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

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

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.
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