[dpdk-dev] [PATCH v4] vhost: Add indirect descriptors support to the TX path

[Yuanhan Liu]

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?

	--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 at redhat.com]
> >>Sent: Monday, October 17, 2016 10:15 PM
> >>To: Yuanhan Liu <yuanhan.liu at linux.intel.com>
> >>Cc: Wang, Zhihong <zhihong.wang at intel.com>; Xie, Huawei
> >><huawei.xie at intel.com>; dev at dpdk.org; vkaplans at redhat.com;
> >>mst at redhat.com; stephen at 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