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

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

On 10/28/2016 02:49 AM, Wang, Zhihong wrote:
>
>> > -----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.
>

Thanks, I never tried this test.
I managed to find an Haswell platform (Intel(R) Xeon(R) CPU E5-2699 v3
@ 2.30GHz), and can reproduce the problem with the loop test you
mention. I see a performance drop about 10% (8.94Mpps/8.08Mpps).
Out of curiosity, what are the numbers you get with your setup?

As I never tried this test, I run it again on my Sandy Bridge setup, and
I also see a performance regression, this time of 4%.

If I understand correctly the test, only 32 packets are allocated,
corresponding to a single burst, which is less than the queue size.
So it makes sense that the performance is lower with this test case.

Thanks,
Maxime