It is a build time trend chart from a Jenkins job building a Maven project. Around build # 480 the job was switched from a bare metal builder to a same size virtual machine. Average build time increased for ~25%.
The job was running on a dedicated Jenkins slave machine running CentOS 6 Linux. Earlier it had an i7 4 core + HT CPU with 8 GB RAM and software RAID0 over 2 rotating disks. Then it moved to a Xen 4 virtual machine (paravirtualized) with the same characteristics, only CPU logical cores number was set to 7 instead of 8. No other load was put on this physical box. So the performance drop is due to virtualization only.
Build time change was similar for the other jobs. Moreover C# builds running on Windows Server 2008 boxes shown the same figures. It means
migration from a physical machine to a Xen VM costs about 25% build time increase.
Given various comparative benchmarks of different hypervisors I don't expect better results from the other virtualization technologies. Of course things like Linux cgroups don't count. Another important note - we use local disks. For networked storage things are completely different.
