Editing Virtual machine (section)

== Definitions ==

=== System virtual machines ===
{{main|System virtual machine}}
{{See also|Hardware virtualization|comparison of platform virtualization software}}

A 'virtual machine' was originally defined by [[Popek and Goldberg virtualization requirements|Popek and Goldberg]] as "an efficient, isolated duplicate of a real computer machine."<ref name="Popek"/> Current use includes virtual machines that have no direct correspondence to any real hardware.<ref name="Smith_Nair_05"/> The physical, "real-world" hardware running the VM is generally referred to as the 'host', and the virtual machine emulated on that machine is generally referred to as the 'guest'. A host can emulate several guests, each of which can emulate different [[Operating system|operating systems]] and hardware platforms.

The desire to run multiple operating systems was the initial motive for virtual machines, so as to allow time-sharing among several single-tasking operating systems. In some respects, a system virtual machine can be considered a generalization of the concept of [[virtual memory]] that historically preceded it. IBM's [[CP/CMS]], the first systems to allow [[full virtualization]], implemented [[time sharing]] by providing each user with a single-user operating system, the [[Conversational Monitor System]] (CMS). Unlike virtual memory, a system virtual machine entitled the user to write privileged instructions in their code. This approach had certain advantages, such as adding input/output devices not allowed by the standard system.<ref name="Smith_Nair_05"/>

As technology evolves virtual memory for purposes of virtualization, new systems of [[memory overcommitment]] may be applied to manage memory sharing among multiple virtual machines on one computer operating system. It may be possible to share ''memory pages'' that have identical contents among multiple virtual machines that run on the same physical machine, what may result in mapping them to the same physical page by a technique termed [[kernel same-page merging]] (KSM). This is especially useful for read-only pages, such as those holding code segments, which is the case for multiple virtual machines running the same or similar software, software libraries, web servers, [[middleware]] components, etc. The guest operating systems do not need to be compliant with the host hardware, thus making it possible to run different operating systems on the same computer (e.g., [[Microsoft Windows|Windows]], [[Linux]], or prior versions of an operating system) to support future software.<ref name="Oliphant"/>

The use of virtual machines to support separate guest operating systems is popular in regard to [[embedded system]]s. A typical use would be to run a [[real-time operating system]] simultaneously with a preferred complex operating system, such as Linux or Windows. Another use would be for novel and unproven software still in the developmental stage, so it runs inside a [[Sandbox (software development)|sandbox]]. Virtual machines have other advantages for operating system development and may include improved debugging access and faster reboots.<ref name="vmwarez_2006"/>

Multiple VMs running their own guest operating system are frequently engaged for server consolidation.<ref name="vmware_2007"/>

=== Process virtual machines ===
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{{Redirect-distinguish|Application virtual machine|application virtualization}}
{{See also|Comparison of application virtualization software}}

A '''process virtual machine''', sometimes called an ''application virtual machine'', or ''Managed Runtime Environment'' (MRE), runs as a normal application inside a host OS and supports a single process. It is created when that process is started and deleted when it is closed. Its purpose is to provide a [[system platform|platform]]-independent programming environment that abstracts away details of the underlying hardware or operating system and allows a program to execute in the same way on any platform.{{Citation needed|date=November 2015}}

A process VM provides a high-level abstraction{{snd}} that of a [[high-level programming language]] (compared to the low-level ISA abstraction of the system VM). Process VMs are implemented using an [[Interpreter (computing)|interpreter]]; performance comparable to compiled programming languages can be achieved by the use of [[just-in-time compilation]].{{Citation needed|date=November 2015}}

This type of VM has become popular with the [[Java (programming language)|Java programming language]], which is implemented using the [[Java virtual machine]]. Other examples include the [[Parrot virtual machine]] and the [[.NET Framework]], which runs on a VM called the [[Common Language Runtime]]. All of them can serve as an [[abstraction layer]] for any computer language.{{Citation needed|date=November 2015}}

A special case of process VMs are systems that abstract over the communication mechanisms of a (potentially heterogeneous) [[computer cluster]]. Such a VM does not consist of a single process, but one process per physical machine in the cluster. They are designed to ease the task of programming concurrent applications by letting the programmer focus on algorithms rather than the communication mechanisms provided by the interconnect and the OS. They do not hide the fact that communication takes place, and as such do not attempt to present the cluster as a single machine.{{Citation needed|date=March 2013}}

Unlike other process VMs, these systems do not provide a specific programming language, but are embedded in an existing language; typically such a system provides bindings for several languages (e.g., [[C (programming language)|C]] and [[Fortran]]).{{Citation needed|date=March 2013}} Examples are [[Parallel Virtual Machine]] (PVM) and [[Message Passing Interface]] (MPI).