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. -vlad >> >> Maybe you remember, we had a similar situation with commit 3e4f8b787370978733ca6cae452720a4f0c296b8 >> (macvtap: Perform GSO on forwarding path), the setup is basically the same. >> >> >> Christian >> >> -- >> To unsubscribe from this list: send the line "unsubscribe netdev" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
