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




On 2013-9-23 13:02, Jason Wang wrote:
On 09/23/2013 07:04 AM, Anirban Chakraborty wrote:
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.

-Anirban
In this case, even if you still want to do GRO. It's better to find the
root cause of why the GSO packet were segmented

Totally agree, we need to find the cause why large packets is segmented only in different host case.

(maybe GSO were not
enabled for netback?), since it introduces extra overheads.

From Anirban's feedback, large packets can be seen on vif interface, and even on guests running on the same host.

Thanks
Annie

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