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

[Dietmar Maurer]

> 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?

qemu-kvm 1.2, compile from source (no patches)

I would also try with latest qemu, but seem qemu site is down?

> Was performance better in previous versions?

We observed the problem only with version 1.2.

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

Sure. Using vhost=on drops performance from 100MB/s to 15MB/s.