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

Maxime Coquelin maxime.coquelin at redhat.com
Fri Nov 4 12:22:47 CET 2016



>>>> Hi Maxime,
>>>>
>>>> I did a little more macswap test and found out more stuff here:
>>> Thanks for doing more tests.
>>>
>>>>
>>>>  1. I did loopback test on another HSW machine with the same H/W,
>>>>     and indirect_desc on and off seems have close perf
>>>>
>>>>  2. So I checked the gcc version:
>>>>
>>>>      *  Previous: gcc version 6.2.1 20160916 (Fedora 24)
>>>>
>>>>      *  New: gcc version 5.4.0 20160609 (Ubuntu 16.04.1 LTS)
>>>
>>> On my side, I tested with RHEL7.3:
>>>  - gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11)
>>>
>>> It certainly contains some backports from newer GCC versions.
>>>
>>>>
>>>>     On previous one indirect_desc has 20% drop
>>>>
>>>>  3. Then I compiled binary on Ubuntu and scp to Fedora, and as
>>>>     expected I got the same perf as on Ubuntu, and the perf gap
>>>>     disappeared, so gcc is definitely one factor here
>>>>
>>>>  4. Then I use the Ubuntu binary on Fedora for PVP test, then the
>>>>     perf gap comes back again and the same with the Fedora binary
>>>>     results, indirect_desc causes about 20% drop
>>>
>>> Let me know if I understand correctly:
>
> Yes, and it's hard to breakdown further at this time.
>
> Also we may need to check whether it's caused by certain NIC
> model. Unfortunately I don't have the right setup right now.
>
>>> Loopback test with macswap:
>>>  - gcc version 6.2.1 : 20% perf drop
>>>  - gcc version 5.4.0 : No drop
>>>
>>> PVP test with macswap:
>>>  - gcc version 6.2.1 : 20% perf drop
>>>  - gcc version 5.4.0 : 20% perf drop
>>
>> I forgot to ask, did you recompile only host, or both host and guest
>> testmpd's in your test?

> Both.

I recompiled testpmd on a Fedora 24 machine using GCC6:
gcc (GCC) 6.1.1 20160621 (Red Hat 6.1.1-3)
Testing loopback with macswap on my Haswell RHEL7.3 machine gives me the 
following results:
  - indirect on: 7.75Mpps
  - indirect off: 7.35Mpps

Surprisingly, I get better results with indirect on my setup (I
reproduced the tests multiple times).

Do you have a document explaining the tuning/config you apply to both 
the host and the guest (isolation, HT, hugepage size, ...) in your
setup?

Regards,
Maxime


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