netdev - Re: [net PATCH v2 2/2] ipv4/GRO: Make GRO conform to RFC 6864

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Message-Id: <20160406.153007.1347001580998282428.davem@davemloft.net>
Date:	Wed, 06 Apr 2016 15:30:07 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tom@...bertland.com
Cc:	ecree@...arflare.com, herbert@...dor.apana.org.au,
	alexander.duyck@...il.com, aduyck@...antis.com, jesse@...nel.org,
	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [net PATCH v2 2/2] ipv4/GRO: Make GRO conform to RFC 6864

From: Tom Herbert <tom@...bertland.com>
Date: Wed, 6 Apr 2016 14:42:26 -0300

> On Wed, Apr 6, 2016 at 12:43 PM, David Miller <davem@...emloft.net> wrote:
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Wed, 6 Apr 2016 10:53:52 -0300
>>
>>> Packets that are forwarded really should not be GRO'ed in the first
>>> place because of the loss of information and added latency.
>>
>> First of all GRO is supposed to be lossless, so please stop saying this
>> would be a reason to turn it off on a router.
>>
>> Second of all, the biggest piece of overhead is the routing lookup,
>> therefore GRO batching helps enormously with routing workloads, and
>> therefore is appropriate to be enabled on routers.
>>
>> Yes, I agree that for locally terminated stuff it helps more, but don't
>> turn this into a "GRO on routers, meh..." type argument.  It simply is
>> not true at all.
>>
> GRO on routers will help in a limited case where there is little load
> and the traffic is nicely groomed high tput TCP connections. But for
> routers with significant load, handling large quantities other
> protocols like UDP, GRO is not necessarily helpful and presents a
> nondeterministic performance improvement. For instance, I cannot
> provision a router with any assumptions that GRO will be effective for
> any % of packets to save any % of CPU, we need to provision based
> purely on ability to forward by pps assuming no benefit from GRO.

Just because you cannot predict how effective a facility will be,
that doesn't mean you shouldn't use it at all.