linux KVM guests network performance is slower than host

Question

i have a KVM linux based virtual machines, as the network performance of the host is running perfect with 1 Gbps (on the internal network) and using iperf tools it show that the network is running perfect on the host, but guests have slow network performance as iperf result was so strange as it is not 100 Mbps and is not 1 Gbps, it's between 350 and 500 Mbps maximum. This is the result of runnin iperf -c servername -P 15:

[ ID] Interval Transfer Bandwidth [ 17] 0.0-10.0 sec 68.6 MBytes 57.5 Mbits/sec [ ID] Interval Transfer Bandwidth [ 4] 0.0-10.0 sec 18.6 MBytes 15.5 Mbits/sec [ ID] Interval Transfer Bandwidth [ 5] 0.0-10.0 sec 61.9 MBytes 51.9 Mbits/sec [ ID] Interval Transfer Bandwidth [ 11] 0.0-10.0 sec 16.0 MBytes 13.4 Mbits/sec [ ID] Interval Transfer Bandwidth [ 10] 0.0-10.0 sec 24.3 MBytes 20.4 Mbits/sec [ ID] Interval Transfer Bandwidth [ 15] 0.0-10.0 sec 20.2 MBytes 16.9 Mbits/sec [ ID] Interval Transfer Bandwidth [ 12] 0.0-10.0 sec 30.2 MBytes 25.3 Mbits/sec [ ID] Interval Transfer Bandwidth [ 3] 0.0-10.0 sec 36.2 MBytes 30.3 Mbits/sec [ ID] Interval Transfer Bandwidth [ 8] 0.0-10.0 sec 23.6 MBytes 19.8 Mbits/sec [ ID] Interval Transfer Bandwidth [ 9] 0.0-10.0 sec 24.2 MBytes 20.3 Mbits/sec [ ID] Interval Transfer Bandwidth [ 14] 0.0-10.0 sec 31.0 MBytes 25.9 Mbits/sec [ ID] Interval Transfer Bandwidth [ 7] 0.0-10.0 sec 16.6 MBytes 13.9 Mbits/sec [ ID] Interval Transfer Bandwidth [ 6] 0.0-10.0 sec 18.8 MBytes 15.7 Mbits/sec [ ID] Interval Transfer Bandwidth [ 13] 0.0-10.0 sec 13.3 MBytes 11.2 Mbits/sec [ ID] Interval Transfer Bandwidth [ 16] 0.0-10.0 sec 29.6 MBytes 24.8 Mbits/sec [SUM] 0.0-10.0 sec 433 MBytes 363 Mbits/sec 

I tested different network models and the best result was with the virtio model any one please can help to get the full performance of network and be 1 Gbps.

I would like to add that when i execute the command brctl show I got the fbridge name

bridge id STP enabled interfaces br0 8000.d067e5fb7dec no eth0 vnet0 vnet1 vnet2 br2 8000.d067e5fb7dee no eth1 virbr0 8000.000000000000 yes 

can this affect the network speed of the guest?

Answer 1

Try changing ethernet driver settings.

/sbin/ethtool --offload eth0 gso off tso off sg off gro off 

If you experience low performance with the para-virtualized network drivers, verify the setting for the GSO and TSO features on the host system. The para-virtualized network drivers require that the GSO and TSO options are disabled for optimal performance.

Answer 2

I suppose you already using virtio driver in your VMs, its a must to have decent network performance.

You can also try using quite new guest netdev model, which uses some advanced memory mapping techniques to achieve even better network performance. You VMs should use the same configuration as with virtio drivers; kvm parameters to set start VM should be like this (fix them for your needs):

-netdev tap,vhost=on,ifname=vnet0,id=guest0,script=/path/to/custom/tap-ifup,downscript=no \ -device virtio-net-pci,netdev=guest0,mac=00:16:3e:47:aa:53 

With this configuration I've achieved 6 Gb/s.

Answer 3

For best network performance, use virtio drivers in the definition of the host in KVM.

For example:

<interface type='bridge'> <source bridge='virbr0'/> <mac address="46:AD:2B:48:00:00"/> <model type='virtio'/> </interface> 

This posssibly requires additionnal virtio drivers. They are found in a CD that can be mounted on the virtual host. (source code https://github.com/virtio-win/kvm-guest-drivers-windows and binary download https://fedorapeople.org/groups/virt/virtio-win/direct-downloads/archive-virtio/virtio-win-0.1.217-1/)