This paper proposes an approach to scaling UNIX-like operating systems for many cores in a backward-compatible way, which still enjoys common wisdom in new operating system designs. The proposed system, called Cerberus, mitigates contention on many shared data structures within OS kernels by clustering multiple commodity operating systems atop a VMM, and providing applications with the traditional shared memory interface. Cerberus extends a traditional VMMwith efficient support for resource sharing and communication among the clustered operating systems. It also routes system calls of an application among operating systems, to provide applications with the illusion of running on a single operating system.