On 11/04/2016 08:57 AM, Maxime Coquelin wrote: > Hi Zhihong, > > On 11/04/2016 08:20 AM, Wang, Zhihong wrote: >> >> >>> -----Original Message----- >>> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com] >>> Sent: Thursday, November 3, 2016 4:11 PM >>> To: Wang, Zhihong <zhihong.wang at intel.com>; Yuanhan Liu >>> <yuanhan.liu at linux.intel.com> >>> Cc: stephen at networkplumber.org; Pierre Pfister (ppfister) >>> <ppfister at cisco.com>; Xie, Huawei <huawei.xie at intel.com>; dev at >>> dpdk.org; >>> vkaplans at redhat.com; mst at 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 at redhat.com] >>>>>> Sent: Friday, October 28, 2016 3:42 PM >>>>>> To: Wang, Zhihong <zhihong.wang at intel.com>; Yuanhan Liu >>>>>> <yuanhan.liu at linux.intel.com> >>>>>> Cc: stephen at networkplumber.org; Pierre Pfister (ppfister) >>>>>> <ppfister at cisco.com>; Xie, Huawei <huawei.xie at intel.com>; >>> dev at dpdk.org; >>>>>> vkaplans at redhat.com; mst at 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 at linux.intel.com] >>>>>>>>> Sent: Thursday, October 27, 2016 6:46 PM >>>>>>>>> To: Maxime Coquelin <maxime.coquelin at redhat.com> >>>>>>>>> Cc: Wang, Zhihong <zhihong.wang at intel.com>; >>>>>>>>> stephen at networkplumber.org; Pierre Pfister (ppfister) >>>>>>>>> <ppfister at cisco.com>; Xie, Huawei <huawei.xie at intel.com>; >>>>>> dev at dpdk.org; >>>>>>>>> vkaplans at redhat.com; mst at 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: > 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: > 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? > >> >> So in all, could you try PVP traffic on HSW to see how it works? > Sadly, the HSW machine I borrowed does not have other device connected > back to back on its 10G port. I can only test PVP with SNB machines > currently. > >> >> >>> >>> 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, > Maxime
