netdev - Re: [Xen-devel] [PATCH net-next] xen-netfront: convert to GRO API and advertise this feature

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Date:	Sun, 22 Sep 2013 23:04:12 +0000
From:	Anirban Chakraborty <abchak@...iper.net>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	Jason Wang <jasowang@...hat.com>,
	"<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	"<xen-devel@...ts.xen.org>" <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH net-next] xen-netfront: convert to GRO API
 and advertise this feature


On Sep 22, 2013, at 5:09 AM, Wei Liu <wei.liu2@...rix.com> wrote:

> On Sun, Sep 22, 2013 at 02:29:15PM +0800, Jason Wang wrote:
>> On 09/22/2013 12:05 AM, Wei Liu wrote:
>>> Anirban was seeing netfront received MTU size packets, which downgraded
>>> throughput. The following patch makes netfront use GRO API which
>>> improves throughput for that case.
>>> 
>>> Signed-off-by: Wei Liu <wei.liu2@...rix.com>
>>> Signed-off-by: Anirban Chakraborty <abchak@...iper.net>
>>> Cc: Ian Campbell <ian.campbell@...rix.com>
>> 
>> Maybe a dumb question: doesn't Xen depends on the driver of host card to
>> do GRO and pass it to netfront? What the case that netfront can receive
> 
> The would be the ideal situation. Netback pushes large packets to
> netfront and netfront sees large packets.
> 
>> a MTU size packet, for a card that does not support GRO in host? Doing
> 
> However Anirban saw the case when backend interface receives large
> packets but netfront sees MTU size packets, so my thought is there is
> certain configuration that leads to this issue. As we cannot tell
> users what to enable and what not to enable so I would like to solve
> this within our driver.
> 
>> GRO twice may introduce extra overheads.
>> 
> 
> AIUI if the packet that frontend sees is large already then the GRO path
> is quite short which will not introduce heavy penalty, while on the
> other hand if packet is segmented doing GRO improves throughput.
> 

Thanks Wei, for explaining and submitting the patch. I would like add following to what you have already mentioned.
In my configuration, I was seeing netback was pushing large packets to the guest (Centos 6.4) but the netfront was receiving MTU sized packets. With this patch on, I do see large packets received on the guest interface. As a result there was substantial throughput improvement in the guest side (2.8 Gbps to 3.8 Gbps). Also, note that the host NIC driver was enabled for GRO already. 

-Anirban
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