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

[Peter Lieven]

Dietmar Maurer wrote:
>> > ./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?
>
> I just tested with latest sources from git://git.qemu-project.org/qemu.git
>
> ./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 --enable-kvm
>
> got incredible bad performance  - can't you reproduce the problem?

I have seen a similar problem, but it seems to be only with extreme old
guest kernels. With Ubuntu 12.04 it seems not to be there. Can you try a
recent guest kernel.