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

["Wang, Zhihong" <zhihong.wang@intel.com>]

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu@linux.intel.com]
> Sent: Thursday, October 27, 2016 6:46 PM
> To: Maxime Coquelin <maxime.coquelin@redhat.com>
> Cc: Wang, Zhihong <zhihong.wang@intel.com>;
> stephen@networkplumber.org; Pierre Pfister (ppfister)
> <ppfister@cisco.com>; Xie, Huawei <huawei.xie@intel.com>; dev@dpdk.org;
> vkaplans@redhat.com; mst@redhat.com
> Subject: Re: [dpdk-dev] [PATCH v4] vhost: Add indirect descriptors support
> to the TX path
> 
> 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.

In fact simple loopback test will also do, without pktgen.

Start testpmd in both host and guest, and do "start" in one
and "start tx_first 32" in another.

Perf drop is about 24% in my test.

> 
> 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