Re: [PATCH v4] vhost: Add indirect descriptors support to the TX path

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Oct 27, 2016 at 12:35:11PM +0200, Maxime Coquelin wrote:
> 
> 
> On 10/27/2016 12:33 PM, Yuanhan Liu wrote:
> >On Thu, Oct 27, 2016 at 11:10:34AM +0200, Maxime Coquelin wrote:
> >>Hi Zhihong,
> >>
> >>On 10/27/2016 11:00 AM, Wang, Zhihong wrote:
> >>>Hi Maxime,
> >>>
> >>>Seems indirect desc feature is causing serious performance
> >>>degradation on Haswell platform, about 20% drop for both
> >>>mrg=on and mrg=off (--txqflags=0xf00, non-vector version),
> >>>both iofwd and macfwd.
> >>I tested PVP (with macswap on guest) and Txonly/Rxonly on an Ivy Bridge
> >>platform, and didn't faced such a drop.
> >
> >I was actually wondering that may be the cause. I tested it with
> >my IvyBridge server as well, I saw no drop.
> >
> >Maybe you should find a similar platform (Haswell) and have a try?
> Yes, that's why I asked Zhihong whether he could test Txonly in guest to
> see if issue is reproducible like this.

I have no Haswell box, otherwise I could do a quick test for you. IIRC,
he tried to disable the indirect_desc feature, then the performance
recovered. So, it's likely the indirect_desc is the culprit here.

> I will be easier for me to find an Haswell machine if it has not to be
> connected back to back to and HW/SW packet generator.

Makes sense.

	--yliu
> 
> Thanks,
> Maxime
> 
> >
> >	--yliu
> >
> >>Have you tried to pass indirect_desc=off to qemu cmdline to see if you
> >>recover the performance?
> >>
> >>Yuanhan, which platform did you use when you tested it with zero copy?
> >>
> >>>
> >>>I'm using RC2, and the CPU is Xeon E5-2699 v3 @ 2.30GHz.
> >>>
> >>>Could you please verify if this is true in your test?
> >>I'll try -rc1/-rc2 on my platform, and let you know.
> >>
> >>Thanks,
> >>Maxime
> >>
> >>>
> >>>
> >>>Thanks
> >>>Zhihong
> >>>
> >>>>-----Original Message-----
> >>>>From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> >>>>Sent: Monday, October 17, 2016 10:15 PM
> >>>>To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> >>>>Cc: Wang, Zhihong <zhihong.wang@intel.com>; Xie, Huawei
> >>>><huawei.xie@intel.com>; dev@dpdk.org; vkaplans@redhat.com;
> >>>>mst@redhat.com; stephen@networkplumber.org
> >>>>Subject: Re: [dpdk-dev] [PATCH v4] vhost: Add indirect descriptors support
> >>>>to the TX path
> >>>>
> >>>>
> >>>>
> >>>>On 10/17/2016 03:21 PM, Yuanhan Liu wrote:
> >>>>>On Mon, Oct 17, 2016 at 01:23:23PM +0200, Maxime Coquelin wrote:
> >>>>>>>On my side, I just setup 2 Windows 2016 VMs, and confirm the issue.
> >>>>>>>I'll continue the investigation early next week.
> >>>>>>
> >>>>>>The root cause is identified.
> >>>>>>When INDIRECT_DESC feature is negotiated, Windows guest uses indirect
> >>>>>>for both Tx and Rx descriptors, whereas Linux guests (Virtio PMD &
> >>>>>>virtio-net kernel driver) use indirect only for Tx.
> >>>>>>I'll implement indirect support for the Rx path in vhost lib, but the
> >>>>>>change will be too big for -rc release.
> >>>>>>I propose in the mean time to disable INDIRECT_DESC feature in vhost
> >>>>>>lib, we can still enable it locally for testing.
> >>>>>>
> >>>>>>Yuanhan, is it ok for you?
> >>>>>
> >>>>>That's okay.
> >>>>I'll send a patch to disable it then.
> >>>>
> >>>>>
> >>>>>>
> >>>>>>>Has anyone already tested Windows guest with vhost-net, which also
> >>>>has
> >>>>>>>indirect descs support?
> >>>>>>
> >>>>>>I tested and confirm it works with vhost-net.
> >>>>>
> >>>>>I'm a bit confused then. IIRC, vhost-net also doesn't support indirect
> >>>>>for Rx path, right?
> >>>>
> >>>>No, it does support it actually.
> >>>>I thought it didn't support too, I misread the Kernel implementation of
> >>>>vhost-net and virtio-net. Acutally, virtio-net makes use of indirect
> >>>>in Rx path when mergeable buffers is disabled.
> >>>>
> >>>>The confusion certainly comes from me, sorry about that.
> >>>>
> >>>>Maxime