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

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Date:	Fri, 28 Feb 2014 17:14:58 -0500
From:	Vlad Yasevich <vyasevic@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.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: Re: macvtap performance regression (bisected) between 3.13 and 3.14-rc1

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.

-vlad

> 
> 
> Christian
> 

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