technical Perspective
a Vm ‘engine’ that
makes a Difference
By Carl Waldspurger

the PASt DecADe has witnessed a renaissance in server virtualization, which is transforming enterprise computing by offering new capabilities and efficien-cies. The following paper by Diwaker Gupta et al. presents a novel approach for significantly improving the efficiency of virtualized servers. Their “ Difference Engine” eliminates memory redundancy by exploiting similarity both within and across virtual machines.

A virtual machine (VM) is a software abstraction that behaves like hardware. The classic definition by Popek and Goldberg is “an efficient, isolated duplicate of a real machine.” For example, a VM that presents the illusion of being a physical x86 server may run an unmodified operating system designed for that platform, such as Windows or Linux. Neither the OS nor its users need be aware they are interacting with a VM instead of dedicated hardware.

Little more than a decade ago, virtual machines were considered a fairly exotic mainframe technology. Today, VMs are pervasive in corporate data-centers, and serve as the foundation for cloud-computing platforms. The commercial success of virtual machines has influenced the design of high-volume processor architectures, which now contain special-purpose hardware to accelerate virtualization.

Why have VMs proliferated so rapidly? One reason is that virtualization is an extremely versatile technology.

There is a well-known adage: “All problems in computer science can be solved by another level of indirection.” The virtualization software layer, known as a hypervisor, provides this level of indirection, decoupling an OS and its applications from physical hardware. Eliminating the traditional “one machine, one OS” constraint opens up numerous possibilities.

Initially, the most compelling use of
VMs was basic partitioning and server
consolidation. In typical unvirtualized
environments, individual servers were
grossly underutilized. Virtualization al-
lowed many servers to be consolidated
as VMs onto a single physical machine,
resulting in significantly lower capi-
tal and management costs. This abil-
ity to “do more with less” fueled the
rapid adoption of virtualization, even
through economic downturns.

Carl Waldspurger ( [e-mail]) is a Principal engineer at VMware, Palo alto, where he oversees core resource management and virtualization technologies.