Re: [PATCH v2] add mtu set in virtio

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Sep 08, 2016 at 09:50:34AM +0200, Maxime Coquelin wrote:
> 
> 
> On 09/08/2016 09:30 AM, Yuanhan Liu wrote:
> >On Wed, Sep 07, 2016 at 11:16:47AM +0200, Maxime Coquelin wrote:
> >>
> >>
> >>On 09/07/2016 05:25 AM, Yuanhan Liu wrote:
> >>>On Tue, Aug 30, 2016 at 09:57:39AM +0200, Maxime Coquelin wrote:
> >>>>Hi Souvik,
> >>>>
> >>>>On 08/30/2016 01:02 AM, souvikdey33 wrote:
> >>>>>Signed-off-by: Souvik Dey <sodey@sonusnet.com>
> >>>>>
> >>>>>Fixes: 1fb8e8896ca8 ("Signed-off-by: Souvik Dey <sodey@sonusnet.com>")
> >>>>>Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
> >>>>>
> >>>>>Virtio interfaces should also support setting of mtu, as in case of cloud
> >>>>>it is expected to have the consistent mtu across the infrastructure that
> >>>>>the dhcp server sends and not hardcoded to 1500(default).
> >>>>>---
> >>>>>drivers/net/virtio/virtio_ethdev.c | 12 ++++++++++++
> >>>>>1 file changed, 12 insertions(+)
> >>>>
> >>>>FYI, there are some on-going changes in the VIRTIO specification
> >>>>so that the VHOST interface exposes its MTU to its VIRTIO peer.
> >>>>It may also be used as an alternative of what you patch achieves.
> >>>>
> >>>>I am working on its implementation in Qemu/DPDK, our goal being to
> >>>>reduce performance drops for small packets with Rx mergeable buffers
> >>>>feature enabled.
> >>>
> >>>Mind to educate me a bit on how that works?
> >>
> >>Of course.
> >>
> >>Basically, this is a way to advise the MTU we want in the guest.
> >>In the guest, if GRO is not enabled:
> >> - In case of Kernel virtio-net, it could be used to
> >>size the SKBs at the expected MTU. If possible, we could disable Rx
> >>mergeable buffers.
> >> - In case of virtio PMD, if the MTU advised by host is lower than the
> >>pre-allocated mbuf size for the receive queue, then we should not need
> >>mergeable buffers.
> >
> >Thanks for the explanation!
> >
> >I see. So, the point is to avoid using mergeable buffers while it is
> >enabled.
> >
> >>Does that sound reasonnable?
> >
> >Yeah, maybe. Just don't know how well it may work in real life. Have
> >you got any rought data so far?
> 
> The PoC is not done yet, only Qemu part is implemented.
> But what we noticed is that for small packets, we have a 50%
> degradation when rx mergeable buffers are on when running PVP
> use-case.
> 
> Main part of the degradation is due an additional cache-miss in
> virtio-pmd receive path, because we fetch the header to get the number
> of buffer.
> 
> When sending only small packets and removing this access, we recover
> 25% of the degradation.
> 
> The 25% remaining part may be reduced significantly with Zhihong series.
> 
> Hope it answer your questions.

Yes, it does and thanks for the info.

	--yliu