Re: Virtio network performance on Debian

[Michael Tokarev <mjt@tls.msk.ru>]

On 16.04.2012 13:01, Stefan Pietsch wrote:
> On 12.04.2012 09:42, Hans-Kristian Bakke wrote:
>> Hi
>>
>> For some reason I am not able to get good network performance using
>> virtio/vhost-net on Debian KVM host (perhaps also valid for Ubuntu
>> hosts then).
>> Disc IO is very good and the guests feels snappy so it doesn't seem
>> like there is something really wrong, just something suboptimal with
>> the networking.
> 
> [......]
> 
>> I have tried:
>> ----------------
>> - Replacing Debian Wheezy with Debian Squeeze (stable, kernel
>> 2.6.32-xx) - even worse results
>> - Replacing kernel 3.2.0-2-amd64 with vanilla kernel 3.4-rc2 and
>> config based on Debians included config - no apparent change
>> - Extracted the kernel-config file from Fedora 17 alphas kernel and
>> used this to compile a new kernel based on Debian Wheezys kernel
>> source - slightly worse results
>> - ...in addition to exchanging Debian with Fedora 17 alpha, Proxmox
>> 1.9 and 2.0 and ESXi 5 which all have expected network performance
>> using virtio.
>>
>> So, I am at a loss here. I does not seem to be kernel config related
>> (as using Fedoras config on Debian kernel source didn't do anything
>> good) so I think it must be either a kernel patch that red hat kernel
>> based distros uses to make virtio/vhost much more efficient or perhaps
>> something with Debians qemu-version, bridging or something.

Hans-Kristian submitted a bugreport to Debian BTS about this,
http://bugs.debian.org/668594 .  In this case it turns out to
be a problem with particular qemu-kvm userspace binary build,
maybe the toolchain used, maybe something else - I don't yet
know.  This is not the first time this version is mentioned
as being slow (slower) with network transfers, here's another
bugreport which I merely ignored till now -- see
http://bugs.debian.org/665046 .

I'm working on this, or trying to anyway.

> I have made some tests with a Debian Squeeze KVM host running with the
> Linux Kernel 2.6.39 from backports and the Kernel version 2.6.32-11-pve
> from Proxmox.
> 
> (http://download.proxmox.com/debian/dists/squeeze/pve/binary-amd64/pve-kernel-2.6.32-11-pve_2.6.32-66_amd64.deb)

These are very different.  Note that 2.6.32 does not have vhost-net
support which is used by Hans-Kristian and speeds up network operations
dramatically.

> Network performance between two virtual machines on the same host is
> significantly slower with the Debian kernel:
> 
> 2.6.39-bpo.2-amd64 : 1.31 Gbits/sec
> 2.6.32-11-pve      : 2.20 Gbits/sec

Please try current debian kernel.  There were a few issues with that
2.6.32 backport, but I don't know the details and don't know if these
affects qemu/kvm in any way or not.  And again, if you really want
good networking speeds, you should use vhost-net which is is not
supported by 2.6.32 kernel.

> iperf tests between a virtual machine and the KVM host connected to the
> same local bridge interface showed similar results.
> 
> Are there other people who can confirm this?

Thanks,

/mjt