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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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 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.

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