Comparison With Other Technologies
User-mode Linux is generally considered to have lower performance than some competing technologies, such as Xen and OpenVZ. Future work in adding support for x86 virtualization to UML may reduce this disadvantage.
Often cited as a strength of Xen (a competing technology) is support for Thread Local Storage (TLS). This is now also supported in the latest UML kernels. Xen concentrates on virtualizing the whole machine, and thus all systems running on a Xen machine are really virtual machines. In UML, the host machine is not virtualized in any way, and only guest systems are true virtual machines. This allows UML guest direct access to host filesystems and hardware, where it is common to map a host directory (e.g. /uml/root -> /).
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