Can you imagine to have 52 Windows XP virtual machines - 1GB RAM each - running on a server with just 16GB? Ok, sounds a lot… but we are near the 16GB barrier. Anyway, this is one of the features in the new 2.6.32 release. Its called Kernel Samepage Merging, KSM. The approach is you start more than one VM and tell the kernel that they probably will use identically memory segments.