My understanding is that memory sharing is generally not done in multi-tenant xen systems, and that this is one of the reasons why Xen is so dominant in that sp

My understanding is that memory sharing is generally not done in multi-tenant xen systems, and that this is one of the reasons why Xen is so dominant in that space. OpenVZ, generally speaking, shares memory, but does not use ballooning; ballooning is specific to systems where each user has their own kernel. Generally speaking, I think it's much less bad to oversubscribe CPU than memory. Among other things, if you take cpu away from a heavy user to give to a user who has not used their fair share, all the new user has to do is reload cpu cache from main memory, which is slow, but super fast compared to reloading main memory cache from disk, which is what the light user has to do when you take memory away from the heavy user. Of course, the equation is quite different when the system is all owned by the same entity.