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: Thursday, November 3, 2016 4:11 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
> 
> 
> 
> On 11/02/2016 11:51 AM, Maxime Coquelin wrote:
> >
> >
> > On 10/31/2016 11:01 AM, Wang, Zhihong wrote:
> >>
> >>
> >>> -----Original Message-----
> >>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> >>> Sent: Friday, October 28, 2016 3:42 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
> >>>
> >>>
> >>>
> >>> 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?
> >>
> >> Hi Maxime,
> >>
> >> Let's align our test case to RC2, mrg=on, loopback, on Haswell.
> >> My results below:
> >>  1. indirect=1: 5.26 Mpps
> >>  2. indirect=0: 6.54 Mpps
> >>
> >> It's about 24% drop.
> > OK, so on my side, same setup on Haswell:
> > 1. indirect=1: 7.44 Mpps
> > 2. indirect=0: 8.18 Mpps
> >
> > Still 10% drop in my case with mrg=on.
> >
> > The strange thing with both of our figures is that this is below from
> > what I obtain with my SandyBridge machine. The SB cpu freq is 4% higher,
> > but that doesn't explain the gap between the measurements.
> >
> > I'm continuing the investigations on my side.
> > Maybe we should fix a deadline, and decide do disable indirect in
> > Virtio PMD if root cause not identified/fixed at some point?
> >
> > Yuanhan, what do you think?
> 
> I have done some measurements using perf, and know understand better
> what happens.
> 
> With indirect descriptors, I can see a cache miss when fetching the
> descriptors in the indirect table. Actually, this is expected, so
> we prefetch the first desc as soon as possible, but still not soon
> enough to make it transparent.
> In direct descriptors case, the desc in the virtqueue seems to be
> remain in the cache from its previous use, so we have a hit.
> 
> That said, in realistic use-case, I think we should not have a hit,
> even with direct descriptors.
> Indeed, the test case use testpmd on guest side with the forwarding set
> in IO mode. It means the packet content is never accessed by the guest.
> 
> In my experiments, I am used to set the "macswap" forwarding mode, which
> swaps src and dest MAC addresses in the packet. I find it more
> realistic, because I don't see the point in sending packets to the guest
> if it is not accessed (not even its header).
> 
> I tried again the test case, this time with setting the forwarding mode
> to macswap in the guest. This time, I get same performance with both
> direct and indirect (indirect even a little better with a small
> optimization, consisting in prefetching the 2 first descs
> systematically as we know there are contiguous).


Hi Maxime,

I did a little more macswap test and found out more stuff here:

 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 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

So in all, could you try PVP traffic on HSW to see how it works?


> 
> Do you agree we should assume that the packet (header or/and buf) will
> always be accessed by the guest application?
> If so, do you agree we should keep indirect descs enabled, and maybe
> update the test cases?


I agree with you that mac/macswap test is more realistic and makes
more sense for real applications.


Thanks
Zhihong



> 
> Thanks,
> Maxime