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

[annie li <annie.li@xxxxxxxxxx>]

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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