[libvirt] qemu-kvm networking speed

Bryan_Coleman at dart.biz Bryan_Coleman at dart.biz
Thu Mar 12 20:18:27 UTC 2009

Thank you for the suggestion.  Switching to virtio and/or e1000 helped but 
still was not able to exceed 450Mb/s.  It took me awhile to figure out you 
had to either restart or reload libvirtd for the changes to take effect. 
Is there anything else I could check/change to approach 1000Mb/s or 

Thank you,

Bryan Coleman

"Daniel P. Berrange" <berrange at redhat.com> 
Sent by: libvir-list-bounces at redhat.com
03/12/2009 09:42 AM
Please respond to
"Daniel P. Berrange" <berrange at redhat.com>

Bryan_Coleman at dart.biz
libvir-list at redhat.com
Re: [libvirt] qemu-kvm networking speed

On Thu, Mar 12, 2009 at 09:28:56AM -0400, Bryan_Coleman at dart.biz wrote:
> How do I change the networking speed provided to my qemu-kvm guest?  I 
> have four gigabit ports bonded; however, my guest is showing just 
>  I have used iperf to confirm.

The reported 'speed' setting for emulated NICs is basically irrelevant
since this is emulated hardware. The original real hardware may have
been specced as 100mb/s, but that has no bearing on its speed in a
virtual environment. The QEMU emulated devices will send/recv data until
the hit a bottleneck, typically the CPU usage of the QEMU process is
hit first. So you want a NIC that entails a low CPU overhead in QEMU.
In order of prefernce you'll want to try virtio, e1000, rtl8139, ne2k_pci
This can be set in libvirt xml with <model type='virtio'/> in the guest
XML <interface> section. Obviously you need to adjust your guest OS to
load correct drivers to match.

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