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

["Pierre Pfister (ppfister)" <ppfister@cisco.com>]

> Le 27 oct. 2016 à 12:19, Wang, Zhihong <zhihong.wang@intel.com> a écrit :
> 
> 
> 
>> -----Original Message-----
>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
>> Sent: Thursday, October 27, 2016 5:55 PM
>> To: Wang, Zhihong <zhihong.wang@intel.com>; Yuanhan Liu
>> <yuanhan.liu@linux.intel.com>; stephen@networkplumber.org; Pierre
>> Pfister (ppfister) <ppfister@cisco.com>
>> Cc: 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 10/27/2016 11:10 AM, 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.
>>> 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.
>> As a first test, I tried again Txonly from the guest to the host (Rxonly),
>> where Tx indirect descriptors are used, on my E5-2665 @2.40GHz:
>> v16.11-rc1: 10.81Mpps
>> v16.11-rc2: 10.91Mpps
>> 
>> -rc2 is even slightly better in my case.
>> Could you please run the same test on your platform?
> 
> I mean to use rc2 as both host and guest, and compare the
> perf between indirect=0 and indirect=1.
> 
> I use PVP traffic, tried both testpmd and OvS as the forwarding
> engine in host, and testpmd in guest.
> 
> Thanks
> Zhihong

From my experience, and as Michael pointed out, the best mode for small packets is obviously
ANY_LAYOUT so it uses a single descriptor per packet.

So, disabling indirect descriptors may give you better pps for 64 bytes packets, but that doesn't mean you should not implement, or enable, it in your driver. That just means that the guest is not taking the right decision, and uses indirect while it should actually use any_layout.

Given virtio/vhost design (most decision comes from the guest), the host should be liberal in what it accepts, and not try to influence guest implementation by carefully picking the features it supports. Otherwise guests will never get a chance to make the right decisions either.

- Pierre

> 
>> 
>> And could you provide me more info on your fwd bench?
>> Do you use dpdk-pktgen on host, or you do fwd on howt with a real NIC
>> also?
>> 
>> Thanks,
>> Maxime
>>> 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