From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: souvikdey33 <sodey@sonusnet.com>,
stephen@networkplumber.org, huawei.xie@intel.com, dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH v2] add mtu set in virtio
Date: Thu, 8 Sep 2016 09:50:34 +0200 [thread overview]
Message-ID: <c0901a43-4904-9498-d22d-21ea4d208021@redhat.com> (raw)
In-Reply-To: <20160908073029.GM23158@yliu-dev.sh.intel.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
next prev parent reply other threads:[~2016-09-08 7:50 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-29 23:02 souvikdey33
2016-08-30 7:57 ` Maxime Coquelin
2016-09-01 22:20 ` Dey, Souvik
2016-09-02 7:05 ` Maxime Coquelin
2016-09-07 2:11 ` Dey, Souvik
2016-09-07 8:28 ` Maxime Coquelin
2016-09-07 3:25 ` Yuanhan Liu
2016-09-07 9:16 ` Maxime Coquelin
2016-09-08 7:30 ` Yuanhan Liu
2016-09-08 7:50 ` Maxime Coquelin [this message]
2016-09-08 7:57 ` Yuanhan Liu
2016-09-09 3:44 ` Dey, Souvik
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