Re: [dpdk-dev] [PATCH] vhost: remove deferred shadow update

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

Hi Marvin,

On 4/15/20 4:55 PM, Liu, Yong wrote:
> 
> 
>> -----Original Message-----
>> From: Maxime Coquelin <maxime.coquelin@redhat.com>
>> Sent: Wednesday, April 15, 2020 10:16 PM
>> To: Liu, Yong <yong.liu@intel.com>; Ye, Xiaolong <xiaolong.ye@intel.com>;
>> Wang, Zhihong <zhihong.wang@intel.com>; eperezma@redhat.com
>> Cc: dev@dpdk.org
>> Subject: Re: [PATCH] vhost: remove deferred shadow update
>>
>>
>>
>> On 4/1/20 11:29 PM, Marvin Liu wrote:
>>> Defer shadow ring update will help overall throughput when frontend
>>> much slower than backend. But that is not all the cases we faced now.
>>> In case like ovs-dpdk + dpdk virtio user, frontend will much faster
>>> than backend. Frontend may not be able to collect available descs when
>>> shadow update is deferred. Thus will harm RFC2544 performance.
>>
>> I don't understand this comment. What is the difference in term of
>> performance between Qemu + Virtio PMD and Virtio-User PMD, as the
>> datapath is the same?
>>
> 
> Hi Maxime,
> The statement is for the different situations between virtio-net + vhost pmd and virtio-user + vhost pmd in ovs. 
> When combination is virtio-user + vhost pmd in ovs, frontend will be much faster than backend.  Defer used ring update won't give benefit when requiring zero packet loss. 

Ok, so you mean Virtio PMD vs. Virtio-net kernel driver.

Regarding who is faster between Virtio PMD and Vhost PMD, it actually
depends on what the applications using them are doing.

If you have OVS on host + testpmd on guest doing IO fowarding, then of
course the frontent is much faster.

But if you have testpmd IO forward on host + tespmd MACSWAP forward in
guest, then the frontend could be slower.

That looks like a benchmark optimization only.

> Regards,
> Marvin
> 
>>> Solution is just remove deferred shadow update, which will help RFC2544
>>> and fix potential issue with virtio net driver.
>>
>> What is the potential issue?
>>
>> Maxime
> 
> It  is napi stops issue which has been fixed by Eugenio.

OK, then I would suggest to change the patch title to:
"vhost: fix shadow update"

Then explicit the commit message to point to Eugenio's bug, and tag it
with the proper Fixes tag, so that the patch gets backported to 19.11
LTS.

Thanks,
Maxime