, David Miller wrote:
>>> From: Eric Dumazet <eric.dumazet@...il.com>
>>> Date: Tue, 29 Oct 2013 17:53:48 -0700
>>>
>>> > So should we apply the first fix to avoid the BUG_ON() ?
>>>
>>> Please be more specific, are you talking about splitting up
>>> this patch in some way?
>>
>> I think Eric is referring to the patch that removes the BUG_ON
>> in skb_segment and deals with the new mega-GRO packets.
>>
>> I think that's fine for stable, but for the long term we should
>> fix it properly as these new meag-GRO packets still retain the
>> existing packet boundaries and are trivially segmentable.
>>
>> If we are indeed able to do that, I doubt we would even need
>> the sysctl patch since the GRO performance should be vastly
>> superior to the non-GRO case, even for a router/bridge.
> 
> Probably not the case for the simple forwarding case. See my
> test result of some small (5-8%) CPU+throughput penalty from
> GRO (over GRE tunnel) posted previously. But I can believe
> the number may be very different if the forwarding path involves
> more work (NAT, iptables filtering,...,etc) resulting in a higher per
> pkt cost.

It's that way because it's not implemented properly.

GRO should always win, even on a router, because it decreases the
number of fundamental operations (routing lookups) that the stack
needs to perform.
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