netdev - macvtap performance regression (bisected) between 3.13 and 3.14-rc1

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Message-ID: <530FA586.3010400@de.ibm.com>
Date:	Thu, 27 Feb 2014 21:52:22 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Vlad Yasevich <vyasevic@...hat.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	KVM list <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: macvtap performance regression (bisected) between 3.13 and 3.14-rc1

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?

Christian

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