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

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Fri, Nov 04, 2016 at 12:22:47PM +0100, Maxime Coquelin wrote:
> 
> 
> >>>>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:

Have you built host DPDK with gcc6 as well?

	--yliu

> 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