Re: kvm network latency, higher with virtio ?

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 15, 2009 at 05:15:09PM +0200, Luca Bigliardi wrote:
> Hi,
> I'm running some tests between two linux instances bridged together.
> 
> If I try to ping 10 times I obtain the following results:
> 
> -net nic,model=virtio -net tap :
>     rtt min/avg/max/mdev = 0.756/0.967/2.115/0.389 ms
> 
> -net nic,model=rtl8139 -net tap :
>     rtt min/avg/max/mdev = 0.301/0.449/1.173/0.248 ms
> 
> So it seems with virtio the latency is higher. Is it normal?

Yes, the main reason is the TX timer it uses for interrupt/vm exit mitigation.

> The results I'm reporting were obtained with
> - host
>   qemu-kvm 0.11-rc2
>   kvm-kmod-2.6.30.1
>   kernel: 2.6.30.5 (HIGH_RES_TIMERS=y as suggested in
>                     http://www.linux-kvm.org/page/Virtio )
> - guest
>   kernel: 2.6.31
> 
> but I also tested older versions always obtaining latency values at least two
> times higher than rtl8139/e1000 .
> 
> Thank you,
> Luca
> 
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