[Qemu-discuss] Network performance issues between VM and host

[Chris Pierzycki]

Hi,

I'm trying to deploy few KVM servers based on the following AMD 6xxx based motherboard:

http://www.supermicro.com/Aplus/motherboard/Opteron6100/SR56x0/H8QG6-F.cfm

I'm using Fedora 17 (kernel 3.4.4-5.fc17.x86_64) for the KVM host and CentOS 6 (kernel 2.6.32-220.el6.x86_64) for the VM.  Everything seems to work fine until it comes to VM's network performance.  I was able to narrow down the problem to host and the VM by running tests using iperf.  Here are the results:

# iperf -c 10.10.11.18 -r

------------------------------------------------------------

Server listening on TCP port 5001

TCP window size: 85.3 KByte (default)

------------------------------------------------------------

------------------------------------------------------------

Client connecting to 10.10.11.18, TCP port 5001

TCP window size:  197 KByte (default)

------------------------------------------------------------

[  5] local 10.10.11.250 port 46737 connected with 10.10.11.18 port 5001

[ ID] Interval       Transfer     Bandwidth

[  5]  0.0-10.0 sec  2.17 GBytes  1.86 Gbits/sec

[  4] local 10.10.11.250 port 5001 connected with 10.10.11.18 port 35905

[  4]  0.0-10.0 sec  6.45 GBytes  5.54 Gbits/sec

So while the performance isn't the same in either direction and I find even  the higher 5.54 Gbps to be slow.  Switching from virtio-net to e1000 driver actually slows the connection down to ~ 280 mbps.  I've been trying to figure out the problem for a while so I used nearly all tunable options to improve the performance and while some do make a difference it's not very big.  

At home I have a budget server with Core i3 processor and the same software setup. I am able to get 16 Gbps:

# iperf -c 192.168.7.60 -r

------------------------------------------------------------

Server listening on TCP port 5001

TCP window size: 85.3 KByte (default)

------------------------------------------------------------

------------------------------------------------------------

Client connecting to 192.168.7.60, TCP port 5001

TCP window size:  279 KByte (default)

------------------------------------------------------------

[  5] local 192.168.7.99 port 54182 connected with 192.168.7.60 port 5001

[ ID] Interval       Transfer     Bandwidth

[  5]  0.0-10.0 sec  19.4 GBytes  16.6 Gbits/sec

[  4] local 192.168.7.99 port 5001 connected with 192.168.7.60 port 43845

[  4]  0.0-10.0 sec  24.2 GBytes  20.8 Gbits/sec

So the little server is getting higher network performance by 4x.  I'm actually not sure what is the issue at all anymore.  I think it could relate to the following:

• Kernel
• Qemu
• Bridge

On another interesting note there is a 10G Myricom card in the server and I am also getting inconsistent numbers when performing a test between two hosts with maximum of 6 Gbps.  Any help would be great.  Thank you!