> Subject: Re: Network throughput limits for local VM <-> VM > communication > > On Tuesday 09 June 2009, Fischer, Anna wrote: > > > I have tried using virtio and using the emulated QEMU virtual NICs. > > It does not make a difference. It seems as if there is an overflow > somewhere > > when QEMU/virtio cannot cope with the network load any more, and then > the > > virtual interfaces don't seem to transmit anything anymore. It seems > to > > mostly work again when I shut down and start up the interfaces of the > router > > inside of the guest. I use two bridges (and VLANs) that pass packets > between > > sending/receiving guests and the routing guest. The set-up works fine > for > > simple ping and other communication that is low-throughput type > traffic. > > Have you tried eliminating VLAN to simplify the setup? No - but there is a relating bug in the tun/tap interface (well, it is not really a bug but simply the way tun/tap works) that will cause packets to be replicated on all the tap interfaces (across all bridges attached to those) if I do not configure VLANs. This will result in a system that is even more overloaded. I had discovered this a while back when running UDP stress tests under 10G. > Does it change when the guests communicate over a -net socket interface > with your router instead of the -net tap + bridge in the host? I have not tried this - I need the bridge in the network data path for some testing, so using the -net socket interface would not solve my problem. However, I have just today managed to get around this bug by using the e1000 QEMU emulated NIC model and this seems to do the trick. Now the throughput is still very low, but that might simply be because my system is too weak. When using the e1000 model instead of rtl8139 or virtio, I do not have any network crashes any more. I have been looking through the RedHat bug lists to search for some hints today, and it seems as if there are a lot of people seeing the network under KVM break down under heavy load. I think that this is something that needs some further investigation. I can provide more detailed system set-up etc, it should be easy to reproduce this. Thanks for your help, Anna -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
