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1、Virtual machineA virtual machine (VM) is a completely isolated guest operating system installation within your normal host operating system.Modern virtual machines are implemented with either software emulation or hardware virtualization.A virtual machine (VM) is a software implementation of a machi
2、ne (i.e. a computer) that executes programs like a physical machine. Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete
3、operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual
4、machineit cannot break out of its virtual world.A virtual machine was originally defined by Popek and Goldberg as an efficient, isolated duplicate of a real machine. Current use includes virtual machines which have no direct correspondence to any real hardware.System virtual machinesmultiple OS envi
5、ronments can co-exist on the same computer, in strong isolation from each otherthe virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machineapplication provisioning, maintenance, high availability and disaster recoverya virtual machine
6、 is less efficient than a real machine when it accesses the hardware indirectlywhen multiple VMs are concurrently running on the same physical host, each VM may exhibit a varying and unstable performance (Speed of Execution, and not results), which highly depends on the workload imposed on the syste
7、m by other VMs, unless proper techniques are used for temporal isolation among virtual machines.Multiple VMs each running their own operating system (called guest operating system) are frequently used in server consolidation, where different services that used to run on individual machines to avoid
8、interference are instead run in separate VMs on the same physical machine.The desire to run multiple operating systems was the original motivation for virtual machines, as it allowed time-sharing a single computer between several single-tasking Operation Systems. In some respects, a system virtual m
9、achine can be considered a generalization of the concept of virtual memory that historically preceded it. IBMs CP/CMS, the first systems to allow full virtualization, implemented time sharing by providing each user with a single-user operating system, the CMS. Unlike virtual memory, a system virtual
10、 machine allowed the user to use privileged instructions in their code. This approach had certain advantages, for instance it allowed users to add input/output devices not allowed by the standard system.The guest OSes do not have to be all the same, making it possible to run different OSes on the sa
11、me computer (e.g., Microsoft Windows and Linux, or older versions of an OS to support software that has not yet been ported to the latest version). The use of virtual machines to support different guest OSes is becoming popular in embedded systems; a typical use is to support a real-time operating s
12、ystem at the same time as a high-level OS such as Linux or Windows.Another use is to sandbox an OS that is not trusted, possibly because it is a system under development. Virtual machines have other advantages for OS development, including better debugging access and faster reboots.Process virtual m
13、achinesA process VM, sometimes called an application virtual machine, runs as a normal application inside a host OS and supports a single process. It is created when that process is started and destroyed when it exits. Its purpose is to provide a platform-independent programming environment that abs
14、tracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.A process VM provides a high-level abstraction that of a high-level programming language (compared to the low-level ISA abstraction of the system VM). Process VMs are i
15、mplemented using an interpreter; performance comparable to compiled programming languages is achieved by the use of just-in-time compilation.This type of VM has become popular with the Java programming language, which is implemented using the Java virtual machine. Other examples include the Parrot v
16、irtual machine, which serves as an abstraction layer for several interpreted languages, and the .NET Framework, which runs on a VM called the Common Language Runtime.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 parallel appli
