From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: macvtap performance regression (bisected) between 3.13 and 3.14-rc1 Date: Mon, 03 Mar 2014 10:13:22 +0100 Message-ID: <531447B2.7040008@de.ibm.com> References: <530FA586.3010400@de.ibm.com> <53110A62.7070109@redhat.com> <5311C142.6040509@de.ibm.com> <53123495.7030902@gmail.com> <531287AE.5080606@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , Jason Wang , "Michael S. Tsirkin" , netdev@vger.kernel.org, KVM list , Linux Kernel Mailing List To: Vlad Yasevich , vyasevic@redhat.com Return-path: In-Reply-To: <531287AE.5080606@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 02/03/14 02:21, Vlad Yasevich wrote: > On 03/01/2014 02:27 PM, Vlad Yasevich wrote: >> On 03/01/2014 06:15 AM, Christian Borntraeger wrote: >>> On 28/02/14 23:14, Vlad Yasevich wrote: >>>> On 02/27/2014 03:52 PM, Christian Borntraeger wrote: >>>>> Vlad, >>>>> >>>>> commit 6acf54f1cf0a6747bac9fea26f34cfc5a9029523 >>>>> macvtap: Add support of packet capture on macvtap device. >>>>> >>>>> causes a performance regression for iperf traffic between two KVM guests >>>>> on my s390 system. Both guests are connected via two macvtaps on the same OSA >>>>> network card. >>>>> Before that patch I get ~20 Gbit/sec between two guests, afterwards I get >>>>> ~4Gbit/sec >>>>> >>>>> Latency seems to be unchanges (uperf 1byte ping pong). >>>>> >>>>> According to ifconfig in the guest, I have ~ 1500 bytes per packet with this >>>>> patch and ~ 40000 bytes without. So for some reason this patch causes the >>>>> network stack to do segmentation. (the guest kernel stays the same, only host >>>>> kernel is changed). >>>>> >>>>> Any ideas? >>>> >>>> I am looking. It shouldn't cause addition segmentations and when I ran >>>> netperf on the code I didn't see any difference in the throughput. >>> >>> Dont know if the different bytes/packets ratio is really the reason or >>> just a side effect. As a hint: the underlying network device does not support >>> segmentation, but this should not matter for traffic between to guests. >> >> Could you post 'ethtool -k' output for both lower-level device and the >> macvtap device? >> >> Thanks >> -vlad >> > > Ok. I think I see what's happening. Since you turn off offloads on > lower device, that's propagated to macvlan device. As a result, when > when we call dev_queue_xmit on the vlan->dev, we end up segmenting since > lower level says it does support segmentation. > > One way to fix this is to never disable offloads on macvlan. macvlan > will always try to use __dev_queue_xmit() with it's lower device, so any > segmentation can happen there. If you have anything that I should test, let me know. Christian