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

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.

Maybe you remember, we had a similar situation with commit 3e4f8b787370978733ca6cae452720a4f0c296b8
(macvtap: Perform GSO on forwarding path), the setup is basically the same.


Christian