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

[Dietmar Maurer]

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?

- Dietmar