Re: [dpdk-dev] [PATCH v2] add mtu set in virtio

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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.

Thanks,
Maxime