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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@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> Signed-off-by: Anirban Chakraborty <abchak@xxxxxxxxxxx>
>>> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> 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. 


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