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

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

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'm using RC2, and the CPU is Xeon E5-2699 v3 @ 2.30GHz.

Could you please verify if this is true in your test?


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