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

["Liu, Yong" <yong.liu@intel.com>]

> -----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. 

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.