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 11:04 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
> 
> 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.
> 

Maxime,
IMHO, it will be more like performance bug fix. Defer shadow ring update method brings performance issue in certain case.

Thanks,
Marvin

> > 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, will do it in next version.

> Thanks,
> Maxime