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Re: [Xen-users] Bad TCP accept performance

I should have probably stated in my previous post that I performed this testing on Domain-0 hoping to achieve best performance.

Will experiment inside VMs tomorrow also.


On 26 May 2011, at 00:09, Iain Kay wrote:

I've done some testing with Xen 4.1.0 and 2.6.39 vanilla kernel. Also tested on CentOS 5.6 stock kernel and 2.6.39 without xen-4.1.gz loading it.

The results speak for themselves. I can definitely confirm the issue you are reporting on current xen stable (4.1.0) with latest 2.6.39 vanilla kernel.

The tests I ran each (per kernel) 3 times and took the mean values. This is most fair I think. I then took those values and plotted a graph showing performance with the different kernels:

For those of you that would like the raw output from the tests, along with /proc/cpuinfo + /proc/meminfo, here's a link to that:

I think it's interesting to observe that the 2.6.39 kernel performs the best of all without Xen loading but when loading kernel xen-4.1.gz with 2.6.39 as a module the performance plummets. 

Pretty knackered for tonight but I will see what I can do tomorrow and see if I can tune anything to get the performance up.


On 25 May 2011, at 11:55, Carl Byström wrote:

2011/5/25 Iain Kay <spam@xxxxxxxxxxx>
None of your Xen tests where performed on a suitably recent kernel... KVM was performed on kernel whereas Xen was performed on 2.6.32... Fair difference. Kernel development has been pretty active lately and I would not be surprised if you see major differences between older and newer kernels when using for virtualisation.

I've tried on EC2 using a 2.6.38 kernel (Ubuntu 11.04 Natty 32-bit, AMI ami-e2af508b), same results there.
Someone also submitted a report from Linode with kernel
I'm attempting to install Xen on top of 2.6.39 vanilla kernel today. Will run your tests on my box and see what the results are...

Please do.
Quite disheartening to see such a drop in accept performance, I for one hope it's just misconfiguration (or requires further configuration) to bring the VMs up to spec.

Yes, I hope so. At least I hope for the possibility to mitigate this by some guest OS tuning (but I'm starting to think that's a bit naïve).

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