On 03/03/2014 04:13 AM, Christian Borntraeger wrote:

On 02/03/14 02:21, Vlad Yasevich wrote:

quoted

On 03/01/2014 02:27 PM, Vlad Yasevich wrote:

quoted

On 03/01/2014 06:15 AM, Christian Borntraeger wrote:

quoted

On 28/02/14 23:14, Vlad Yasevich wrote:

quoted

On 02/27/2014 03:52 PM, Christian Borntraeger wrote:

quoted

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.

Hi Christian

Just sent out a patch to fix this.  I tried it with namespaces and
kvm guests and it seems to restore performance for me.

Please give it a try.

Thanks
-vlad

Christian