[CentOS-virt] performance differences between kvm/xen

Tue Oct 26 20:15:40 UTC 2010
Eric Searcy <emsearcy at gmail.com>

On Oct 25, 2010, at 8:14 AM, Todd Deshane wrote:

> I was also going to mention that we should look at scalability and
> performance isolation.
> Some references and previous studies here:
> http://todddeshane.net/research/Xen_versus_KVM_20080623.pdf
> http://clarkson.edu/~jnm/publications/isolation_ExpCS_FINALSUBMISSION.pdf
> http://clarkson.edu/~jnm/publications/freenix04-clark.pdf

I only got as far as the top one.  One concern: the nestled comment "We believe that KVM may have performed better than Xen in terms of I/O due to disk caching" makes me skeptical of the value of the results if this wasn't taken into consideration (in other words I think it is a much bigger problem than the aforementioned comment gives credit to, such that it ought to be at least addressed in the concluding remarks) ... for instance if my VM load-outs use all but ~384M of total memory (that being the amount I leave to the host, most of it used) then there's not going to be much extra RAM for memory cache/buffers with on the host side (depending greatly on what vm.swappiness camp you are in).  Based on the author's result output [1] (since the VM parameters aren't given in the paper), as relates to a disk-intensive test they in effect gave 2G potential caching to Xen but ~4G to KVM.  Based at least on the amount of free memory on my Xen/KVM hosts I don't think this "host memory cache bias" can be assumed to be a bonus trait that would normally be present for KVM.  (And of course a cache bias would be even more noticeable in the 256MB Phoronix test and in the 4x128M isolation tests [2] ...)

[1] http://web2.clarkson.edu/projects/virtualization/benchvm/results/performance/
[2] http://web2.clarkson.edu/projects/virtualization/benchvm/results/isolation/xen/memory/specweb1/SPECweb_Support.20080614-100931.html

BTW, I do realize you're pointing out that we should look at scalability and isolation, and here I am just giving critical feedback on a 3 year old paper ... yes you're right those are important!  I just want to make sure the tests are fair ;-)