Re: [Qemu-devel] slow virtio network with vhost=on and multiple cores

[Stefan Hajnoczi]

On Mon, Nov 5, 2012 at 12:51 PM, Dietmar Maurer <address@hidden> wrote:
> Network performance with vhost=on is extremely bad if a guest uses multiple 
> cores:
>
> HOST kernel: RHEL 2.6.32-279.11.1.el6
> KVM 1.2.0
> GuestOS: Debian Squeeze (amd64 or 686), CentOS 6.2
>
> Test with something like (install Debian Squeeze first):
>
> ./x86_64-softmmu/qemu-system-x86_64 -smp sockets=1,cores=2 -m 512 -hda 
> debian-squeeze-netinst.raw -netdev type=tap,id=net0,ifname=tap111i0,vhost=on 
> -device virtio-net-pci,netdev=net0,id=net0
>
> Downloading a larger file with wget inside the guest will show the problem. 
> Speed drops from 100MB/s to 15MB/s.
>
> Can someone reproduce that bug? Any ideas how to fix that?

Which exact QEMU are you using: qemu or qemu-kvm?  Distro package or
from source?  Any patches applied?

Was performance better in previous versions?

Is the network path you are downloading across reasonably idle so that
you get reproducible results between runs?

Stefan