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

["Wang, Zhihong" <zhihong.wang@intel.com>]

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> Sent: Friday, November 4, 2016 7:23 PM
> To: Wang, Zhihong <zhihong.wang@intel.com>; Yuanhan Liu
> <yuanhan.liu@linux.intel.com>
> Cc: 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
> 
> 
> 
> >>>> 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?


The setup where it goes wrong:
 1. Xeon E5-2699, HT on, turbo off, 1GB hugepage for both host and guest
 2. Fortville 40G
 3. Fedora 4.7.5-200.fc24.x86_64
 4. gcc version 6.2.1
 5. 16.11 RC2 for both host and guest
 6. PVP, testpmd macswap for both host and guest

BTW, I do see indirect_desc gives slightly better performance for loopback
in tests on other platforms, but don't know how PVP performs yet.


> 
> Regards,
> Maxime